A capacitor feedback low noise amplifier circuit

Abstract

The invention discloses a capacitor feedback low noise amplifier circuit, comprising an operational amplifier, an input capacitor C in , feedback capacitor C f , common mode signal feedback resistor R fp , NMOS tube M f , switch SW 1 and switch SW 2 . And the signals related to the capacitor feedback low noise amplifier circuit disclosed by the present invention include, input signal V in , output signal V out , common mode signal V CM , subthreshold voltage signal V Sub‑TH , MOS tube turn-on voltage signal V ON . The capacitor feedback low noise amplifier circuit proposed in the present invention adopts capacitors for signal feedback, thereby overcoming the defect of deterioration of thermal noise performance of the inverting amplifier circuit caused by the feedback path formed by resistors in the traditional inverting amplifier circuit. At the same time, the inverting amplifier circuit has the ability to obtain DC common mode signals while maintaining low noise performance. The present invention can be applied to medical, audio, sensitive signal capture and other scenarios to improve the noise performance of the low noise amplifier circuit and reduce the design cost of the low noise amplifier circuit.

Description

Technical Field

[0001] The invention relates to the technical field of electronic circuits, and in particular to a capacitor feedback low noise amplifier circuit. Background Art

[0002] Traditional low noise amplifier circuits usually use Figure 1 The circuit structure of the inverting amplifier shown in the figure uses a negative feedback loop to make the voltage V at the inverting input terminal of the operational amplifier U1, that is, point A, A It is approximately equal to the voltage V at the non-inverting input terminal of the operational amplifier U1, that is, point B. B At this time, the voltage V A Approximately equal to 0V, point A forms a virtual ground. The formation of virtual ground has two advantages. First, the input differential signal amplitude of operational amplifier U1 is small, so that U1 works in the state of optimal linearity and noise performance; second, the formation of virtual ground is conducive to setting the DC common mode signal range of the amplifier circuit more easily. However, as a low-noise amplifier circuit application, such as Figure 1 The input resistance R of the inverting amplifier circuit shown in and feedback resistor R f Thermal noise will be generated, which will deteriorate the noise performance of the entire operational amplifier circuit. The calculation formula for resistor thermal noise is sqrt(4KTRB), where K is the Boltzman constant, T is the ambient temperature, R is the resistance value, and B is the noise bandwidth. Therefore, a 1KΩ resistor will generate 4nV / sqrt(Hz) of noise, and increasing the resistance value will increase the noise. A 100KΩ resistor will generate 40nV / sqrt(Hz) of noise.

[0003] In addition to the thermal noise generated by the resistor, we also need to know that the operational amplifier U1 itself also has noise, which requires the concept of noise gain. Noise gain is a physical quantity used to indicate how the noise signal source at the input end of the amplifier is converted to the output end. To describe the calculation method of noise gain, Figure 2 We introduce the input noise equivalent resistance R of the operational amplifier U1 nop , R nop This is the modeling of the equivalent noise of the operational amplifier U1. We assume that the operational amplifier U1 will generate a voltage of 2nV / sqrt(Hz). As mentioned above, a 1KΩ resistor will generate a noise of 4nV / sqrt(Hz). Therefore, the input noise equivalent resistance R of the operational amplifier is nop The value of is determined to be 250 ohms. At this time, assuming that the input resistance R in and feedback resistor R f The noise is zero, R in =800Ω, R f=800Ω, according to the calculation formula of the noise gain of the in-phase and inverting operational amplifier circuits: 1+(R f / Ri), we calculated that the equivalent input noise of the inverting operational amplifier circuit is 4nV / sqrt(Hz), which is completely determined by the input noise equivalent resistance R nop When we change the input resistance R in and feedback resistor R f When the noise of the resistor is taken into account at the same time, the equivalent input noise of the inverting operational amplifier circuit is calculated to be 6.6nV / sqrt(Hz). As mentioned above, we know that resistors will generate noise, and the noise generated by each resistor will be superimposed on the total input equivalent noise of the inverting operational amplifier circuit through the calculation of the noise gain.

