Front-end electronic circuit for electromagnetic radiation sensor application examples

Abstract

A front-end electronic circuit (10) for an electromagnetic radiation sensor application includes a signal shaping circuit (1000) having an amplifier circuit (1100) and an active dynamic feedback circuit (1200) disposed in a feedback path (1001) of the signal shaping circuit (1000). The active dynamic feedback circuit (1200) includes a first input transistor (100) disposed in a first current path (1201) of the active dynamic feedback circuit and a second input transistor (200) (1200) disposed in a second current path (1202) of the active dynamic feedback circuit. The first input transistor (100) has a control node for receiving an output signal (Vout_shaper) of the signal shaping circuit (1000), and the second input transistor (200) has a control node for receiving a reference signal (Vref). The active dynamic feedback circuit (1200) includes a buffer circuit (300) arranged to decouple first and second current paths (1201, 1202).

Description

Technical Field

[0001] The present disclosure relates to a front-end electronic circuit for electromagnetic radiation sensor applications, and more particularly to X-ray imaging applications such as multi-energy spectrum CT (Computed Tomography) or photon counting applications. The present disclosure further relates to a photon counting sensor circuit using the front-end electronic circuit, and a medical diagnostic device.

Background Art

[0002] In typical CT devices and X-ray imaging products, indirect conversion type sensors are used. The indirect conversion type sensor includes a scintillator that converts X-rays into visible light. The visible light is captured by a photodetector or a photodiode, and generates an electrical signal in response to the X-rays hitting the material of the scintillator.

[0003] In contrast to conventional computed tomography using the indirect detection principle, in a direct conversion type sensor, an incident photon can generate an electrical signal when hitting a direct conversion material such as CdTe / CZT. Together with the direct conversion material and continuous time asynchronous front-end electronics, a CT device can utilize the advantages of a photon counting system. Photon counting medical imaging has many advantages compared to typical approaches, such as better resolution and / or lower dose, as well as spectral information.

[0004] A photon counting imaging system requires high-speed and asynchronous continuous time processing of the input signal (input current pulse). Specifically, the photon counting approach requires a front-end circuit that receives an input current and supplies a voltage shaped at the output to facilitate further processing by a discriminator.

[0005] The front-end topology is usually a single-stage approach and a two-stage approach. When the input capacitance is small and fixed, a single-stage architecture can be used. This incorporates an amplifier circuit followed by a signal shaping circuit including a discriminator and a counter.

[0006] Figure 1 shows a front-end electronic circuit 10 by a single-stage approach. The front-end electronic circuit 10 includes a signal shaping circuit 1000. The signal shaping circuit 1000 has an input terminal I10 for receiving an input signal Iin from an electromagnetic radiation sensor, for example, a photon detector. The signal shaping circuit 1000 includes an amplifier circuit 1100. The amplifier circuit 1100 has an input side for receiving the input signal Iin from the sensor and a reference signal Vref, and an output side for supplying an output signal Vout_shaper at the output terminal O10 of the front-end circuit. A capacitor 1300 is disposed within a feedback path 1002 between the input side and the output side of the amplifier circuit 1100. A feedback element 1400, which can be configured as a resistor, is disposed within a feedback path 1001 in parallel with the capacitor 1300.

[0007] When incident photons hit the direct conversion material of the photon detector, a transient current signal is generated. This current is proportional to the energy of the incident photons. The current is applied as an input signal / input current pulse Iin to the input terminal I10, and then processed by the signal shaping circuit 1000, and a shaped voltage is supplied to the output terminal O10. The shaped voltage is proportional to the input current and thus proportional to the energy of all single incident photons. The output voltage of the shaper can be further processed by several discriminators and counters. The count number is proportional to the number of incident photons. With multiple discriminators and counters, information on the energy level of each incident photon can be further obtained.

[0008] When the input capacitance is large and variable, a two-stage architecture approach of the front-end electronic circuit can be used. Instead of a single-stage signal shaping circuit, the signal shaping circuit of the two-stage front-end approach includes a charge-sensitive amplifier coupled to a shaping amplifier, followed by a discriminator and a counter. In the two-stage topology of the front-end electronic circuit, the signal shaping stage is disconnected from the input capacitance at the input of the front-end electronic circuit. However, since the charge-sensitive amplifier functions as a buffer, the two-stage front-end circuit is accompanied by higher noise and power penalty.

[0009] The baseline of the output of the signal shaping circuit is defined by a reference voltage connected to the positive input terminals of both the charge-sensitive amplifier and the shaping amplifier. These amplifiers are typically operational transconductance amplifiers (OTAs) with differential inputs and single-ended outputs.

[0010] The most important performance parameters of the photon counting front end are low power, low noise, high counting rate, small FWHM (Full Width at Half Maximum) related to the pulse width of the shaped output, small silicon area, high linearity, and low ballistic deficit.

[0011] To reduce the pulse width, and thus the FWHM performance, and increase the counting rate, a large amount of power needs to be consumed in the aforementioned topology. This causes other problems such as undesirable thermal and thermal stability effects in such systems.

[0012] The most important components of the photon counting front end, together with the signal shaping circuit, are its feedback elements. The configuration of this block is crucial for determining the performance of the front end and thus the performance of the entire system. The signal shaping circuit itself needs to exhibit a high transconductance gm to achieve the required bandwidth. However, to achieve a sufficiently high bandwidth to enable a high counting rate, the shaping circuit needs to consume a significant amount of power.

[0013] Here, the feedback elements, which are the feedback resistor and the feedback capacitor, come into play. To reduce the FWHM to obtain a high counting rate and minimize pile-up, the feedback capacitor needs to be made sufficiently small. However, a minimum capacitance is required to maintain sufficient phase margin and avoid stability problems.

