A high speed operational amplifier with feedforward compensation

Abstract

This invention discloses a feedforward compensated high-speed operational amplifier, comprising: a first-stage amplifier circuit, a second-stage amplifier circuit, a common-mode feedback circuit signal-connected to the second-stage amplifier circuit, and at least two feedforward amplifier circuits symmetrically arranged about and signal-connected to the second-stage amplifier circuit; wherein, the second-stage amplifier circuit includes a second input N-type transistor, a second input P-type transistor, and a second tail current source, the gates of the second input N-type transistor and the second input P-type transistor are connected to the output terminal of the first stage, and their drains are connected together as the output terminal of the operational amplifier; the second input N-type transistor and the second input P-type transistor form a push-pull input structure. According to this invention, the operational amplifier has a sufficiently high unity gain and a high gain at 250MHz, does not oscillate in the closed-loop operation, and has a minimal current.

Description

Technical Field

[0001] This invention relates to the technical field of communication system-on-a-chip and integrated circuits, and particularly to a feedforward compensated high-speed operational amplifier. Background Technology

[0002] In UWB receiver analog baseband systems, high-speed operational amplifiers are typically used to construct transimpedance amplifiers, programmable gain amplifiers, or filters to improve receiver linearity. To meet receiver linearity and noise figure requirements, operational amplifiers need high low-frequency gain, high unity-gain bandwidth, low noise, and low power consumption. If the operational amplifier's unity-gain bandwidth is insufficient, it will directly affect the receiver's linearity and reduce the amplification effect on high-frequency signals; therefore, designing a high-speed operational amplifier is essential.

[0003] The traditional operational amplifier structure in a receiver is a Miller-compensated two-stage op-amp, such as... Figure 1 As shown. The first stage of the op-amp, g... m1 High gain is typically achieved using a folded cascode structure or a five-transistor amplifier, with the input signal V... in The output signal V is generated after amplification by the first-stage amplifier circuit. o1 This serves as the input signal for the second-stage amplifier circuit. (Second-stage amplifier circuit g) m2 Typically implemented using a common-source amplifier to provide a large output swing, the output voltage Vout is generated after a second-stage amplification. A Miller capacitor C is used between the input and output of the second stage. c and zero-adjustment resistor R c To ensure stability. Compensation capacitor C c Connected between the input and output of the second stage, according to the Miller effect, this capacitor C c Equivalent to V o1 After the end, it will become the original (1+ g) m2 R o2 ) times. Therefore, V o1 When the terminal is connected to ground, you will see a large Miller capacitance and a large output resistance R. o1 Therefore, the dominant pole of the op-amp is at V. o1 Location, principal pole p1 =1 / [(1+ g m2 R o2 C c R o1 The impedance C of the compensation capacitor at high frequencies. c The value decreases, which is equivalent to short-circuiting the input and output of the second stage. At this point, V... out The output impedance seen at ground is 1 / g m2 Therefore, the secondary point of the op-amp is at V. outSecond pole p2 =g m2 / C2. Add zero-adjustment resistor R c This is to eliminate the effect of compensation capacitor C c The impact of the feedforward effect on the stability of the right-half-plane zeros. Zeros of the op-amp after adding a zero-adjustment resistor. pz =1 / [Cc(1 / g m2 -R c The low-frequency gain of the op-amp is g. m1 g m2 R o1 R o2 The input-to-output transfer function of the op-amp is:

[0004] Unity gain frequency c = g m1 / C c .