[0004] Let's consider the noise generated by the capacitor. The noise value generated by the capacitor is equal to kT / C per sample. K is the Boltzman constant, T is the ambient temperature, and C is the capacitance value. The rule of thumb for capacitor noise is that a 1pF capacitor generates 64uV of noise for each sample. A 4pF capacitor generates 32uV of noise for each sample, and so on. Therefore, according to the above rule of thumb, any switched capacitor circuit has exactly the same noise as a resistor circuit. However, this only applies to switched capacitor circuits, that is, capacitors are performing discontinuous sampling. For capacitors working in an AC coupled state, no noise is generated. We use this feature to design a low-noise capacitor feedback amplifier circuit. Figure 2 The input resistance R in and feedback resistor R f Replaced by input capacitor C in and feedback capacitor C f , we can get Figure 3 The capacitor feedback low noise amplifier circuit shown in FIG. As known above, Figure 3 The input equivalent noise of the capacitor feedback low noise amplifier circuit shown is completely determined by the input noise equivalent resistance R of the operational amplifier U1. nop Therefore, the total input equivalent noise of the capacitor feedback low noise amplifier circuit is 4nV / sqrt(Hz). Figure 3 The circuit shown uses a capacitor feedback loop, and the DC common mode signal cannot be set, so this capacitor feedback low noise amplifier circuit cannot be used. Figure 4 As shown, in Figure 3 Based on the capacitor feedback low noise amplifier circuit, we add feedback resistor R f For setting the DC common mode signal, we set C in =C f =100pF, R f =250KΩ, Rnop =250Ω, then if Figure 4 The noise performance of the low noise amplifier circuit shown is as follows Figure 5 As shown in Figure 2, the noise at 1KHz reaches 415nV / sqrt(Hz), which is Figure 5 The other two flat noise curves in 6.625nV (such as Figure 2 Medium R nop and R f The noise contributed by both) and 4.125nV (such as Figure 3 Medium R nop The noise contributed by the individual f The addition of R greatly deteriorates the noise performance of the amplifier. f The resistance value increases from 250KΩ to 50MΩ, such as Figure 4 The noise performance of the low noise amplifier circuit shown is as follows Figure 6 As shown in the figure, the noise at 1KHz reaches 29.65nV / sqrt(Hz). f As the noise level of the low noise amplifier increases, the overall noise of the low noise amplifier circuit decreases. f tends to infinity, at this time, R f Circuit breaker, Figure 4 The noise performance of the LNA circuit shown will be Figure 3 The capacitor feedback low noise amplifier circuit shown is the same as 4nV / sqrt(Hz). Therefore, in order to achieve Figure 3 To achieve the low noise performance of the capacitor feedback low noise amplifier circuit shown in the figure, and to set the DC common mode signal of the low noise amplifier circuit at the same time, a resistor R approaching infinity is required. f However, in actual circuits, infinite resistance does not exist, so we consider using other devices other than resistors to achieve this. Figure 7 As shown, by using two diodes D1 and D2 instead of R f , realize the setting of DC common mode signal, and at the same time realize the low noise amplification characteristics of capacitor feedback low noise amplifier, because a huge resistance will be formed between the cathodes of the two diodes D1 and D2. Figure 7 The capacitance coupled low noise amplifier circuit shown in the figure has a common mode DC signal V CM The capacitor feedback low noise amplifier circuit can be Figure 8 As shown, any common-mode DC working state within a certain range can be achieved, while having the characteristics of capacitor feedback low-noise signal amplification. Summary of the invention