[0014] The other feedback element is a resistor, which can be implemented by a standard resistor (e.g., a polysilicon resistor) or a MOS transistor operating in the linear region. The latter is preferred because it not only saves area and can make the resistor value very large, but also a saturation behavior can be obtained when the incident radiation hits the target material of the sensor and when the shaper output deviates from the baseline during the pulse activity.

[0015] In other feedback embodiments, an active transconductor is used. However, when there is a large input energy leading to a large amplitude of the input pulse or when there are pile-up events, the existing active feedback topologies will result in current depletion and thus the input signal cannot be further processed.

[0016] There is a need to provide an approach for a front-end electronic circuit for electromagnetic radiation sensor applications that reduces the pulse width of the shaper output, and thus the FWHM, and further improves the recovery from pile-up without consuming an excessive amount of power in the signal shaping circuit. SUMMARY OF THE INVENTION

[0017] A front-end electronic circuit for electromagnetic radiation sensor applications, which enhances the pulse width of the signal shaper output of the front-end electronic circuit, and thus the FWHM, and further improves the count rate during pile-up, is described in claim 1.

[0018] The front-end electronic circuit includes an input terminal configured to be coupled to an electromagnetic radiation sensor to receive an input signal therefrom, and an output terminal for supplying an output signal. The front-end electronic circuit includes a signal shaping circuit. The signal shaping circuit includes an amplifier and an active dynamic feedback circuit disposed within a feedback path of the amplifier circuit. The amplifier circuit has an input node coupled to the input terminal of the front-end electronic circuit and an output node for supplying the output signal. The output node of the amplifier circuit is coupled to the output terminal of the front-end electronic circuit.

[0019] The active dynamic feedback circuit includes a first input transistor disposed within a first current path of the active dynamic feedback circuit. The active dynamic feedback circuit further includes a second input transistor disposed within a second current path of the active dynamic feedback circuit. The first input transistor has a control node for receiving the output signal. The second input transistor has a control node for receiving a reference signal. The active dynamic feedback circuit includes a buffer circuit disposed to disconnect the first and second current paths.

[0020] The proposed active dynamic feedback circuit can improve the input dynamic range of the signal shaping circuit and the current depletion limit during pile-up events by enhancing the pulse shape and FWHM. To achieve this, the proposed active dynamic feedback circuit intentionally introduces non-linearity by implementing a non-linear feedback resistance (1 / gm) in the feedback path of the front-end electronic circuit. As a result, the counting rate is significantly improved without consuming an excessive amount of power in the signal shaping amplifier circuit. The topology is very robust against PVT and mismatches. Further, independently of PVT, the shaper output can be adjusted to the voltage level of the reference signal.

[0021] According to an embodiment of the front-end electronic circuit, the first current path and the second current path are each connected between a terminal supplying a reference potential and a common node of the active dynamic feedback circuit. The buffer circuit is disposed between the common node and the first input transistor to disconnect the first and second current paths.

[0022] Since the buffer circuit can disconnect the bias currents of the first input transistor and the second input transistor respectively, it is possible to process a large signal or a large deviation from the baseline at the shaper output without starving the second input transistor and causing output clipping.

[0023] According to an embodiment of the front-end electronic circuit, the active dynamic feedback circuit includes a third current path. The active dynamic feedback circuit further includes a current source disposed within the third current path between a terminal supplying a supply potential and the common node. The third current path is connected in series to each of the first and second current paths.

[0024] According to an embodiment of the front-end electronic circuit, the buffer circuit includes a first input node, a second input node, and an output node. The first input node of the buffer circuit is connected to the common node. The second input node of the buffer circuit is coupled to the output node of the buffer circuit. According to a possible embodiment, the second input node of the buffer circuit is directly connected to the output node of the buffer circuit, or according to another possible embodiment, the second input node of the buffer circuit is coupled to the output node of the buffer circuit via a feedback network. The output node of the buffer circuit is connected to the first input transistor.

[0025] According to an embodiment of the front - end electronic circuit, the buffer circuit includes a transistor and a current source. The transistor is disposed within a fourth current path of the active dynamic feedback circuit. The first current path and the fourth current path are connected in parallel between a terminal supplying a reference potential and a second common node of the active dynamic feedback circuit. The current source is disposed within a fifth current path of the active dynamic feedback circuit between a terminal supplying a supply potential and the second common node. The current source coupled to the second common node ensures a stable current definition against PVT.

[0026] According to an embodiment of the front - end electronic circuit, the buffer circuit includes an amplifier, and the amplifier has a first input node connected to a common node of the active dynamic feedback circuit. The amplifier of the buffer circuit further includes a second input node connected to a second common node of the active dynamic feedback circuit. The amplifier of the buffer circuit has an output node connected to a control node of the transistor of the buffer circuit.

[0027] The amplifier of the buffer circuit can separately disconnect the bias currents of the first input transistor and the transistor of the buffer circuit respectively, so that without starving the second input transistor and causing output clipping, and without causing saturation of the shaper output, it can process large signals or large deviations from the baseline at the shaper output. By disconnecting the first current path and the fourth current path by the buffer circuit, a low - impedance node can be formed at the second common node of the active dynamic feedback circuit.

[0028] By disconnecting the respective bias currents in the first and fourth current paths, a small current in the first current path becomes possible, and a low mutual conductance, and thus a high resistance, in the feedback path of the signal shaping circuit is obtained. On the other hand, by disconnecting the respective bias currents in the first and fourth current paths, a large current in the fourth current path becomes possible, and it is possible to supply the overcurrent required by the large signal generated at the output node of the amplifier circuit of the signal shaping circuit without depletion and causing output clipping.

