Self-adaptive transimpedance structure applied to current feedback type operational amplifier

Abstract

The invention belongs to the field of integrated circuits, and particularly discloses a self-adaptive transimpedance structure applied to a current feedback type operational amplifier, which comprises an operational amplifier gain stage circuit, and the operational amplifier gain stage circuit comprises two half-side circuits which are connected in parallel at an input end voltage and are symmetrically arranged up and down, the half-side circuit comprises a cascode structure, an equal-proportion current mirror and the like. According to the invention, the gain of the gain stage can be effectively improved when a small-amplitude signal acts, and crossover distortion introduced by an output stage driving tube is reduced; and meanwhile, the gain of the gain stage is stabilized when a large-amplitude signal acts, and distortion caused by transconductance nonlinearity is reduced.

Description

Technical Field

[0001] The invention belongs to the field of integrated circuits, and in particular relates to an adaptive transimpedance structure applied to a current feedback operational amplifier. Background Art

[0002] As a type of operational amplifier, the current feedback op amp has become the first choice for today's mainstream high-performance line drivers due to its advantages in bandwidth, linearity, and current driving capability. Compared with the voltage feedback op amp, it has a nearly constant closed-loop bandwidth and ultra-high slew rate that does not change with the closed-loop gain. The traditional current feedback op amp consists of a transimpedance gain stage and a tapered output stage, where the gain stage is divided into two types: single-tube amplification and common-source common-gate amplification.

[0003] Figure 1 The figure shows the circuit structure of a traditional single-tube amplifier gain stage. The circuit is symmetrical from top to bottom. Q1 and Q2 are input buffers used to raise the voltage at the in-phase input terminal. Q3 and Q4 are input transistors. Q7 and Q8 provide transresistance. The gain of the circuit is expressed as:

[0004]

[0005] Among them, V A is the Early voltage, V T is the thermal voltage. If the influence of the output stage is not considered, the circuit gain of the single-tube amplifier gain stage is approximately stable under constant temperature conditions. However, the crossover distortion introduced by the output stage is also the main factor affecting the overall linearity of the current feedback operational amplifier, especially when the signal amplitude is small, which plays a decisive role in the overall linearity. However, due to the insufficient gain of the single-tube amplifier gain stage, after the overall circuit is closed-loop connected, the suppression effect of the crossover distortion introduced by the output stage is limited.

[0006] Figure 2 This is the circuit structure of the traditional tapered output stage. 14 and Q 15 If the current bias is used to reduce the crossover distortion, it will inevitably lead to a sharp increase in the static power consumption of the circuit. Figure 3 This is the circuit structure of the traditional common source and common gate amplifier gain stage. 24 ~Q 27 The common source and common gate transresistance is formed, and the gain of the circuit can be expressed as:

[0007] A cas =g m18 g m26 r o26 r o24 ≈V A 2 / V T 2

[0008] It can be seen that the circuit gain of the common source and common gate amplifier gain stage is sufficient to suppress the crossover distortion introduced by the output stage, but after the cascade output stage, the terminal load R L The transimpedance of the gain stage is limited by the tapered structure, so that the circuit gain of the gain stage becomes:

[0009] A act_cas =g m18 (g m26 r o26 r o24 Pβ 10 β 12 β 14 R L )

[0010] Among them, β 10 β 12 β 14 R L is the transimpedance parallel term caused by the current transfer characteristics of the bipolar transistor and is related to the load R L The size of g is directly related to that of β, which represents the current gain. In most line driver applications, such as power line carrier communication, the equivalent impedance of the power line is typically very small, making g m26 r o26 r o24 Much larger than β 10 β 12 β 14 R L , so the above formula can be simplified to:

[0011] A act_cas ≈g m18 β 10 β 12 β 14 R L

[0012] Obviously, in practical applications, due to the transistor transconductance g m Due to the strong nonlinearity of the common-source common-gate amplifier, the gain of the common-source common-gate amplifier gain stage cannot be kept approximately constant, but exhibits obvious nonlinearity. Especially when the signal amplitude is large, the distortion introduced by the transconductance nonlinearity of the gain stage will exceed the crossover distortion introduced by the output stage. Therefore, within the entire signal amplitude range, the gain stages of traditional current feedback op amps have their own limitations. Summary of the invention

[0013] In order to address the deficiencies of the prior art, the present invention aims to provide an adaptive transimpedance structure applied to a current feedback operational amplifier, which can effectively improve the gain of the gain stage when a small amplitude signal acts and reduce the crossover distortion introduced by the output stage driver tube; at the same time, when a large amplitude signal acts, it can stabilize the gain of the gain stage and reduce the distortion caused by transconductance nonlinearity.

