Substrate driven rail-to-rail operational amplifier design

With the continuous development of portable electronic products and ultra-deep submicron integrated circuit technology, low power supply voltage and low power consumption design has become the development trend of modern CMOS operational amplifiers. The most direct and effective way to reduce power consumption is to reduce the power supply voltage. However, the reduction of the power supply voltage also reduces the common-mode input range and output dynamic range of the operational amplifier. At the same time, the reduction of the circuit power supply voltage will be limited by the threshold voltage of the MOSFET. In response to this problem, substrate-driven rail-to-rail technology came into being, which not only effectively reduced the threshold voltage of the MOSFET, thereby directly reducing the power supply voltage of the circuit, but also enabled the common-mode input range to reach the full swing. However, the input transconductance of the substrate-driven MOSFET is small and the input capacitance is large, which limits the maximum operating frequency of the circuit. Therefore, the introduction of the substrate-driven input stage will inevitably reduce the first-stage gain of the operational amplifier. To this end, this paper adopts an improved feedforward class AB output stage to increase the effective input stage transconductance, thereby avoiding the shortcomings of the substrate-driven technology and making the circuit have the characteristics of low voltage, low power consumption and high gain.

The circuit designed in this paper adopts substrate drive technology to reduce the power supply voltage to 0.8 V. At the same time, the circuit combines a constant transconductance control circuit and an improved feedforward class AB output stage, which can effectively improve the dynamic range and response speed, making the circuit input and output stages reach rail-to-rail, which is very suitable for low-voltage and low-power analog integrated circuit applications.

1 Circuit Implementation

Substrate Rail-to-Rail Operational Amplifier The implementation is shown in Figure 1.

Figure 1: Substrate rail-to-rail operational amplifier

1.1 Amplifier Input Stage

In order to ensure that the circuit can work properly when the common-mode input of the op amp changes within the entire power supply range, a complementary differential pair structure of NMOS and PMOS in parallel is used to achieve rail-to-rail input stage. As shown in Figure 1, the input stages M1 to M4 all use substrate-driven MOSFETs. For gate-driven transistors, the minimum power supply voltage required for the input stage is Vsup min = Vgsp + Vgsn + 2Vdsat = 2Vth + 4Vdsat, while the minimum power supply voltage required for the substrate-driven differential pair is Vsup min = Vsbp + Vbsn + 2Vdsat ≈ Vth + 2Vdsat. Therefore, the minimum power supply voltage required for the substrate-driven input stage is lower than that of the traditional differential structure. At the same time, since the substrate-driven MOS tube usually works in the depletion region, its depletion characteristics are conducive to achieving a rail-to-rail common-mode input range under low power supply voltage. Among them, Vgsp and Vgnp are the gate-source voltages of PMOS and NMOS tubes respectively, Vdsat is the drain-source saturation voltage of the MOS tube, Vsbp and Vbsn are the source-substrate voltage and substrate-source voltage of the PMOS tube and NMOS tube respectively, and Vth is the turn-on voltage of the MOS tube.

The total transconductance of a typical rail-to-rail operational amplifier changes by nearly one-fold over the entire common-mode input variation range. The change in transconductance brings about changes in gain and unity gain bandwidth, and also brings great difficulties to the frequency compensation of the operational amplifier. To this end, this paper uses redundant differential pairs (M1a~M4a) and inverted cascode summing circuits to control the input stage transconductance to keep it constant. Both the redundant tubes and the summing circuit use substrate-driven MOSFETs to meet the low operating voltage requirements. The input stage after adding redundant tubes has a significant advantage, that is, it provides a constant output current for the summing circuit, thereby effectively eliminating the impact of the input stage transconductance on the ideal frequency compensation caused by the change of input voltage. The summing circuit uses a substrate-driven inverted cascode structure to increase the common-mode input range and improve the power supply rejection ratio (PSRR), while increasing the differential gain of the circuit, reducing offset, and achieving rail-to-rail characteristics under low voltage. The main disadvantages of substrate-driven MOSFETs are small input transconductance and large input capacitance, which lead to a decrease in the characteristic frequency fT of the MOSFET, thereby limiting the maximum operating frequency of the circuit. Therefore, the introduction of substrate driven input stage will inevitably reduce the first stage gain (-gmbr0) of the op amp. This paper adopts an improved feed-forward class AB output stage to increase the effective input stage transconductance and avoid the disadvantages of substrate driven technology.

1.2 Amplifier Output Stage

In the design of rail-to-rail operational amplifiers, in order to give full play to the characteristics of rail-to-rail operational amplifiers, a good output stage must be designed. In order to achieve higher conversion efficiency and output full swing, the output stage of the rail-to-rail operational amplifier usually adopts a feedforward class AB output stage.