[0029] The amplifier of the buffer circuit provides an impedance conversion that enables the second common node of the active dynamic feedback circuit at the drain of the current source of the buffer circuit to be converted from a high impedance to a low impedance.

[0030] According to an embodiment of the front-end electronic circuit, the first input transistor operates in weak inversion. As described above, the active dynamic feedback circuit introduces non-linearity by realizing a non-linear feedback resistance. The non-linearity is achieved, for example, by biasing the first input transistor in weak inversion, as opposed to, for example, MOS resistors, which are often provided in the feedback path of signal shaping circuits conventionally.

[0031] The MOS resistor in the feedback path of the signal shaping circuit needs to be biased in strong inversion to reduce the resistance change due to its gate bias that spreads due to PVT variations of the bias generator. Furthermore, the non-linearity is achieved by the fact that the potential of the second common node is fixed by a large current by the transistor of the buffer circuit in the fourth current path, and thus the second common node becomes like a low impedance node. Disconnecting the current in the first current path from the current in the fourth current path is achieved by the amplifier of the buffer circuit.

[0032] By intentionally introducing non-linearity into the feedback path of the signal shaping stage by means of an active dynamic feedback circuit, the entire large signal amplitude of the output signal supplied at the output node of the amplifier circuit of the signal shaping circuit can be applied to the overdrive of the first input transistor without being degraded by the voltage drop at the second common node of the buffer circuit that would otherwise occur.

[0033] According to an embodiment of the front-end electronic circuit, the first and second input transistors are matched to each other. Further, the first current source of the active dynamic feedback circuit and the second current source of the buffer circuit can be matched to each other.

[0034] According to an embodiment of the front-end electronic circuit, the active dynamic feedback circuit is arranged between the first current path and the sixth current path of the active dynamic feedback circuit to couple the current from the first current path of the active dynamic feedback circuit into the sixth current path, and includes a current mirror. The sixth current path of the active dynamic feedback circuit is connected to the input node of the amplifier circuit of the signal shaping circuit.

[0035] By arranging the current mirror between the first current path and the sixth current path of the active dynamic feedback circuit, a small effective mutual conductance, i.e., a high equivalent resistance, can be realized while biasing the first input transistor with a current high enough to obtain speed and low offset.

[0036] According to an embodiment, the front-end electronic circuit includes a reference signal generation circuit coupled to the control node of the second input transistor to supply a reference signal.

[0037] In addition to the active dynamic feedback circuit, the signal shaping circuit includes a feedback capacitor disposed between the input node of the amplifier circuit and the output node of the amplifier circuit of the signal shaping circuit. Further, the signal shaping circuit includes a third current source disposed between the terminal for supplying the supply potential and the input terminal of the front-end electronic circuit. The first current source is matched with the third current source. The amplifier circuit can be embodied in a single-input, single-output configuration, or a differential-input, single-output configuration.

[0038] An embodiment of a possible application example of the front-end electronic circuit in a photon counting circuit is described in claim 14.

[0039] The photon counting circuit includes a front-end electronic circuit according to one of the foregoing embodiments. The photon counting circuit further includes a photon detector having a photon sensing region. The photon detector is configured to generate a current pulse when a photon hits the photon sensing region. The photon counting circuit further includes an energy discriminator connected to the output terminal of the front-end electronic circuit.

[0040] The photon detector is connected to the input terminal of the front-end electronic circuit. The front-end electronic circuit is configured to generate a voltage pulse at the output node of the front-end electronic circuit when a current pulse is applied to the input node of the front-end electronic circuit. The energy discriminator is configured to generate a digital signal according to the level of the voltage pulse.

[0041] A medical diagnostic device using the principle of photon counting is described in claim 15.

[0042] The device includes a photon counting circuit as described above. The device may be configured as an X-ray apparatus or a computed tomography scanner.

[0043] Further features and advantages of the front-end electronic circuit will be described in the following detailed description. Of course, both the foregoing summary and the following detailed description are merely exemplary and are intended to provide an overview or framework for understanding the nature and characteristics of the claims.

[0044] The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. Accordingly, the present disclosure is more fully understood by taking into account the following detailed description in conjunction with the accompanying drawings.

Brief Description of the Drawings

[0045]

Figure 1

Figure 2A

Figure 2B

Figure 3A

Figure 3B

Figure 4A

Figure 4B

Figure 4C

Figure 5A

Figure 5B

Figure 6

[0046] FIGS. 2A and 2B each show an embodiment of a front-end electronic circuit 10 for an electromagnetic radiation sensor application. The front-end electronic circuit 10 may be used as a photon counting shaper in a photon counting circuit. The front-end electronic circuit 10 includes an input terminal I10 coupled to a sensor sensitive to electromagnetic radiation (e.g., X-ray irradiation) and configured to receive an input signal Iin from the sensor. The sensor may be configured as a photon detector. The front-end electronic circuit further includes an output terminal O10 that supplies an output signal Vout_shaper.

[0047] The front-end electronic circuit includes a signal shaping circuit 1000. The signal shaping circuit 1000 includes an amplifier circuit 1100 and an active dynamic feedback circuit 1200. The active dynamic feedback circuit 1200 is disposed within a feedback path 1001 of the front-end electronic circuit 1000. The amplifier circuit 1100 has an input node I1100a coupled to an input terminal I10 of the front-end electronic circuit and an output node O1100 that supplies an output signal Vout_shaper. The output node O1100 of the amplifier circuit 1100 is coupled to an output terminal O10 of the front-end electronic circuit 10. A feedback capacitor 1300 is disposed between the input node I1100a and the output node O1100 of the amplifier circuit 1100.