[0014] In order to achieve the above object, the technical solution adopted by the present invention is:

[0015] An adaptive transimpedance structure applied to a current feedback operational amplifier comprises an operational amplifier gain stage circuit, the operational amplifier gain stage circuit comprises two half-circuits symmetrically arranged in parallel at an input terminal voltage, the half-circuits comprise a common source and a common gate structure and a proportional current mirror, the common source and a common gate structure comprises a third transistor and a fifth transistor, the proportional current mirror comprises a ninth transistor, an eleventh transistor, a thirteenth transistor, a fifteenth transistor, a third resistor and a fifth resistor; the bases of the third transistor and the fifth transistor are correspondingly connected to an on-chip reference source or a bias voltage generated by a self-bias circuit; the collector of the third transistor is connected to the emitter of the first transistor and the base of the seventh transistor, the base of the first transistor is connected to the positive electrode of the input terminal, and the collector of the first transistor is grounded; The emitter of the third transistor is connected to the collector of the fifth transistor, and the emitter of the fifth transistor is connected to one end of the first resistor; the emitter of the seventh transistor is connected to the negative electrode of the input terminal, and the collector of the seventh transistor is respectively connected to the collector and base of the ninth transistor and the base of the thirteenth transistor, the emitter of the ninth transistor is respectively connected to one end of the third resistor, the base and collector of the eleventh transistor, the emitter of the thirteenth transistor is respectively connected to one end of the fifth resistor, the base and collector of the fifteenth transistor, the other end of the third resistor, the emitter of the eleventh transistor, the other end of the fifth resistor and the emitter of the fifteenth transistor are all connected to the other end of the first resistor; the collector of the thirteenth transistor is connected to the collector of the transistor corresponding to the other half circuit.

[0016] Furthermore, the collector of the thirteenth transistor is also grounded via a gain stage compensation capacitor.

[0017] Preferably, the first transistor, the third transistor, the fifth transistor, the ninth transistor, the eleventh transistor, the thirteenth transistor and the fifteenth transistor are all PNP transistors; and the corresponding transistors on the other half circuit are all NPN transistors.

[0018] Preferably, the seventh transistor is an NPN transistor, and the transistor corresponding to the seventh transistor on the other half circuit is a PNP transistor.

[0019] Furthermore, the gain expression of the half-circuit gain stage of the operational amplifier gain stage circuit under different signal amplitudes is:

[0020]

[0021] Among them, A gt_s , A gt_m , A gt_lRepresents the gain stage gains at small, medium, and large signal amplitudes, g m7 , g m13 , g m15 denote the transconductance of the seventh transistor, the thirteenth transistor, and the fifteenth transistor, respectively, t represents the transimpedance of the gain stage, r o13 is the collector resistance of the thirteenth transistor; V A is the Early voltage, V T is the thermal voltage; R5 represents the resistance value of the fifth resistor.

[0022] Among them, affected by the signal amplitude, r t The value range is divided into 3 categories:

[0023] ① When the signal is at a small amplitude, the emitter-base voltage V EB15 is less than the on-state voltage, the fifteenth transistor is in the off state, g m15 Approximately 0,1 / g m15 Much larger than R5; within this amplitude range, r t is approximately equal to R5, and the transconductance g of the seventh transistor as the input transistor m7 The change is small, A gt_s Approximately remain unchanged;

[0024] ②When the signal is at medium amplitude, V EB15 is greater than the on-state voltage, the fifteenth transistor is in the on-state, g m15 Gradually increase, so that 1 / g m15 The order of magnitude is similar to that of R5; within this amplitude range, r t As the signal amplitude increases, A decreases. gt_m It increases first and then decreases with the increase of signal amplitude, and the fluctuation is small;

[0025] ③When the signal is at a large amplitude, the fifteenth transistor is further turned on, g m15 Further increase, so that 1 / g m15 Much smaller than R5; within this amplitude range, r t Approximately equal to 1 / g m15 , A gt_l Approaching the intrinsic gain of the transistor.