[0005] To ensure a 45° phase margin in the loop, traditional Miller-compensated two-stage op-amps require sufficiently wide separation between the dominant and secondary poles. Typically, the secondary pole is placed outside the unity-gain frequency, while the dominant pole, due to the large capacitance equivalent to the Miller effect, appears at a very low frequency. The loop gain decreases at a rate of -20 dB / dec after the dominant pole, resulting in excessively low loop gain at the useful signal frequency (250 MHz) in UWB systems. This excessively low loop gain affects the linearity of the UWB receiver. To maintain a higher loop gain at 250 MHz, the dominant pole of the traditional op-amp must be placed at a higher frequency. To ensure stability, the secondary pole frequency must also be increased accordingly, requiring a very high current draw. The unity-gain frequency of a traditional Miller-compensated two-stage op-amp... c = g m1 / C c To lower the dominant pole, the Miller compensation capacitor typically needs a large capacitance value, which limits the unity gain of the loop to the Miller capacitance. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a feedforward compensated high-speed operational amplifier with sufficiently high unity gain and high gain at 250MHz, which does not oscillate in the closed-loop operation while minimizing the operational amplifier's current. To achieve the above-mentioned objectives and other advantages of the present invention, a feedforward compensated high-speed operational amplifier is provided, comprising: A first-stage amplifier circuit, a second-stage amplifier circuit connected to the first-stage amplifier circuit, a common-mode feedback circuit connected to the second-stage amplifier circuit, and at least two feedforward amplifier circuits symmetrically arranged about the second-stage amplifier circuit and connected to the second-stage amplifier circuit. The second-stage amplifier circuit includes a second input N-type transistor, a second input P-type transistor, and a second tail current source. The gates of the second input N-type transistor and the second input P-type transistor are connected to the output terminal of the first stage, and their drains are connected together as the output terminal of the operational amplifier. The second tail current source provides bias current to the second input N-type transistor. The second input N-type transistor and the second input P-type transistor form a push-pull input structure.

[0007] Preferably, the first-stage amplifier circuit includes a first input transistor, a first load transistor, a first tail current source, a first common-mode feedback resistor, and a first common-mode feedback capacitor; the gate of the first input transistor serves as the positive and negative input terminals of the operational amplifier, and the drain serves as the first-stage output terminal and is connected to the input terminal of the second-stage amplifier circuit; the first tail current source provides bias current to the first input transistor; the first common-mode feedback resistor and the first common-mode feedback capacitor are connected between the gate and drain of the first load transistor to provide first-stage common-mode feedback.

[0008] Preferably, the common-mode feedback circuit includes a common-mode detection resistor, a common-mode compensation capacitor, a common-mode feedback input transistor, and a common-mode feedback load transistor; the common-mode detection resistor extracts the common-mode information output by the operational amplifier and compares it with a reference voltage, and the comparison result is fed back to the feedforward amplifier circuit to stabilize the output common-mode level.

[0009] Preferably, the feedforward amplifier circuit includes a feedforward N-type transistor, a feedforward P-type transistor, a feedforward tail current source, a compensation capacitor, and a compensation resistor; the gate of the feedforward N-type transistor is connected to the input terminal of the operational amplifier as the input terminal of the feedforward stage; the gate of the feedforward P-type transistor is connected to the input terminal of the operational amplifier through a high-pass filter network composed of the compensation capacitor and the compensation resistor; the drains of the feedforward N-type transistor and the feedforward P-type transistor are connected together and connected to the output terminal of the operational amplifier; the feedforward tail current source provides bias current for the feedforward N-type transistor.

[0010] Preferably, the second input N-type transistor generates a drive current in response to a positive input signal, and the second input P-type transistor generates a drive current in response to an inverted input signal. The two are superimposed at the output terminal to form a complementary driving capability.

[0011] Preferably, the feedforward N-type transistor and the feedforward P-type transistor form a push-pull input structure for feeding the input signal to the output terminal.

[0012] Preferably, the left half-plane zero generated by the feedforward amplifier circuit is used to compensate for the phase shift caused by the two poles generated by the second stage amplifier circuit; the frequency position of the left half-plane zero is determined by the values ​​of the compensation capacitor and the compensation resistor, specifically, the zero frequency fz is equal to 1 divided by the product of the compensation capacitor and the compensation resistor.

[0013] Preferably, both the second input P-type transistor and the feedforward P-type transistor are used as input transistors in parallel with the corresponding N-type transistors to achieve current multiplexing. This allows the current flowing through these transistors to contribute to the signal amplification simultaneously, thereby improving current utilization efficiency and reducing the overall power consumption of the operational amplifier.