[0048] Figure 2A shows a differential input shaping approach. The amplifier circuit 1100 is embodied in a differential input, single output configuration. The differential input amplifier circuit 1100 has an input node I1100a that receives an input signal Iin and a second input node I1100b that receives a reference signal Vref.

[0049] Figure 2B shows a single input approach of the front-end electronic circuit 10. The amplifier circuit 1100 is embodied in a single input, single output configuration. Compared to the differential input shaping approach shown in Figure 2A, the amplifier circuit 1100 of Figure 2B includes only a single input node I1100a that receives the input signal Iin. The amplifier circuit 1100 includes an input transistor 1110 having a control node coupled to the single input node I1100a. The amplifier circuit 1100 further includes a current source 1120 disposed in series with the input transistor 1110.

[0050] In contrast to the front-end electronic circuit including resistive feedback as shown in FIG. 1, the front-end electronic circuits 10 shown in FIGS. 2A and 2B include an active dynamic feedback circuit 1200 within a feedback path 1001 of the signal shaping circuit 1000. The active dynamic feedback circuit 1200 replaces the feedback resistor of the signal shaping circuit in the front-end circuit of FIG. 1.

[0051] The proposed active dynamic feedback circuit 1200 introduces non-linearity by realizing a non-linear feedback resistance (1 / gm). With this approach, the pulse width of the shaper output, and thus the full-width at half-maximum (FWHM), is significantly reduced, and thus the counting rate during pile-up is improved. Also, the front-end electronic circuit 10 improves the recovery from pile-up. Thus, the input dynamic range as well as the counting rate increases without consuming an excessive amount of power in the signal shaping circuit.

[0052] Also, since the proposed active dynamic feedback circuit does not require a control loop within the high-speed path of the signal shaping circuit, the highest speed and phase margin can be obtained. Further, with the proposed active dynamic feedback circuit, as shown in FIG. 2B, a single-input amplifier circuit 1100 that may be configured as an operational transconductance amplifier can be utilized, and the power consumption as well as the front-end noise will be further reduced. Without using the proposed active dynamic feedback amplifier in the feedback path 1001 that replaces the resistive element 1400 of the front-end circuit approach of FIG. 1, a single-input amplifier circuit of the signal shaping circuit cannot be realized.

[0053] The active dynamic feedback circuit 1200 not only functions as a resistance through its transconductance gm, but also adjusts the baseline at the output node O1100 / output terminal O10 to the reference voltage Vref. Further, with the proposed active dynamic feedback circuit, the performance is also significantly improved in terms of the power consumption and FWHM of the signal shaping circuit.

[0054] Implementing an active dynamic feedback circuit within the feedback path 1001 of the signal shaping circuit 1000 having the aforementioned requirements is difficult. The reason behind the problem is the following requirements that such a circuit must satisfy.

[0055] First, the active dynamic feedback circuit 1200 must resemble a resistor. Assuming that the active dynamic feedback circuit 1200 is implemented as a transconductance amplifier including a pair of input transistors sharing the same tail node, this can be achieved using the resistance 1 / gm of one of the input transistors of the amplifier. However, since the active dynamic feedback circuit 1200 must provide a very high resistance value, the transconductance gm of the input transistors must be very small, which can be achieved by a combination of a small bias current and a large overdrive voltage. The large overdrive voltage is usually limited by the saturation of the current source coupled to the supply voltage and the common tail node, but the small bias current affects the speed at which the active dynamic feedback circuit can operate as well as its dynamic range. When dealing with a large dynamic range, i.e., a large pulse or a pulse-up event, there is a starvation of the other transistor of the input pair of transistors sharing the same common-mode tail current.

[0056] Second, the active dynamic feedback circuit 1200 must improve the FWHM. The active dynamic feedback circuit can improve the FWHM by reducing the pulse width for a pulse with a large amplitude or the pulse width during a pulse-up event. This is done in a non-linear scheme, and the FWHM is significantly improved compared to using a resistor as a feedback element. However, to perform this role, the active dynamic feedback circuit must process the shaper output pulse very quickly and must have a high dynamic range to effectively respond and reshape the output pulse. Therefore, a high transconductance gm, which means a relatively large current in the input device, is required. This conflicts with the first aforementioned requirement that a small current is needed to obtain a sufficiently large feedback resistance.

[0057] Thirdly, the active dynamic feedback circuit 1200 must achieve a low offset. The active dynamic feedback circuit further serves to track the reference voltage and adjust the output baseline of the signal shaping circuit via feedback. Therefore, the offset caused by the mismatch induces a deviation from the baseline at the output node of the signal shaping circuit. A certain level of deviation is acceptable because it can be corrected by the DAC at the comparator input. However, excessive deviation is unacceptable because it takes the transconductance amplifier / comparator out of the input common-mode range.

[0058] FIG. 3A shows an embodiment of a front-end electronic circuit 10 for an electromagnetic sensor application (e.g., for a photon counting front-end). The signal shaping circuit 1000 included in this embodiment comprises an amplifier circuit 1100, an active dynamic feedback circuit 1200, and a feedback capacitor 1300. The signal shaping circuit 1000 includes a current source 1500 disposed between a terminal for supplying the supply potential VDD and the input terminal I10 of the front-end electronic circuit 10. The front-end electronic circuit 10 can improve the performance of the differential input amplifier circuit of the signal shaping circuit. Further, the proposed embodiment of the front-end electronic circuit 10 enables the realization of a single-input amplifier circuit for the signal shaping circuit.