[0026] The present invention has the following beneficial effects:

[0027] Compared with the traditional current feedback op amp gain stage, the present invention achieves a significant improvement in circuit linearity within the entire signal amplitude range, effectively integrating the respective advantages of the traditional current feedback op amp gain stage and avoiding the disadvantages; it can improve the gain of the gain stage when a small amplitude signal is applied, and effectively reduce the crossover distortion introduced by the output stage driver tube; at the same time, it can stabilize the gain of the gain stage when a large amplitude signal is applied, and reduce the distortion caused by transconductance nonlinearity. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a circuit structure of a traditional single-tube amplifier gain stage;

[0029] Figure 2 It is the circuit structure of the traditional tapered output stage;

[0030] Figure 3 It is a circuit structure of a traditional common source and common gate amplifier gain stage;

[0031] Figure 4 The present invention adopts a gain stage circuit structure with adaptive transimpedance;

[0032] Figure 5 This is a small signal model diagram of the adaptive transimpedance of the present invention;

[0033] Figure 6 This is a curve diagram showing the amplitude simulation comparison between the present invention and the traditional single-tube amplifier structure;

[0034] Figure 7 This is a frequency simulation comparison curve diagram of the present invention and the traditional single-tube amplifier structure;

[0035] Figure 8 This is a curve diagram showing the amplitude simulation comparison between the present invention and the traditional common-source and common-gate amplification structure;

[0036] Fig. 9 It is a frequency simulation comparison curve diagram of the present invention and the traditional common-source and common-gate amplifier structure. DETAILED DESCRIPTION

[0037] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not intended to limit the scope of use of the present invention.

[0038] like Figures 4-5As shown, the present invention proposes an adaptive transimpedance structure applied to a current feedback operational amplifier, including an operational amplifier gain stage circuit, the operational amplifier gain stage circuit including two upper and lower symmetrically arranged half-circuits connected in parallel at the input voltage, and both half-circuits include a common source and common gate structure and a proportional current mirror. The present invention aims to solve the following two defects in the circuit linearity of the conventional current feedback operational amplifier gain stage: when the signal output amplitude is small, the gain provided by the gain stage is insufficient to effectively reduce the crossover distortion introduced by the output stage; when the signal output amplitude is large, the transconductance nonlinearity of the input tube of the gain stage itself will deteriorate the overall linearity of the circuit.

[0039] The common source and common gate structure in the half circuit of the upper half of the present invention includes a third transistor Q3 and a fifth transistor Q5, which provide current bias for input transistors Q7 and Q8, and resistors R1 and R2 are used to increase the output impedance of the bias circuit. The proportional current mirror includes a ninth transistor Q9, an eleventh transistor Q 11 、Thirteenth transistor Q 13 , the fifteenth transistor Q 15 , the third resistor R3 and the fifth resistor R5, as the load of the input tube. Transistor Q 11 , Q 12 , Q 15 , Q 16 Connected in parallel with resistors R3, R4, R5, and R6 respectively, so that the parallel impedance changes with the signal amplitude.

[0040] Specifically, the base of the first transistor Q1 is connected to the positive input terminal V in + , the collector of the first transistor Q1 is grounded; the collector of the third transistor Q3 is connected to the emitter of the first transistor Q1 and the base of the seventh transistor Q7, and the bases of the third transistor Q3 and the fifth transistor Q5 are respectively connected to the bias voltage V generated by the self-bias circuit BP2 and V BP1 The emitter of the third transistor Q3 is connected to the collector of the fifth transistor Q5, and the emitter of the fifth transistor Q5 is connected to one end of the first resistor R1; the emitter of the seventh transistor Q7 is connected to the negative input terminal V in - The collector of the seventh transistor Q7 is connected to the collector and base of the ninth transistor Q9 and the base of the thirteenth transistor Q 13 The base of the ninth transistor Q9 is connected to one end of the third resistor R3, the emitter of the eleventh transistor Q 11 The base and collector of the thirteenth transistor Q 13 The emitter of the fifth transistor Q 15 The base and collector of the third resistor R3, the other end of the eleventh transistor Q 11The emitter of the fifth resistor R5 and the other end of the fifteenth transistor Q 15 The emitters of are connected to the other end of the first resistor R1.