[0005] A capacitor feedback low noise amplifier circuit includes an operational amplifier U1, an input capacitor C in , feedback capacitor Cf , common mode signal feedback resistor R fp , NMOS tube M f , switch SW1 and switch SW2. The signals related to the capacitor feedback low noise amplifier circuit disclosed in the present invention include: input signal V in , output signal V out , common mode signal V CM , subthreshold voltage signal V Sub-TH , MOS tube turn-on voltage signal V ON The input signal V in The input capacitor C in , the input capacitor C in , one end is connected to the input signal V in The other end is connected to the feedback capacitor C f One end is connected to the inverting input end of the operational amplifier U1, and the NMOS tube M f The feedback capacitor C f The other end is connected to the output end of the operational amplifier U1 and the output signal V out connected, and at the same time with the common-mode signal feedback resistor R fp Common mode signal feedback resistor R fp The other end is connected to the NMOS tube M f The drain of NMOS tube M f The substrate is connected to the non-inverting input of the operational amplifier and the common-mode signal V CM Connected. NMOS tube M f The gate of the switch SW1 is connected to one end of the switch SW2. The other end of the switch SW1 is connected to the subthreshold voltage signal V Sub-TH The other end of switch SW2 is connected to the MOS tube start voltage signal V ON are connected.

[0006] This capacitor feedback low noise amplifier circuit has the following two working states:

[0007] (1) Common-mode signal acquisition working state: In the common-mode signal acquisition working state, switch SW1 is open, switch SW2 is closed, and the NMOS tube turn-on voltage signal V ON Can be applied to NMOS tube M f On the gate of NMOS tube M f The source and drain are turned on, and the common-mode signal V CM The common-mode signal is fed back through the resistor R fp Feedback to the inverting input of operational amplifier U1, input capacitor C in and feedback capacitor C fOne end, and the output end of the operational amplifier V out .

[0008] (2) Low-noise amplifier working state: In the low-noise amplifier working state, switch SW2 is open, switch SW1 is closed, and the subthreshold voltage V Sub-TH Can be applied to NMOS tube M f On the gate of NMOS tube M f Working in the subthreshold working area, NMOS tube M f The source and drain of the amplifier are disconnected, and a resistance of the order of 1012 ohms is formed between the source and the drain, making the inverting input terminal of the operational amplifier, that is, the NMOS tube M f The source of the operational amplifier output V out Directly approximate the open circuit, the input capacitor feeds back the input signal V of the low noise amplifier circuit in That is, through the input capacitor C in and feedback capacitor C f Realize feedback amplification, and the amplification factor is -C f / C in , due to the input capacitance C in and feedback capacitor C f No thermal noise caused by resistance is generated, so the capacitor feedback low-noise amplifier circuit proposed in the present invention realizes low-noise amplification of the signal.

[0009] The switch SW1 and the switch SW2 are composed of digital high and low level controlled transmission gate circuits.

[0010] In order to maintain the low noise characteristic of the capacitor feedback low noise amplifier circuit according to claim 1, the operational amplifier U1 should be designed in a low noise design manner. in The value of the feedback capacitor C f The value of should be determined by the gain factor of the capacitor feedback low noise amplifier circuit, which is -C f / C in .

[0011] The feedback capacitor C f It is a capacitor, or composed of multiple capacitors, to achieve different amplification factors.

[0012] When the capacitor feedback low noise amplifier circuit is implemented by designing an integrated circuit, the input capacitor C in and the feedback capacitor C f It needs to be placed in a separate P-well isolated from other devices to ensure the low noise characteristics of the capacitive feedback low noise amplifier circuit.

[0013] Beneficial effects:

[0014] The capacitor feedback low noise amplifier circuit proposed in the present invention adopts capacitors for signal feedback, thereby overcoming the defect of deterioration of thermal noise performance of the inverting amplifier circuit caused by the feedback path formed by resistors in the traditional inverting amplifier circuit. At the same time, by controlling switches SW1 and SW2, the NMOS tube works in a subthreshold state, and the large resistance equivalent resistor formed is used for common mode signal feedback, so that the inverting amplifier circuit has the ability to obtain DC common mode signals while maintaining low noise performance. The present invention can be applied to medical, audio, sensitive signal capture and other scenarios, improve the noise performance of the low noise amplifier circuit, and greatly reduce the design cost of the low noise amplifier circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is the topology diagram of the resistor feedback inverting amplifier circuit.

[0016] Figure 2 The topology diagram of the resistor feedback inverting amplifier circuit with input equivalent noise resistance added.

