Design of an Improved Gain-Enhanced Cascode Amplifier

Abstract: A fully differential transconductance amplifier suitable for pipeline A/D converter is designed. By adopting the gain enhancement method of single-ended amplifier, the operational amplifier has a higher DC gain, a smaller area and better layout matching. By improving the common switch constant capacitance common mode negative feedback circuit, the settling time is improved and the jitter of the amplifier output common mode is reduced. The circuit adopts SMIC 0.18μm CMOS process, and the circuit and layout are simulated under Cadence. The results show that: small signal low frequency voltage gain is 119.3 dB; unity gain bandwidth is 378.1 MHz; phase margin is 60°.
Keywords: pipeline ADC; gain enhancement; transconductance amplifier; switch capacitor common mode negative feedback; layout0

Introduction
With the continuous development of integrated circuit technology, high-performance operational amplifiers are widely used in various circuit systems. It has become the core unit circuit of analog and mixed signal integrated circuit design, and its performance directly affects the overall performance of the circuit system. As an important part of modern analog integrated circuits, A/D converters have developed with the development of integrated circuit technology. With the introduction of theories such as digital radio, high-speed and high-precision analog-to-digital converters have become the target of research, which has also put forward higher requirements for operational amplifiers. When traditional structures increasingly limit the indicators of amplifiers, the gain enhancement structure proposed by Bult. K. can effectively improve the open-loop gain of the operational amplifier without affecting the bandwidth, making it easier to design high-performance amplifiers.
This paper designs a folded common-source common-gate transconductance operational amplifier with a switch capacitor common-mode feedback using a gain enhancement structure, which can be used in the A/D of the pipeline structure. Considering the performance and layout factors, a single-ended amplifier is used as a gain-enhanced auxiliary amplifier. And by improving the common-mode negative feedback circuit, the amplifier output common-mode feedback voltage stabilizes faster and has less jitter. This design simulates the circuit and layout of the operational amplifier in the Cadence environment. The results show that the performance parameters of the amplifier have achieved ideal results.

1 Analysis and design of circuit structure
The common structures of CMOS transconductance operational amplifiers include two-stage amplification structure, sleeve structure and folded common-source common-gate structure. Although the op amp circuit structure of the two-stage amplification structure has the advantages of high gain and high swing, since at least one pole is introduced in each stage, in order to ensure that the phase-frequency characteristics of the entire amplifier meet the requirements, an additional frequency compensation circuit is required, which increases the current and power consumption of the amplifier, limits the amplifier bandwidth, and reduces the amplifier speed. Therefore, it cannot meet the requirements of the bandwidth and speed of the op amp in this design. Although the sleeve structure has higher gain, better frequency characteristics and lower power consumption, it is limited by the structure, and its output swing and common-mode input range are small, which does not meet the design requirements. The folded cascode structure improves the shortcomings of the sleeve structure with small output swing. By increasing the number of circuit branches and improving power consumption, it meets the requirements of large bandwidth, high swing and high speed while providing higher gain. By applying gain enhancement technology to the folded cascode structure, the DC gain of the circuit can be further improved without affecting the signal bandwidth, slew rate and phase characteristics. Therefore, in view of the special requirements of this design, the folded cascode structure with gain enhancement technology is selected.
1.1 Main op amp circuit
The folded cascode operational amplifier designed in this paper is shown in Figure 1. M0 and M1 are differential input pairs; M2 is a differential pair constant current source; M4 and M5 are current sources; M6 and M7 are common-gate transistors; M8, M10, M58, and M59 are common-source and common-gate current source loads. Since NMOS transistors have higher carrier mobility, using NMOS transistors as differential input stages can improve the gain and bandwidth of the op amp.

When there is no gain-enhanced auxiliary op amp, the small signal voltage gain of the main op amp is:

It can be seen that compared with the basic constant current source load amplifier circuit, the output resistance of the output node increases by gmRout times, so the common source and common gate structure operational amplifier can provide high gain.

1.2 Switched capacitor common-mode negative feedback circuit
Since the folded common-source common-gate amplifier requires an extremely precise bias voltage to stabilize the circuit output common mode at a fixed value, a common-mode negative feedback circuit must be introduced to stabilize the output common mode of the entire circuit at the required output voltage common mode. Commonly used common-mode negative feedback circuits are divided into two types: continuous-time common-mode negative feedback and switched capacitor common-mode negative feedback. Since the switched capacitor common-mode negative feedback has no static power consumption and has a small impact on the amplifier itself, the switched capacitor common-mode negative feedback circuit is selected in this design to stabilize the output common mode. Figure 2 is a traditional switched capacitor common-mode feedback circuit, out+, out- are differential output voltage signals, clock1, clock2 are two-phase non-overlapping clock signals, Vcm is a reference voltage for comparison, which is equal to the desired output common-mode voltage; in order to provide a large output swing, half of the power supply voltage is usually taken, Vt is the bias voltage generated by the bias circuit, and Vb1 is the adjustment voltage generated to stabilize the output common-mode voltage.