[0041] The common source and common gate structure of the lower half circuit includes a fourth transistor Q4 and a sixth transistor Q6, and the proportional current mirror includes a tenth transistor Q 10 , the fourteenth transistor Q 14 , the twelfth transistor Q 12 , the sixteenth transistor Q 16 , a fourth resistor R4 and a sixth resistor R6. The base of the second transistor Q2 is connected to the positive input terminal V in + , the collector of the second transistor Q2 is grounded; the collector of the fourth transistor Q4 is connected to the emitter of the second transistor Q2 and the base of the eighth transistor Q8, and the bases of the fourth transistor Q4 and the sixth transistor Q6 are respectively connected to the bias voltage V BN2 and V BN1 The emitter of the fourth transistor Q4 is connected to the collector of the sixth transistor Q6, and the emitter of the sixth transistor Q6 is connected to one end of the second resistor R2; the emitter of the eighth transistor Q8 is connected to the negative electrode of the input terminal V in - The collector of the eighth transistor Q8 is connected to the tenth transistor Q 10 The collector, base and fourteenth transistor Q 14 The base of the tenth transistor Q 10 The emitter of the transistor is connected to one end of the fourth resistor R4, the twelfth transistor Q 12 The base and collector of the fourteenth transistor Q 14 The emitter of the sixth transistor Q 16 The base and collector of the fourth resistor R4, the other end of the twelfth transistor Q 12 The emitter of the sixth resistor R6 and the other end of the sixteenth transistor Q 16 The emitters of are connected to the other end of the second resistor R2.

[0042] The thirteenth transistor Q 13 The collector of the fourteenth transistor Q 14 The base of the gain stage is also connected through the gain stage compensation capacitor C t Ground.

[0043] Among them, the first transistor Q1, the third transistor Q3, the fifth transistor Q5, the eighth transistor Q8, the ninth transistor Q9, the eleventh transistor Q 11 、Thirteenth transistor Q 13 , the fifteenth transistor Q 15All of them are PNP transistors, the second transistor Q2, the fourth transistor Q4, the sixth transistor Q6, the seventh transistor Q7, the tenth transistor Q 10 , the twelfth transistor Q 12 , the fourteenth transistor Q 14 , the sixteenth transistor Q 16 It is an NPN transistor.

[0044] Since the circuit is symmetrical from top to bottom, we only need to consider half of the circuit. Figure 4 From point A in the figure, the small signal model of adaptive transimpedance is as follows Figure 5 According to the circuit structure of the gain stage and the small signal model of the adaptive transimpedance, the gain expression of the gain stage of the operational amplifier gain stage circuit of the present invention under different signal amplitudes can be derived as follows:

[0045]

[0046] Among them, A gt_s , A gt_m , A gt_l Represents the gain stage gains at small, medium, and large signal amplitudes, g m7 , g m13 , g m15 They represent the seventh transistor Q7 and the thirteenth transistor Q 13 , the fifteenth transistor Q 15 The transconductance, r t represents the transimpedance of the gain stage, r o13 The thirteenth transistor Q 13 The collector resistance of V A is the Early voltage, V T is the thermal voltage; R5 represents the resistance of the fifth resistor. Affected by the signal amplitude, r t The value range is divided into 3 categories, and in the entire signal amplitude range, the fifteenth transistor Q 15 Due to the diode connection method, the Early effect can be ignored. Specifically:

[0047] ① When the signal is at a small amplitude, the fifteenth transistor Q 15 The emitter-base voltage V EB15 is less than the on-state voltage, the Q 15 The tube is disconnected, g m15 Approximately 0,1 / g m15 Much larger than R5; within this amplitude range, r t is approximately equal to R5, and the transconductance g of the seventh transistor Q7 as the input transistor m7 The change is small, A gt_s Remains approximately unchanged.