[0014] Compared with the prior art, the advantages and positive effects of the present invention are: A high-speed operational amplifier with feedforward compensation was designed without consuming excessive current. This is achieved by superimposing the feedforward stage signal and the forward amplification stage signal in phase at the output, generating a left-half-plane zero to compensate for the phase shift caused by the two poles of the forward amplification stage. This ensures a phase shift of less than -135° within the loop unity-gain frequency, achieving a system phase margin greater than 45°. Both the second stage and the feedforward stage of the operational amplifier utilize a push-pull input structure to increase the swing of the feedforward and second stage inputs, while also improving the output swing and thus enhancing the operational amplifier's linearity. Current multiplexing is implemented in the second stage and the feedforward stage, eliminating the load transistor and using each transistor as an input transistor, thereby improving current utilization efficiency and reducing the operational amplifier's power consumption. Attached Figure Description

[0015] Figure 1 The small-signal diagram of a conventional Miller-compensated op-amp with feedforward compensation according to the present invention is shown. Figure 2 The small-signal diagram of the feedforward compensated operational amplifier of the high-speed operational amplifier according to the present invention is shown. Figure 3 The circuit diagram of the feedforward compensated op-amp for the high-speed operational amplifier according to the present invention is shown. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Reference Figure 3 A feedforward compensated high-speed operational amplifier, comprising: A first-stage amplifier circuit, a second-stage amplifier circuit connected to the first-stage amplifier circuit, a common-mode feedback circuit connected to the second-stage amplifier circuit, and at least two feedforward amplifier circuits symmetrically arranged about the second-stage amplifier circuit and connected to the second-stage amplifier circuit.

[0018] The first-stage amplifier circuit includes M N1 M P1 M N5 , R 1 and C 1. M N5 For M N1 The tail current source transistor is used for M N1 Provide bias current, M N5 The gate is connected to a bias voltage. V BN1 M N5 The source is grounded, M N5 The drain is connected to two M N1 The source level of the tube. M N1 M is the input tube for the first stage. N1 The gates are respectively connected to the positive input terminal. V in+ and negative input terminal V in- M N1 The source is connected to M N5 The leak level, M N1 The drain is connected to M P1 The drain is used as the positive output terminal of the first stage. V o1+ and negative output terminal V o1- Simultaneously connected to M N2 and M P2 The gate is used as the input of the second stage, while M N1 The drain is connected to the resistor. R 1 and C One end of 1. M P1 M is the load transistor for the first stage. P1 The gate is connected to two resistors. R 1 and capacitor C The center point of 1, M P1 The source is connected to the power supply, M P1 The drain is connected to M N1 The drain. Resistor. R 1 provides common-mode feedback for the first stage; the capacitor improves the common-mode stability of the first stage; the resistor... R 1 and capacitor C The two ends of 1 are respectively connected to M P1 The gate and drain of the electrode.

[0019] The second-stage amplifier circuit includes M N2 M P2 and M N7 M N7 For M N2 The tail current source transistor is used for M N2 Provide bias current, M N7 The gate is connected to a bias voltage. V BN1 M N7 The source is grounded, M N7 The drain is connected to two M N2 The source level of the tube. M N2 M is the second-stage input transistor. N2 The gates are respectively connected to the positive input terminal. V o1+ and negative input terminal V o1- (i.e. M) N1 and M P1 (drain), M N2 The drain is connected to M P2 The drain of the second stage serves as the positive output terminal. V o+ and negative output terminal V o- Simultaneously connected to M N3 and M P3 The drain electrode. M N2 The source is connected to M N7 The drain electrode. M P2 Also serving as the input tube for the second stage, M P2 The gates are respectively connected to the positive input terminal. V o1+ and negative input terminal V o1- (i.e. M) N1 and M P1 (drain), M P2 The source is connected to the power supply, M P2 The drain is connected to M N2 The drain electrode.