[0059] Referring to the front - end electronic circuit 10 of FIG. 3A, the active dynamic feedback circuit 1200 includes a first input transistor 100 disposed within a first current path 1201 of the active dynamic feedback circuit. The active dynamic feedback circuit 1200 includes a second input transistor 200 disposed within a second current path 1202 of the active dynamic feedback circuit 1200. The first input transistor 100 has a control node for receiving an output signal Vout_shaper generated at the output node O1100 of the amplifier circuit 1100. The second input transistor 200 has a control node for receiving a reference signal / voltage Vref. The active dynamic feedback circuit 1200 further includes a buffer circuit 300 arranged to disconnect the first current path 1201 and the second current path 1202.

[0060] Accordingly, the buffer circuit 300 can disconnect the bias currents in the first and second current paths 1201, 1202, thereby enabling further processing of large signals or large deviations from the baseline at the output node O1100 of the amplifier circuit 1100 of the signal shaping circuit 1000 without starving the second input transistor 200 and without causing output clipping.

[0061] The first current path 1201 and the second current path 1202 are each connected between a terminal supplying the reference potential VSS and a common node 1210 of the active dynamic feedback circuit 1200.

[0062] The active dynamic feedback circuit 1200 includes a third current path 1203. The active dynamic feedback circuit 1200 further includes a current source 400 disposed within the third current path 1203 between a terminal supplying the supply potential VDD and the common node 1210. The third current path 1203 is connected in series to each of the first and second current paths 1201 and 1202.

[0063] The buffer circuit 300 is disposed between the common node 1210 and the first input transistor 100 to disconnect the first current path 1201 from the second current path 1202. The buffer circuit 300 includes a first input node I300a, a second input node I300b, and an output node O300. The first input node I300a of the buffer circuit 300 is connected to the common node 1210. The second input node I300b of the buffer circuit 300 is coupled to the output node O300. This means that the second input node I300b of the buffer circuit 300 can be directly connected to the output node O300 or can be coupled to the output node O300 via a feedback network.

[0064] Nonlinearity is introduced by the active dynamic feedback circuit 1200. This is achieved, inter alia, by the fact that the first input transistor 100 can operate in weak inversion, in contrast to a conventional MOS resistor that can be placed within the feedback path of a conventional signal shaping circuit. A conventional MOS resistor needs to be biased in strong inversion to reduce the resistance variation due to its Vgate bias that spreads due to PVT variations of the bias generator.

[0065] The active dynamic feedback circuit 1200 may include a reference signal generation circuit 500 coupled to the control node of the second input transistor 200 to supply a reference signal / voltage Vref. The offset of the active dynamic feedback circuit can be corrected by the reference signal generation circuit 500. The reference signal generation circuit may be configured as a resistive DAC (digital-to-analog converter) to correct the offset of the active dynamic feedback circuit amplifier.

[0066] FIG. 3B shows the front-end electronic circuit 10 of FIG. 3A with an advantageous embodiment of the buffer circuit 300.

[0067] The buffer circuit 300 includes a transistor 310 disposed within a fourth current path 1204 of the active dynamic feedback circuit 1200. The first current path 1201 and the fourth current path 1204 are connected in parallel between a terminal supplying the reference potential VSS and a second common node 1220 of the active dynamic feedback circuit 1200. The buffer circuit 300 further includes a current source 320. The current source 320 is disposed within a fifth current path 1205 of the active dynamic feedback circuit 1200 between a terminal supplying the supply potential VDD and the second common node 1220.

[0068] According to an advantageous embodiment of the front-end electronic circuit 10, the first and second input transistors 100, 200 are matched to each other. Also, the current source 400 is matched to the current source 1500. Further, the current source 400 and the current source 320 are matched to each other.

[0069] The buffer circuit 300 further includes an (auxiliary) amplifier 330. The amplifier 330 has a first input node I330a connected to a common node 1210 of the active dynamic feedback circuit 1200. The amplifier 330 has a second input node I330b connected to the second common node 1220 of the active dynamic feedback circuit 1200 and an output node O330 connected to the control node of the transistor 310.

[0070] By disconnecting the bias current in the first current path 1201 from the bias current in the fourth current path 1204, a low impedance can be formed at the second common node 1220. The (auxiliary) amplifier 330 provides impedance conversion. Specifically, the common tail node 1220 at the drain of the current source 320 can be converted from a high impedance to a low impedance.

[0071] Nonlinearity is achieved by the fact that the potential of the common tail node 1220 can be fixed by a large current by the (auxiliary) amplifier 330, making the common tail node 1220 act like a low impedance node. Disconnecting the input current in the first current path 1201 from the auxiliary current in the fourth current path 1204 is achieved via the (auxiliary) amplifier 330. Thus, the shaper output can exercise the overdrive of the first input transistor without degrading its entire large signal amplitude due to the voltage drop that would otherwise occur at the tail common node 1220.

[0072] Finally, without the (auxiliary) amplifier, the negative input node I1100b of the amplifier 1100 of the active dynamic feedback circuit would deplete, especially in high energy and / or pile-up events, thus clipping the output. By disconnecting the positive and negative input bias currents, a small current at the positive input, i.e., a small current in the first current path 1201, becomes possible, resulting in a low transconductance, thus a high resistance, in the feedback path 1001, and also a large current at the negative input, i.e., in the fourth current path 1204, which can supply the overcurrent required by the large signal of the shaper output without depleting and causing output clipping.