[0048] ②When the signal is at medium amplitude, V EB15 is greater than the turn-on voltage, the fifteenth transistor Q 15 In the on state, g m15 Gradually increase, so that 1 / g m15 The order of magnitude is similar to that of R5; within this amplitude range, r t As the signal amplitude increases, A decreases. gt_m It increases first and then decreases with the increase of signal amplitude, and the fluctuation is small;

[0049] ③When the signal is at a large amplitude, the fifteenth transistor Q 15 Further conduction, g m15 Further increase, so that 1 / g m15 Much smaller than R5; within this amplitude range, r t Approximately equal to 1 / g m15 , A gt_l Approaching the intrinsic gain of the transistor.

[0050] In the whole signal amplitude range, the gain of the current feedback amplifier gain stage of the present invention increases first and then decreases as the signal amplitude increases. At the same time, since the maximum value of the inherent transimpedance g of the gain stage is m13 r o13 R5 is always much smaller than the terminal load R L The parallel term β conducted through the output stage 3 R L (β represents current gain), so the effect of the load on the gain stage can be basically ignored. In general, when the signal amplitude is small, the gain is much greater than the gain of the traditional single-tube amplifier gain stage; when the signal amplitude is large, the gain stability is much higher than the traditional common source and common gate gain stage.

[0051] Simulation experiment:

[0052] The present invention Figure 6 to Figure 9 The simulation results of the spurious free dynamic range (SFDR) of the current feedback operational amplifier using the present invention and the traditional structure at different output amplitudes and signal frequencies are demonstrated, which are consistent with the derived results.

[0053] on the one hand, Figure 6 The display amplifier is biased at low current, the terminal load is 15Ω, and the signal frequency is 1MHz. Compared with the traditional single-tube amplifier, the gain level of the present invention is 6V. pp The maximum SFDR improvement within the output amplitude is 8dB. Figure 8 It shows that the amplifier is biased at high current, the terminal load is 1KΩ, and the signal frequency is 1MHz. Compared with the traditional common source and common gate amplifier, the gain level of the present invention is 8V. pp The maximum SFDR improvement is 5dB above the output amplitude.

[0054] on the other hand, Figure 7 and Fig. 9 It is shown that when the signal frequency increases, the influence of the compensation capacitor and the parasitic capacitor in the circuit will gradually increase, so that the improvement effect of the present invention on SFDR will fluctuate.

[0055] The above simulation is performed using dual op amp channels when the power supply voltage is 12V and the crossover distortion introduced by the output stage is small. As the maximum output amplitude or the output stage crossover distortion increases, the improvement effect of the present invention on SFDR will also increase significantly.

[0056] In summary, compared with the traditional current feedback op amp gain stage, the present invention achieves a significant improvement in circuit linearity within the entire signal amplitude range. In addition, the present invention can effectively integrate the advantages of the traditional current feedback op amp gain stage and avoid the disadvantages; it can increase the gain of the gain stage when a small amplitude signal is applied, and reduce the crossover distortion introduced by the output stage driver tube; at the same time, it can stabilize the gain of the gain stage when a large amplitude signal is applied, and reduce the distortion caused by transconductance nonlinearity.

[0057] Obviously, the embodiments described above are only some embodiments of the present application, rather than all embodiments. The preferred embodiments of the present application are given in the accompanying drawings, but they do not limit the patent scope of the present application. The present application can be implemented in many different forms. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive. Although the present application is described in detail with reference to the aforementioned embodiments, for those skilled in the art, it is still possible to modify the technical solutions recorded in the aforementioned specific implementation methods, or to perform equivalent replacement of some of the technical features therein. Any equivalent structure made using the contents of the specification and drawings of this application, directly or indirectly used in other related technical fields, is similarly within the scope of patent protection of this application.