This design uses folded common gate and common source as the active load and combines it with a feedforward class AB output stage. While improving the voltage gain and increasing the voltage output dynamic range, it ensures the total voltage gain of the operational amplifier within the entire common-mode input voltage range. However, the disadvantage of this traditional structure is that the bias current source and the common source and common gate load of the class AB control circuit are in parallel, thereby reducing the output impedance and gain of the input stage. In addition, the current source will also introduce greater noise and offset to the operational amplifier. Therefore, the following measures were taken:

(1) As shown in Figure 1, M17 and M18 are output transistors, and M15, M16, M17, M21 and M13, M14, M18, and M22 form two linear loops to control the output transistor current. M7, M8, M9, and M10 all use substrate-driven MOSFETs to meet the needs of low power supply voltage. M21 and M22 are floating class AB control circuits embedded in the common source and common gate summing circuit, and their bias is provided by the common source and common gate structure to reduce the noise and offset introduced by the bias current source in the traditional structure.

(2) The feedforward AB class output stage can obtain a higher maximum current to quiescent current ratio and improve the utilization rate of power supply power consumption. If the gates of M17 and M18 are biased close to VDD-Vth and VSS+Vth respectively, the output dynamic range of the voltage can reach VSS+Vdsat~VDD-Vdsat. In this way, the quiescent current of M17 and M18 is very small, which will reduce the speed of the output stage. Therefore, the compromise between maximum output current, quiescent power consumption, frequency response performance and circuit area should be comprehensively considered. In this circuit, M21 and M22 are used as the circuit for fixing the gate voltage of the output tube, which saves more circuit area than using resistors. At the same time, it has the advantages of reducing the sensitivity of the gate voltage to the process and power supply.

(3) In the other branch of the common-source and common-gate structure, floating current sources M19 and M20 with the same structure as the class AB control circuit are added. Through the common-source and common-gate current mirror, they can provide a stable bias for the class AB control circuit to reduce the impact of common-mode input voltage changes on the class AB output stage.

The dimensions of the operational amplifier MOS tube designed in this paper are shown in Table 1.

Table 1 Substrate rail-to-rail operational amplifier MOS tube dimensions

Simulation results #e# 2 Simulation results

Based on the BSIM3 model of PTM 0.18 μm CMOS process, Hspice is used to simulate the characteristics of the substrate-driven rail-to-rail op amp. The redundant differential input signal is 0.4 V. Figure 2 shows the input common-mode voltage range curve. The linear part of the transfer curve with a slope of about 1 is the input common-mode voltage range. From Figure 2, it can be measured that the common-mode input voltage range is -0.36 V to 0.39 V, achieving rail-to-rail input.

Figure 2 Input common mode voltage range curve

The operational amplifier is connected in a closed loop with a reverse gain of 10, and its output voltage range is measured. The output voltage swing curve is shown in Figure 3. As can be seen from the figure, when the output voltage swing is about -0.39 V to 0.395 V, it basically reaches rail-to-rail output. So far, the operational amplifier has met the design requirements of rail-to-rail input and rail-to-rail output.

Figure 3 Output voltage swing curve

Figure 4 shows the amplitude-frequency characteristic curve of the operational amplifier. When the power supply voltage is 0.8 V, the DC open-loop gain is 62.1 dB, the unity gain bandwidth is 2.14 MHz, the phase margin is 52°, and the power consumption is 65.9 μW.

Figure 4 Amplitude-frequency characteristic curve of operational amplifier

The same signal is added to the two input terminals of the operational amplifier, and the AC small signal analysis is performed to measure the common-mode voltage gain of the circuit as shown in Figure 5. At low frequency, the common-mode gain of the circuit is -114 dB. Combined with the results of the previous AC small signal analysis, it can be concluded that the common-mode rejection ratio of the circuit is 176.1 dB. Figure 6 is the voltage rejection ratio simulation curve. At low frequency, the voltage rejection ratio is about -73.8 dB.

Figure 5 Common-mode voltage gain

Figure 6 Voltage suppression ratio simulation curve

The simulation results show that the substrate-driven operational amplifier has good performance. Although the frequency bandwidth and linearity of the operational amplifier are reduced, it can effectively avoid the limitation of threshold voltage, reduce the power supply voltage to 0.8 V, and the power consumption is 65.9 μW. At the same time, the rail-to-rail input/output voltage range is achieved. In the traditional gate-driven rail-to-rail operational amplifier signal path, there is the influence of MOS tube threshold voltage, which limits its application under ultra-low power supply voltage.

Summary: This paper uses a substrate-driven complementary differential pair circuit to effectively reduce the power supply voltage requirements of CMOS analog integrated circuits, and improves the voltage gain through an improved feedforward class AB output stage, achieving ultra-low voltage operational amplifier signal amplification, and obtaining a common-mode input range of -0.36 V to 0.39 V and an output voltage range of -0.39 V to 0.395 V. The simulation results show that the operational amplifier has good performance indicators, can effectively drive resistive loads, and has a simple structure, which is suitable for low-voltage and low-power analog integrated circuit applications.