[0073] The main technical advantages of the front-end electronic circuit including the active dynamic feedback circuit 1200 within the feedback path 1001 of the signal shaping circuit 1000 can be summarized as follows.

[0074] By providing an active dynamic feedback circuit 1200 in the feedback path 1001 of the signal shaping circuit 1000, the performance of the differential input shaper can be improved, and as shown in FIG. 3B, a single input shaper can be realized. The proposed active dynamic feedback circuit achieves a lower FWHM by improving, i.e., reducing, the pulse width, and thus operates at a higher count rate. This is achieved by intentionally introducing a highly non-linear feedback resistance (1 / gm) between the input voltage and the output current, in contrast to the linear relationship in the case of polysilicon resistors or the quadratic relationship in the case of MOS resistors (NMOS) where the bulk and source are connected to the shaper output.

[0075] The non-linearity of the differential pair of input transistors is enhanced by using an active feedback loop to force its tail node 1220 to a constant voltage such that the first input transistor 100 receives the full shaper output voltage as its gate-source voltage. Thus, the complete non-linear relationship of the (MOS) first input transistor 100 drain current versus gate-source voltage is utilized.

[0076] When biased in weak inversion, an exponential characteristic of the resistance in the feedback path can be realized, improving the FWHM. This improves the performance of the speed of the photon counting system and the count rate during pile-up. The ability to bias in weak inversion is due to the appropriate reference voltage generation for the active feedback loop, i.e., by adding the gate-source voltage of the input transistor 200 to the reference voltage Vref, the overdrive of the first input transistor 100 can be decoupled from the shaper baseline.

[0077] At the same time, similar to the case of a standard differential transistor pair, the tail current source 320 ensures a current definition that is stable with respect to PVT. The active feedback loop disconnects the current in the fourth current path 1204 from the current in the first current path 1201 in order to define the tail potential at the second common node 1220. This enables a highly asymmetric biasing between the first input transistor 100 and the transistor 310 of the buffer circuit 300 without the matching limitations as in the case of a standard differential pair.

[0078] On the other hand, the first input transistor 100 can be biased with a lower current according to the high feedback resistance requirement, while the transistor 310 of the buffer circuit 300 can be biased with a large current in order to achieve a low tail node impedance at the second common node 1220 and to achieve tail potential clamping even in the case of high frequencies where the active feedback loop is not very effective.

[0079] Due to the active feedback loop and the low tail node impedance, the tail node potential at the second common node 1220 is kept constant regardless of the shaper output. Therefore, similar to the case of a typical differential pair approach, there is no possibility of current depletion for large shaper peaks. When the first input transistor 100 draws excessive current in response to a large shaper peak, that current is supplied either by the active feedback loop or by the asymmetrically highly biased transistor 310 of the buffer circuit 300. As a result, since the active feedback loop and the transistor 310 of the buffer circuit 300 can supply overcurrent within the first current path 1201, there is no current depletion for large shaper peaks.

[0080] As a result, there is no limitation for pile-up protection. Also, there is no need to provide a mutual conductance controller for controlling the mutual conductance of the active dynamic feedback circuits in the high-speed path, and the active dynamic feedback circuit 1200 can operate at high speed.

[0081] The active dynamic feedback circuit 1200 further includes a current mirror 600 disposed between a first current path 1201 and a sixth current path 1206 of the active dynamic feedback circuit 1200. The current mirror 600 includes a transistor 610 disposed within the first current path 1201 and a transistor 620 disposed within the sixth current path. The current mirror 600 can couple the current from the first current path 1201 of the active dynamic feedback circuit 1200 into the sixth current path 1206. The sixth current path 1206 of the active dynamic feedback circuit 1200 is connected to the input node I1100a of the amplifier circuit 1100 of the signal shaping circuit 100. The respective gate nodes of the transistors 610 and 620 may be connected via at least one resistor 630 to provide mirror pole compensation.

[0082] The current mirror 600 enables downscaling of the current in the first current path 1201. As a result, by downscaling the current in the current path 1201, a small effective transconductance can be achieved while biasing the first input transistor 100 with a current that is high enough for speed and low offset.

[0083] Figures 4A, 4B, and 4C show possible embodiments for the active dynamic feedback circuit 1200 shown in Figures 3A and 3B. In Figures 4A - 4C, the same elements as in Figures 3A and 3B are labeled with the same reference numerals.

[0084] Figure 4A shows an embodiment for the active dynamic feedback circuit 1200 with a folding auxiliary amplifier 330 of the buffer circuit 300. The auxiliary amplifier includes a differential transistor pair 331, 332 coupled to a bias current source 333. The folding auxiliary amplifier 330 further includes a bias transistor 334, a cascode transistor 335, and a current mirror 336.

[0085] FIG. 4B shows an embodiment of an active dynamic feedback circuit 1200 with a folded cascode auxiliary amplifier 330 of a buffer circuit 300. The folded cascode auxiliary amplifier 330 includes differential transistor pairs 331, 332, a bias current transistor 333, a bias transistor 334, transistors of a first cascode stage 335, a second cascode stage 337, and a current mirror 336.

[0086] FIG. 4C shows a possible embodiment of an active dynamic feedback circuit 1200 with a single-stage auxiliary amplifier 330 of a buffer circuit 300. The auxiliary amplifier 330 includes differential transistor pairs 331, 332, a bias transistor 333, and a current mirror 338.

[0087] The proposed design of a front-end electronic circuit 10 including a signal shaping circuit 1000 with an active dynamic feedback circuit 1200 in a feedback path 1001 can be provided in a single-stage architecture as shown in FIG. 5A or a two-stage architecture approach as shown in FIG. 5B within a photon counting circuit. FIGS. 5A and 5B show the active dynamic feedback circuit 1200 in a differential input configuration. Embodiments of the active dynamic feedback circuit 1200 in a single-input configuration are also possible.