[0017] Figure 3 This is the topology diagram of the capacitor feedback inverting amplifier circuit with input equivalent noise resistance added.

[0018] Figure 4 The topology diagram of the capacitor feedback inverting amplifier circuit with input equivalent noise resistance and feedback resistance added.

[0019] Figure 5 To add feedback resistor R f =250KΩ after the inverting amplifier circuit noise performance and capacitor feedback inverting amplifier circuit noise performance comparison chart.

[0020] Figure 6 For; add feedback resistor R f =50MΩ after the inverting amplifier circuit noise performance and R f =250KΩ inverting amplifier circuit noise performance and capacitor feedback inverting amplifier circuit noise performance comparison chart.

[0021] Figure 7 A capacitive feedback inverting amplifier is added with diodes D1 and D2.

[0022] Figure 8 To add diodes D1 and D2 to the common-mode signal V CM A capacitive feedback inverting amplifier.

[0023] Fig. 9 To add NMOS tube M f With subthreshold voltage V Sub-TH A capacitive feedback inverting amplifier.

[0024] Fig.10 NMOS tube M is added to show the cross section f With subthreshold voltage V Sub-TH A capacitive feedback inverting amplifier.

[0025] Fig.11 For NMOS tube M f The connection relationship of the three equivalent resistors in the capacitive feedback inverting amplifier.

[0026] Fig.12 Add NMOS tube M f With subthreshold voltage V Sub-TH and the turn-on voltage V ON A capacitive feedback inverting amplifier.

[0027] Fig.13 The utility model discloses a capacitor feedback inverting amplifier circuit with adjustable gain (amplification factor).

[0028] Fig.14 A class AB operational amplifier circuit is provided to implement the U1 operational amplifier.

[0029] Fig.15 A controllable transmission gate circuit for implementing switching.

[0030] Fig.16 FIG. 1 is a diagram showing the relationship between different amplification factors and frequencies in the capacitor feedback low noise amplifier circuit embodiment of the present invention.

[0031] Fig.17 The figure shows the noise performance of the capacitor feedback low noise amplifier circuit according to an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The present invention is further described in detail below with reference to the accompanying drawings:

[0033] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] like Fig.12 A capacitor feedback low noise amplifier circuit as shown includes an operational amplifier, an input capacitor C in , feedback capacitor C f , common mode signal feedback resistor R fp , NMOS tube M f, switch SW1 and switch SW2. And signals related to a capacitor feedback low noise amplifier circuit disclosed in the present invention include an input signal V in , output signal V out , common mode signal V CM , subthreshold voltage signal V Sub-TH , MOS tube turn-on voltage signal V ON The capacitor feedback low noise amplifier circuit proposed in the present invention can overcome the defect of deterioration of thermal noise performance of the inverting amplifier circuit caused by the feedback path formed by resistors in the traditional inverting amplifier circuit by using capacitors for signal feedback. At the same time, by controlling switches SW1 and SW2, the NMOS tube is in a subthreshold working state, and the large resistance equivalent resistor formed performs common mode feedback, so that the inverting amplifier circuit has the ability to obtain DC common mode signals while maintaining low noise performance.

[0035] NMOS tube M f is used to replace Figure 7 and Figure 8 When switch SW1 is closed and SW2 is open, the NMOS tube operates at a subthreshold voltage V where the gate voltage is lower than the threshold voltage of the NMOS tube. Sub-TH hour, Fig.12 The circuit shown can be equivalent to Fig. 9 The circuit shown, further, as Fig.10 As shown, the NMOS tube M f The cross-sectional view of the NMOS tube is shown, and it is found that the source S and drain D of the NMOS tube form two diodes D1 and D2 with the substrate of the NMOS tube, namely BUILK, respectively. Therefore Fig. 9 , Fig.10 The circuit shown can be equivalent to Figure 7 , Figure 8 The capacitor feedback low noise amplifier circuit shown in the figure. When switch SW1 is closed and SW2 is open, the NMOS tube operates at a subthreshold voltage V where the gate voltage is lower than the threshold voltage of the NMOS tube. Sub-TH When the NMOS tube is equivalent to Fig.11 The three equivalent resistors R DB , R SB With R DS , respectively represent the resistance between the drain and the substrate, the resistance between the source and the substrate, and the resistance between the drain and the source. f Working at subthreshold voltage V Sub-TH In the state of modern CMOS technology, R DB , R SB With R DS The resistance value is not less than 10 12 Ω is 1000GΩ, NMOS tube M fA connection that is close to an open circuit is formed between the source S and the drain D, but current still flows, that is, the feedback of the DC common-mode signal is realized while maintaining the capacitive coupling inverting low-noise amplifier, thereby determining the DC common-mode signal value of the inverting amplifier circuit.