[0020] The feedforward amplifier circuit includes M N6 M N3 M P3 , R c and C c M N6 For M N3 The tail current source transistor is used for M N3 Provide bias current, M N6 The gate is connected to a bias voltage V BN1 M N6The source is grounded, M N6 The drain is connected to two M N3 The source level of the tube. M N3 M is the input transistor of the feedforward stage. N3 The gates are respectively connected to the positive input terminal. V in+ and negative input terminal V in- M N3 The source is connected to M N6 The drain, M N3 The drain is connected to M P3 The drain of the feedforward stage is used as the positive output terminal. V o+ and negative output terminal V o- Simultaneously connected to M N2 and M P2 The drain electrode. M P3 Also used as the input transistor of the feedforward stage, M P3 The gate is connected through a compensation resistor. R c and compensation capacitor C c The high-pass filter network is connected to the positive input terminal. V in+ and negative input terminal V in- M P3 The source is connected to the power supply, M P3 The drain is connected to MN3, M P2 and M N2 The drain. Two compensation capacitors. C c One end is connected to the positive input terminal V o1+ and negative input terminal V o1- The other end is connected to two compensation resistors. R c One end is connected to M. P3 The gate. Compensation resistor. R c The other end is connected to M P3 Gate and compensation capacitor C c The other end is connected to the common-mode feedback output voltage. V CMFB .

[0021] The common-mode feedback circuit includes M N8 M N4A M N4B M P4A M P4B MP5A M P5B , R 2 and C 2. M N8 For M N4A and M N4B The tail current source transistor is used for M N4A and M N4B Provide bias current, M N8 The gate is connected to a bias voltage V BN1 M N8 The source is grounded, M N8 The drain is connected to M N4A and M N4B The source level. M N4A For common-mode feedback input transistors, M N4A The gate is connected to the common-mode sense resistor. R 2A , R 2B and common mode compensation capacitor C 2A , C 2B At one end of the network, M N4B The gate is connected to the reference voltage. V ref M N4A and M N4B The source is connected to M N8 The leak level. M N4A The drain is connected to M P4A and M P5A The drain electrode. M N4B The drain is connected to M P4B and M P5B The drain is connected to the compensation resistor. R c One end. M P4A and M P4B The load transistor acts as a common-mode feedback. M P4A The gate is connected to M P4A The drain, M P4A The source is connected to the power supply. M P4B The gate is connected to M P4B The drain, M P4B The source is connected to the power supply. M P5A and M P5B As a load transistor for common-mode feedback, M P5A The gate is connected to a bias voltage V BP M P5A The source is connected to the power supply, M P5A The drain is connected to M P4A and M N4A The drain electrode. MP5B The gate is connected to a bias voltage V BP M P5B The source is connected to the power supply, M P5B The drain is connected to M P4B and M N4B The drain electrode.

[0022] Furthermore, such as Figure 2 As shown, the input signal V in Connect to the first level g m1 and the first-level feedforward g mf1 The input, simultaneously passing through the compensation capacitor C c and compensation resistor R c The resulting high-pass circuit is connected to the second-stage feedforward g. mf2 Input. V in The output signal V after the first stage of amplification o1 As the input to the second stage, the amplified signal from the second stage is superimposed with the amplified signals from the two feedforward stages to generate the output signal V. out .

[0023] Furthermore, the working principle of the feedforward compensation op-amp is as follows: The input signal is amplified by the first stage and then output. V o1 After the second stage of transconductance gain amplification, current is generated. g m2 V o1 This current, combined with the current from the feedforward stage, flows through the output resistor at the output terminal to generate the output voltage. V out Ignoring the influence of the feedforward stage, the input-to-output transfer function of the forward amplifier stage is:

[0024] in p1 =1 / R o1 C 1, p2 =1 / R o2 C 2. g m1 For M N1 transconductance, g m2 For M N2 and M P2 The sum of transconductances. R o1 The output resistor of the first stage,R o1 = r o,N1 / / r o,P1 / / R1, R o2 The parallel connection of the second-stage output resistor and the feedforward stage output resistor R o2 =r o,N2 / / r o,P2 / / r o,N3 / / r o,P3 Ignoring the influence of the forward amplifier stage, the input-to-output transfer function of the feedforward stage is:

[0025] in p2 =1 / R c C c . g mf1 For M N3 transconductance, g mf2 For M P3 The transconductance. The final input-to-output transfer function is the sum of the two:

[0026] in A 0= g m1 g m2 R o1 R o2 , z1 = p3 =1 / R c C c , z2 = g m1 g m2 / [( g mf1 + g mf2 ) C 1). System unity-gain bandwidth c =( g mf1 + gmf2 ) / C 2.

[0027] The feedforward compensated high-speed operational amplifier proposed in this paper can be simplified into a two-pole, one-zero system, and has no low-frequency dominant pole, ensuring sufficient gain even at higher frequencies. Furthermore, by designing the zero at half the unity-gain frequency, the loop has at least a 45° phase margin, thus guaranteeing sufficient stability in closed-loop operation. Moreover, the unity-gain frequency of the proposed feedforward compensated operational amplifier is independent of the compensation capacitor, depending only on the load capacitor size and the transconductance of the feedforward stage; therefore, the proposed feedforward compensated operational amplifier can achieve a high unity-gain frequency.

[0028] Feedforward compensation high-speed operational amplifier circuit implementation To ensure sufficient gain for the operational amplifier (op-amp) to improve receiver linearity within the UWB useful signal frequency range (0-250MHz) while minimizing op-amp power consumption, this paper proposes a feedforward compensated high-speed circuit. This circuit uses the in-phase superposition of the feedforward path (fast path) and the forward amplification path (slow path) to generate a left-half-plane zero to compensate for the phase shift introduced by the forward amplification path. The proposed feedforward compensated high-speed op-amp has no low-frequency dominant poles, ensuring high gain even at higher frequencies. Furthermore, a push-pull input stage and current multiplexing structure are used to improve current utilization efficiency, thereby reducing the overall power consumption of the op-amp. Figure 3 This is the schematic diagram of the feedforward compensated high-speed operational amplifier circuit proposed in this paper. The circuit consists of a two-stage amplifier circuit, a feedforward stage, and a common-mode feedback stage. The first-stage amplifier is mainly used to provide higher gain, and the common-mode feedback of the first stage is provided by a resistor. R 1 and capacitor C 1. To achieve, in order to ensure R 1. This will not significantly affect the first-stage gain; therefore, resistor R1 >> r. o,N1 / / r o,P1 The second-stage amplifier primarily provides moderate gain and a high output swing; the feedforward stage mainly provides a left-half-plane zero to ensure closed-loop stability; the common-mode feedback circuit is mainly used to stabilize the output common-mode level, and the resistor... R 2. Extract output common-mode information and reference voltage V ref Compare, and finally the results of the comparison. V CMFB The input fed back to the feedforward stage ensures that the output common-mode level equals the reference voltage, and the capacitor... C 2 is used to ensure the stability of the common-mode feedback loop. Add M. P5 To reduce M P4 The current in the tube decreases, M P4 The transconductance of the transistor can be increased, thereby improving the gain of the common-mode feedback loop and reducing the error between the output common-mode level and the reference voltage.

[0029] In summary, the feedforward compensated high-speed operational amplifier proposed in this invention has a low-frequency gain of 40dB and a gain of 28dB at 300MHz, with a total power consumption of only 0.92mA, including the bias circuit.

[0030] The feedforward compensated high-speed operational amplifier designed in this invention has a unity-gain bandwidth greater than 2.4 GHz and a phase margin greater than 50° during unity-gain feedback, and there is no risk of oscillation in closed-loop operation.