[0088] Referring to the single-stage architecture of the photon counting circuit 2 in FIG. 5A, a photodetector 20 having a photon sensing region 21 is connected to an input terminal I10 of the front-end electronic circuit 10. The photodetector 20 is configured to generate a current pulse when a photon hits the photon sensing region 21.

[0089] When a current pulse generated by the photon detector 20 is applied to the input terminal I10 of the front-end electronic circuit 10, the front-end electronic circuit 10 is configured to generate a voltage pulse of the output signal Vout_shaper at the output node O10. Thereafter, the output voltage Vout_shaper of the shaper is further processed by some discriminator circuits 30a, ..., 30n of the energy discriminator 30. The energy discriminator 30 is configured to generate a digital signal according to the level of the voltage pulse at the output terminal O10. Thereafter, the output of the discriminator is supplied into the counter circuits 40a, ..., 40n of the counter 40. The count number is proportional to the number of incident photons. Having a plurality of discriminator circuits 30a, ..., 30n and counter circuits 40a, ..., 40n, information regarding the energy level of each incident photon can be obtained. The counter output may be processed by a DSP (Digital Signal Processor) not shown in FIG. 5A.

[0090] When the input capacitance is small and fixed, the single-stage architecture of the photon counting circuit 2 shown in FIG. 5A can be used. When the input capacitance is large and variable, a two-stage architecture approach of the photon counting circuit 2 as shown in FIG. 5B can be used.

[0091] Referring to the two-stage architecture approach of the photon counting circuit 2 in FIG. 5B, the signal shaping circuit 1000 is coupled to the input terminal I10 via the charge-sensitive amplifier circuit 2000. The charge-sensitive amplifier circuit 2000 functions as a buffer that isolates the signal shaping circuit 1000 from the input capacitance at the input terminal I10. The charge-sensitive amplifier circuit 2000 includes an operational transconductance amplifier 2100, a feedback resistor 2200, and a feedback capacitor 2300. The charge-sensitive amplifier circuit 2000 and the signal shaping circuit 1000 are coupled by a coupling network 3000 including a parallel connection of a resistor 3100 and a capacitor 3200.

[0092] When using the active dynamic feedback circuit 1200 in the signal shaping circuit of the front-end electronic circuit for photon counting application examples, a small FWHM that provides a budget in terms of speed and count rate is achieved, and the capacitance of the feedback capacitor 1300 can be increased. By increasing the capacitance of the capacitor 1300, low ballistic deficits can be achieved.

[0093] In addition to using the front-end electronic circuit 10 in photon counting application examples, the proposed configuration of the front-end electronic circuit 10 including the active dynamic feedback circuit 1200 as shown in FIGS. 3A, 3B, or 4A - 4C may be used in various X-ray imaging application examples, such as computed tomography, security, food or luggage inspection, defect inspection of materials and electronic devices, etc.

[0094] FIG. 6 shows an example of an application in which a photon counting circuit 2 equipped with a front-end electronic circuit 10 according to one of the approaches shown in FIGS. 3A, 3B, or 4A - 4C is provided in a medical diagnostic device 1. The medical diagnostic device 1 may be configured as, for example, an X-ray device or a computed tomography scanner.

[0095] The embodiments of the front-end electronic circuit disclosed in this specification have been described for the purpose of enabling the reader to know a novel aspect of the front-end circuit design. Although the preferred embodiments have been illustrated and described, many modifications, changes, equivalents, and substitutions of the disclosed ideas can be made by those skilled in the art without unnecessarily departing from the scope of the claims.

[0096] Specifically, the design of the front-end electronic circuit is not limited to the disclosed embodiments, and examples of as many alternatives as possible are shown for the features included in the described embodiments. However, any changes, equivalents, and substitutions of the disclosed ideas are intended to be included within the scope of the claims appended to this specification.

[0097] The features described in the dependent claims may advantageously be combined. Also, the reference signs used in the claims are not to be construed as limiting the claims.

[0098] Furthermore, as used herein, the term “comprising” does not exclude other elements. Further, as used herein, the article “a” is intended to include one or more components or elements and is not to be construed as limited to meaning only one.

[0099] This patent application claims the priority of German Patent Application No. 102021111362.8. The disclosure of this document is incorporated herein by reference.

Explanation of Signs

[0100] 1 Medical diagnostic device 2 Photon counting circuit 10 Front-end electronic circuit 20 Photon detector 30 Energy discriminator 40 Counter 100 First input transistor 200 Second input transistor 300 Buffer circuit 310 Transistor 320 Current source 330 Amplifier 400 Current source 500 Reference signal generation circuit 600 Current mirror 1000 Signal shaping circuit 1100 Amplification circuit 1200 Active dynamic feedback circuit 1300 Capacitor 2000 Charge-sensitive amplification circuit 2100 Operational transconductance amplifier 2200 Feedback resistor 2300 Capacitor 3000 Combined Network 3100 Capacitor 3200 Resistor