Claims

1. An adaptive transimpedance structure applied to a current feedback operational amplifier, characterized in that: The invention comprises an operational amplifier gain stage circuit, wherein the operational amplifier gain stage circuit comprises two half-circuits symmetrically arranged in parallel at the input terminal voltage, the half-circuits comprise a common source and a common gate structure and a proportional current mirror, the common source and a common gate structure comprises a third transistor and a fifth transistor, the proportional current mirror comprises a ninth transistor, an eleventh transistor, a thirteenth transistor, a fifteenth transistor, a third resistor and a fifth resistor; the bases of the third transistor and the fifth transistor are correspondingly connected to the bias voltage generated by the on-chip reference source or the self-bias circuit; the collector of the third transistor is connected to the emitter of the first transistor and the base of the seventh transistor, the base of the first transistor is connected to the positive electrode of the input terminal, and the collector of the first transistor is grounded; the emitter of the third transistor The collector of the fifth transistor is connected, and the emitter of the fifth transistor is connected to one end of the first resistor; the emitter of the seventh transistor is connected to the negative electrode of the input terminal, the collector of the seventh transistor is respectively connected to the collector and base of the ninth transistor and the base of the thirteenth transistor, the emitter of the ninth transistor is respectively connected to one end of the third resistor, the base and collector of the eleventh transistor, the emitter of the thirteenth transistor is respectively connected to one end of the fifth resistor, the base and collector of the fifteenth transistor, the other end of the third resistor, the emitter of the eleventh transistor, the other end of the fifth resistor and the emitter of the fifteenth transistor are all connected to the other end of the first resistor; the collector of the thirteenth transistor is connected to the collector of the transistor corresponding to the other half circuit.

2. The adaptive transimpedance structure for a current feedback operational amplifier according to claim 1, characterized in that: The collector of the thirteenth transistor is also grounded via a gain stage compensation capacitor.

3. The adaptive transimpedance structure for a current feedback operational amplifier according to claim 1, characterized in that: The first transistor, the third transistor, the fifth transistor, the ninth transistor, the eleventh transistor, the thirteenth transistor, and the fifteenth transistor are all PNP transistors; and the corresponding transistors on the other half circuit are all NPN transistors.

4. The adaptive transimpedance structure for a current feedback operational amplifier according to claim 1, characterized in that: The seventh transistor is an NPN transistor, and the transistor corresponding to the seventh transistor on the other half circuit is a PNP transistor.

5. The adaptive transimpedance structure for a current feedback operational amplifier according to claim 1, characterized in that: The gain expression of the half-side circuit gain stage of the operational amplifier gain stage circuit under different signal amplitudes is: Among them, A gt_s , A gt_m , A gt_l Represents the gain stage gains at small, medium, and large signal amplitudes, g m7 , g m13 , g m15 denote the transconductance of the seventh transistor, the thirteenth transistor, and the fifteenth transistor, respectively, t represents the transimpedance of the gain stage, r o13 is the collector resistance of the thirteenth transistor; V A is the Early voltage, V T is the thermal voltage; R5 represents the resistance value of the fifth resistor.

6. The adaptive transimpedance structure applied to a current feedback operational amplifier according to claim 5, characterized in that: Affected by the signal amplitude, r t The value range is divided into 3 categories: ① When the signal is at a small amplitude, the emitter-base voltage V EB15 is less than the on-state voltage, the fifteenth transistor is in the off state, g m15 Approximately 0,1 / g m15 Much larger than R5; within this amplitude range, r t is approximately equal to R5, and the transconductance g of the seventh transistor as the input transistor m7 The change is small, A gt_s Approximately remain unchanged; ②When the signal is at medium amplitude, V EB15 is greater than the on-state voltage, the fifteenth transistor is in the on-state, g m15 Gradually increase, so that 1 / g m15 The order of magnitude is similar to that of R5; within this amplitude range, r t As the signal amplitude increases, A decreases. gt_m It increases first and then decreases with the increase of signal amplitude, and the fluctuation is small; ③When the signal is at a large amplitude, the fifteenth transistor is further turned on, g m15 Further increase, so that 1 / g m15 Much smaller than R5; within this amplitude range, r t Approximately equal to 1 / g m15 , A gt_l Approaching the intrinsic gain of the transistor.

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