[0031] The number of devices and processing scale described herein are for simplification purposes. Applications, modifications, and variations of this invention will be readily apparent to those skilled in the art. Although embodiments of the invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. It can be applied to various fields suitable for this invention, and further modifications can be readily implemented by those skilled in the art. Therefore, without departing from the general concept defined by the claims and their equivalents, this invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A feedforward compensated high-speed operational amplifier, characterized in that, Includes the following steps: A first-stage amplifier circuit, a second-stage amplifier circuit connected to the first-stage amplifier circuit, a common-mode feedback circuit connected to the second-stage amplifier circuit, and at least two feedforward amplifier circuits symmetrically arranged about the second-stage amplifier circuit and connected to the second-stage amplifier circuit. The second-stage amplifier circuit includes a second input N-type transistor, a second input P-type transistor, and a second tail current source. The gates of the second input N-type transistor and the second input P-type transistor are connected to the output terminal of the first stage, and their drains are connected together as the output terminal of the operational amplifier. The second tail current source provides bias current to the second input N-type transistor. The second input N-type transistor and the second input P-type transistor form a push-pull input structure.

2. The feedforward compensated high-speed operational amplifier as described in claim 1, characterized in that, The first-stage amplifier circuit includes a first input transistor, a first load transistor, a first tail current source, a first common-mode feedback resistor, and a first common-mode feedback capacitor. The gate of the first input transistor serves as the positive and negative input terminals of the operational amplifier, and the drain serves as the first-stage output terminal and is connected to the input terminal of the second-stage amplifier circuit. The first tail current source provides bias current to the first input transistor. The first common-mode feedback resistor and the first common-mode feedback capacitor are connected between the gate and drain of the first load transistor to provide first-stage common-mode feedback.

3. The feedforward compensated high-speed operational amplifier as described in claim 1, characterized in that, The common-mode feedback circuit includes a common-mode detection resistor, a common-mode compensation capacitor, a common-mode feedback input transistor, and a common-mode feedback load transistor. The common-mode detection resistor extracts the common-mode information output by the operational amplifier and compares it with a reference voltage. The comparison result is fed back to the feedforward amplifier circuit to stabilize the output common-mode level.

4. The feedforward compensated high-speed operational amplifier as described in claim 1, characterized in that, The feedforward amplifier circuit includes a feedforward N-type transistor, a feedforward P-type transistor, a feedforward tail current source, a compensation capacitor, and a compensation resistor. The gate of the feedforward N-type transistor is connected to the input of the operational amplifier as the input of the feedforward stage. The gate of the feedforward P-type transistor is connected to the input of the operational amplifier through a high-pass filter network composed of the compensation capacitor and the compensation resistor. The drains of the feedforward N-type transistor and the feedforward P-type transistor are connected together and connected to the output of the operational amplifier. The feedforward tail current source provides bias current to the feedforward N-type transistor.

5. A feedforward compensated high-speed operational amplifier as described in claim 1, characterized in that, The second input N-type transistor generates a drive current in response to a positive input signal, and the second input P-type transistor generates a drive current in response to an inverted input signal. The two are superimposed at the output to form a complementary driving capability.

6. A feedforward compensated high-speed operational amplifier as described in claim 4, characterized in that, The feedforward N-type transistor and the feedforward P-type transistor form a push-pull input structure, which is used to feed the input signal forward to the output terminal.

7. A feedforward compensated high-speed operational amplifier as described in claim 6, characterized in that, The left half-plane zero generated by the feedforward amplifier circuit is used to compensate for the phase shift caused by the two poles generated by the second stage amplifier circuit; the frequency position of the left half-plane zero is determined by the values ​​of the compensation capacitor and the compensation resistor, specifically, the zero frequency fz is equal to 1 divided by the product of the compensation capacitor and the compensation resistor.

8. A feedforward compensated high-speed operational amplifier as described in claim 1 or 4, characterized in that, The second input P-type transistor and the feedforward P-type transistor are both used as input transistors in parallel with the corresponding N-type transistors to achieve current multiplexing. This allows the current flowing through these transistors to contribute to the signal amplification simultaneously, thereby improving current utilization efficiency and reducing the overall power consumption of the operational amplifier.

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