[0036] Fig.12 When switch SW1 is open and SW2 is closed, the circuit is equivalent to Figure 4 The capacitor feedback low noise amplifier circuit shown in the figure, at this time the NMOS tube turn-on voltage signal V ON Can be applied to NMOS tube M f On the gate of NMOS tube M f The source and drain are turned on, and the common-mode signal V CM The common-mode signal is fed back through the resistor R fp =100K is fed back to the inverting input of the operational amplifier U1, and the input capacitor C in and feedback capacitor C f One end, and the output end of the operational amplifier V out In this state, the capacitive coupling low noise amplifier circuit of the present invention operates in a DC common mode signal acquisition state and can quickly acquire a DC common mode signal.

[0037] In order to make the amplification factor of the capacitor feedback low noise amplifier circuit of the present invention adjustable, as Fig.12 The feedback capacitor C f Can be designed as Fig.13 The 10 groups of adjustable capacitor arrays shown are connected by switches SW3 to SW 13 By adjusting, in a progressive manner, closing 1, 2, 3, 4 to 10 switches each time, 10 different magnifications can be achieved. in =100pF, C fb =6pF, C f1 =2.6pF, C f2 =3.6pF, C f3 =5.2pF, C f4 =7.3pF, C f5 =10.4pF, C f6 =14.7pF, C f7 =20.7pF, C f8 =29.2pF, C f9 =41.3pF, C f10 =58.3pF.

[0038] Fig.12 , 13 The amplifier U1 in the Fig.14 The two-stage class AB amplifier shown in the figure realizes 19 With M20 The two output tubes C that form the AB output stage m1 , R m1 , C m2 , R m2 Two compensation networks are formed respectively, M 10 , M 11 , M 13 , M14 is used for M 20 Bias available, M9, M 16 , M 17 , M 15 For M 19 Providing bias, M4 and M5 are the input pair of operational amplifier U1, and the remaining MOS tubes are current mirror bias MOS tubes.

[0039] Fig.12 , 13 The switch in Fig.15 The transmission gate switch circuit shown is implemented as Fig.15 As shown, A and B are the two ends of the switch, C is the control signal, the switch is closed when the level is high, and the switch is open when the level is low.

[0040] This embodiment is implemented using SMIC 180nm CMOS technology. DD =3V, Fig.16 The circuit amplification factor of the capacitor feedback low noise amplifier circuit embodiment proposed by the present invention at different frequencies is shown, the horizontal axis represents the frequency, the unit is Hz, and the vertical axis represents the amplification factor, the unit is dB. By controlling SW3 to SW 13 , achieving 10 different magnifications. Fig.16 As shown, within 100Khz, the capacitor feedback low noise amplifier circuit embodiment proposed by the present invention has excellent gain flatness. The noise performance of the capacitor feedback low noise amplifier described in this embodiment is as follows Fig.17 As shown, the horizontal axis is the gain of the low noise amplifier, the unit is dB, and the vertical axis is the root mean square (RMS) value of the A-weighted noise, the unit is V. Fig.17 It can be seen that as the gain of the low noise amplifier increases, the feedback capacitance increases, and the noise of the capacitor feedback low noise amplifier circuit tends to decrease, reaching an excellent performance of less than 500nV. Fig.17 It can also be seen that the noise performance obtained by postlayout simulation is better than that by prelayout simulation, because the parasitic capacitance further filters the noise.