Claims

1. A front-end electronic circuit for an electromagnetic radiation sensor application, an input terminal (I10) coupled to the electromagnetic radiation sensor and configured to receive an input signal (Iin) from the sensor, an output terminal (O10) for supplying an output signal (Vout_shaper), a signal shaping circuit (1000) including an amplifier circuit (1100) and an active dynamic feedback circuit (1200), wherein the active dynamic feedback circuit (1200) is disposed within a feedback path (1001) of the signal shaping circuit (1000), the signal shaping circuit (1000) being included, the amplifier circuit (1100) has an input node (I1100a) coupled to the input terminal (I10) and an output node (O1100) for supplying the output signal (Vout_shaper), and the output node (O1100) is coupled to the output terminal (O10), the active dynamic feedback circuit (1200) includes a first input transistor (100) having a first source-drain path disposed within a first current path (1201) of the active dynamic feedback circuit and a second input transistor (200) having a second source-drain path disposed within a second current path (1202) of the active dynamic feedback circuit, the first input transistor (100) has a control node for receiving the output signal (Vout_shaper), the second input transistor (200) has a control node for receiving a reference signal (Vref), the active dynamic feedback circuit (1200) includes a buffer circuit (300) disposed between the first and second current paths (1201, 1202), the first current path (1201) is connected between a terminal for supplying a reference potential (VSS) and the buffer circuit (300), the second current path (1202) is connected between the terminal for supplying the reference potential (VSS) and a first node (1210) of the active dynamic feedback circuit (1200), the buffer circuit (300) has an input side (I300a, I300b) connected to the first node (1210) and an output side (O300) connected to the first input transistor (100), The source of the first input transistor (100) is connected to the output side (O300) of the buffer circuit (300), The source of the second input transistor (200) is connected to the first node (1210), The active dynamic feedback circuit (1200) includes a third current path (1203), The active dynamic feedback circuit (1200) includes a current source (400) disposed within the third current path (1203) between a terminal supplying a supply potential (VDD) and the first node (1210), The third current path (1203) is connected in series to each of the first and second current paths (1201, 1202), The active dynamic feedback circuit (1200) is disposed between the first current path (1201) and the sixth current path (1206) of the active dynamic feedback circuit (1200), and includes a current mirror (600) that combines the current from the first current path (1201) of the active dynamic feedback circuit (1200) into the sixth current path (1206), The sixth current path (1206) of the active dynamic feedback circuit (1200) is connected between the input node (I1100a) of the amplifier circuit (1100) of the signal shaping circuit (1000) and the terminal supplying the reference potential (VSS), the front-end electronic circuit.

2. The buffer circuit (300) includes first and second input nodes (I300a, I300b) and an output node (O300), The first input node (I300a) of the buffer circuit (300) is connected to the first node (1210), The second input node (I300b) of the buffer circuit (300) is coupled to the output node (O300) of the buffer circuit (300), The output node (O300) of the buffer circuit (300) is connected to the first input transistor (100), the front-end electronic circuit according to claim 1.

3. The buffer circuit (300) includes a transistor (310) and a current source (320), The transistor (310) is disposed within the fourth current path (1204) of the active dynamic feedback circuit (1200) between the terminal supplying the reference potential (VSS) and the second node (1220), The first current path (1201) and the fourth current path (1204) are connected in parallel between the terminal supplying the reference potential (VSS) and the second node (1220) of the active dynamic feedback circuit (1200). The current source (320) is disposed within a fifth current path (1205) of the active dynamic feedback circuit (1200) between a terminal supplying a supply potential (VDD) and the second node (1220), the front-end electronic circuit according to claim 1 or 2. **Claim 4** The buffer circuit (300) includes an amplifier (330), the amplifier (330) having a first input node (I330a) connected to the first node (1210) of the active dynamic feedback circuit (1200), a second input node (I330b) connected to the second node (1220) of the active dynamic feedback circuit (1200), and an output node (O330) connected to a control node of the transistor (310) of the buffer circuit (300), the front-end electronic circuit according to claim 3. **Claim 5** The front-end electronic circuit according to claim 1, including a reference signal generation circuit (500) coupled to the control node of the second input transistor (200) to supply the reference signal (Vref). **Claim 6** The current mirror (600) includes a first mirror transistor (610) disposed within the first current path (1201) and a second mirror transistor (620) disposed within the sixth current path (1206). The current mirror (600) includes at least one resistor (630) disposed between a gate node of the first mirror transistor (610) and a gate node of the second mirror transistor (620), the front-end electronic circuit according to claim 1. **Claim 7** The first input transistor (100) operates in a weak inversion region, the front-end electronic circuit according to claim 1. **Claim 8** The signal shaping circuit (1000) includes a feedback capacitor (1300) disposed between the input node (I1100a) of the amplifier circuit (1100) and the output node (O1100) of the amplifier circuit (1100). The front-end electronic circuit according to claim 1, wherein the signal shaping circuit (1000) includes a third current source (1500) disposed between a terminal for supplying a supply potential (VDD) and the input terminal (I10).

9. The front-end electronic circuit according to claim 1, wherein the amplifier circuit (1100) is embodied in a single-input, single-output configuration or a differential-input, single-output configuration.

10. A photon counting circuit, comprising the front-end electronic circuit (10) according to claim 1, a photon detector (20) having a photon sensing region (21), wherein the photon detector (20) is configured to generate a current pulse when a photon hits the photon sensing region (21), and an energy discriminator (30) connected to the output terminal (O10) of the front-end electronic circuit (10), wherein the photon detector (20) is connected to the input terminal (I10) of the front-end electronic circuit (10), wherein the front-end electronic circuit (10) is configured to generate a voltage pulse at the output terminal (O10) of the front-end electronic circuit (10) when the current pulse is applied to the input terminal (I10) of the front-end electronic circuit (10), and the energy discriminator (30) is configured to generate a digital signal according to the level of the voltage pulse.

11. A medical diagnostic device, comprising the photon counting circuit (2) according to claim 10, wherein the device (1) is configured as an X-ray device or a computed tomography scanner.

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