Since the switch capacitor common-mode negative feedback needs to continuously calculate the difference between the output common mode and Vcm to control the amplifier so that its output common mode is stable at the required voltage value. For the traditional switch capacitor common-mode negative feedback circuit, half of the clock cycle in a clock cycle requires two capacitors C1 and C2 to take the difference between Vcm and Vt, and cannot be used to interact with the output common mode to generate feedback voltage. Therefore, the common-mode level is slow to establish. Therefore, we introduce another set of sampling capacitors to make the two sets of sampling capacitors collect the difference between Vcm and Vt, and calculate with the common mode of the output voltage in different clock cycles. In this way, the circuit reduces the establishment time of the common-mode feedback voltage and reduces the jitter of the feedback voltage caused by the switch on and off. The improved common-mode negative feedback circuit is shown in Figure 3.
Since the capacitor in the switch constant capacitance common-mode negative feedback circuit is directly hung on the output node, too large a capacitance value will reduce the bandwidth and slew rate of the amplifier. At the same time, in order to reduce the clock feedthrough effect caused by dynamic switching action and other parasitic stray capacitance effects and back-end process accuracy, the capacitance value cannot be too small. Therefore, in this design, we take all capacitors to be 0.5pF.
1.3 Auxiliary amplifier circuit for gain enhancement The
use of gain enhancement technology can effectively improve the DC gain of the operational amplifier without affecting its speed. Considering the contribution of layout symmetry to reducing amplifier offset, this design introduces four single-ended current mirror cascode amplifiers as gain enhancement amplifiers, which are divided into two groups to increase the equivalent resistance from the cascode tube and the equivalent resistance of the cascode current mirror, thereby greatly improving the DC gain. The auxiliary amplifier uses current input, and the current input from the main amplifier circuit to the auxiliary amplifier is determined by the ratio of the input tube size to the corresponding cascode device size. Compared with the traditional differential structure, the single-ended amplifier can be better symmetrically distributed on both sides of the main amplifier layout, and because the amplifier uses a current input proportional to the main branch, compared with the voltage input amplifier, the influence caused by the input common mode voltage change is eliminated. The auxiliary amplifier structure is shown in Figure 4.

Since the auxiliary amplifier has limited output swing and high gain, a common-mode negative feedback structure using differential pair sampling is selected. This structure will limit the amplifier output swing, but will not affect the amplifier gain, and has low power consumption, so it is suitable for use in the auxiliary amplifier. The common-mode negative feedback circuit is shown in Figure 5.
1.4 Bias circuit Since there are many common-source and common-gate tubes in the entire circuit that need to provide bias, a common-source and common-gate wide-swing current mirror is used to provide bias for these tubes. The wide-swing common-
source and common-gate current mirror increases the swing while ensuring the current replication accuracy, so that the circuit maintains normal operation while ensuring the output swing. The wide-swing common-source and common-gate current mirror circuit diagram is shown in Figure 6, where I2=I1.

2 Circuit Simulation Results
The entire op amp and its bias circuit are designed using SMIC 0.18μm CMOS mixed signal process and simulated using Spectre in the Cadence environment, with a power supply voltage of 3.3V and a load capacitance of 3 pF. The circuit is AC simulated, and the simulation results show that the circuit has a DC gain of 119.3 dB, a unity gain bandwidth of 378.1 MHz, and a phase margin of 60°, as shown in Figure 7.

The amplifier takes 7.9 ns to settle to an output voltage with an accuracy of 0.1%. The test waveform is shown in Figure 8.

Common mode input range 600mV～3.3V; voltage output range 0.6～3.1V; power consumption 39mW.

3 Layout Design
The overall circuit includes 1 main amplifier, 2 gainboosts and 1 common mode negative feedback. The main amplifier and gainboost each have their own bias circuit. The bias circuit of gainboost and the gai-nboost amplifier are placed close to each other to make the connection as short as possible. The 2 gainboots are placed on both sides of the main amplifier to make the overall layout symmetrical. The switched capacitor common mode negative feedback is placed under the main amplifier to make the connection between out+, out- and Vb1 as short as possible. The main amplifier is ABAB matched and uses double-sided power supply to ensure better matching of differential mode signals. The main amplifier bias is split on both sides of the main amplifier to make the overall layout more regular and neat, saving area.

4 Conclusion This paper
introduces the design of a folded cascode operational amplifier. The actual design simulation values ​​are: small signal low frequency voltage gain 119.3 dB; unity gain bandwidth 378.1 MHz; phase margin 60°; settling time 7.9 ns; power supply voltage 3.3 V; common mode input range 600 mV～3.3 V; voltage output range 0.6～3.1 V; load capacitance 3 pF; power consumption 39 mW. The whole design meets the design index requirements and is applied to the design of 12 b, 60 MHz pipeline ADC with undersampling technology.