Claims

1. A capacitor feedback low noise amplifier circuit, characterized in that: Including operational amplifier U1, input capacitor C in , feedback capacitor C f , common mode signal feedback resistor R fp , NMOS tube M f , switch SW1 and switch SW2, capacitor feedback low noise amplifier circuit related signals, including, input signal V in , output signal V out , common mode signal V CM , subthreshold voltage signal V Sub-TH , MOS tube turn-on voltage signal V ON , the input signal V in The input capacitor C in , the input capacitor C in , one end is connected to the input signal V in The other end is connected to the feedback capacitor C f One end is connected to the inverting input end of the operational amplifier U1, and the NMOS tube M f The source of the feedback capacitor C f The other end is connected to the output end of the operational amplifier U1 and the output signal V out connected, and at the same time with the common-mode signal feedback resistor R fp Connected, common mode signal feedback resistor R fp The other end is connected to the NMOS tube M f The drain of NMOS tube M f The substrate is connected to the non-inverting input of the operational amplifier and the common-mode signal V CM Connected, NMOS tube M f The gate of the switch SW1 is connected to one end of the switch SW2, and the other end of the switch SW1 is connected to the subthreshold voltage signal V Sub-TH The other end of switch SW2 is connected to the MOS tube start voltage signal V ON Connected; This capacitor feedback low noise amplifier circuit has the following two working states: (1) Common-mode signal acquisition working state: In the common-mode signal acquisition working state, switch SW1 is open, switch SW2 is closed, and the NMOS tube turn-on voltage signal V ON Can be applied to NMOS tube M f On the gate of NMOS tube M f The source and drain are turned on, and the common-mode signal V CM The common-mode signal is fed back through the resistor R fp Feedback to the inverting input of operational amplifier U1, input capacitor C in and feedback capacitor C f One end, and the output end of the operational amplifier V out ; (2) Low-noise amplification working state: In the low-noise amplification working state, the switch SW2 is open, the switch SW1 is closed, and the subthreshold voltage V Sub-TH Can be applied to NMOS tube M f On the gate of NMOS tube M f Working in the subthreshold working area, NMOS tube M f The source and drain are disconnected, forming a 10 12 The resistance of the order of ohms makes the inverting input terminal of the operational amplifier, that is, the NMOS tube M f The source of the operational amplifier output V out Directly approximate the open circuit, the input capacitor feeds back the input signal V of the low noise amplifier circuit in That is, through the input capacitor C in and feedback capacitor C f Realize feedback amplification, and the amplification factor is -C f / C in , since the input capacitance C in and the feedback capacitor C f No thermal noise due to resistance is generated, so the proposed capacitor feedback low-noise amplifier circuit achieves low-noise amplification of the signal.

2. A capacitive feedback low noise amplifier circuit according to claim 1, characterized in that: The switch SW1 and the switch SW2 are composed of digital high and low level controlled transmission gate circuits.

3. The capacitor feedback low noise amplifier circuit according to claim 1, characterized in that: In order to maintain the low noise characteristic of the capacitive feedback low noise amplifier circuit according to claim 1 , the operational amplifier U1 should be designed in a low noise design manner.

4. The capacitor feedback low noise amplifier circuit according to claim 1, characterized in that: The input capacitance C in The value of the feedback capacitor C f The value of should be determined by the gain factor of the capacitor feedback low noise amplifier circuit, which is -C f / C in .

5. The capacitor feedback low noise amplifier circuit according to claim 4, characterized in that: The feedback capacitor C f It is a capacitor, or composed of multiple capacitors, to achieve different amplification factors.

6. The capacitor feedback low noise amplifier circuit according to claim 1, characterized in that: When the capacitor feedback low noise amplifier circuit is implemented by designing an integrated circuit, the input capacitor C in and the feedback capacitor C f It needs to be placed in a separate P-well isolated from other devices to ensure the low noise characteristics of the capacitive feedback low noise amplifier circuit.

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