Frequency compensation of a multistage operational amplifier (op-amp) is normally performed through solving nodal equations of an equivalent circuit to obtain the op-amp's final transfer function. The process is often very tedious and offers little insight into the roles of the selected compensation scheme. In this paper, we present a graphical design approach for two-stage and three-stage op-amps with active feedback Miller compensation. By viewing frequency compensation as a standard feedback problem, we can utilize the well-known graphical tools such as the root locus and Bode plot to understand the effects of the compensation and to estimate the locations of the closed-loop poles and zeros of the op-amp. Intuitive graphical design procedures for two-stage and three-stage op-amps are also formulated. To show its effectiveness, we illustrate our design approach through the design of a three-stage op-amp in a standard 0.18-μm complementary metal-oxide-semiconductor (CMOS) process. With a load capacitance of 500 pF, post-layout simulations show that the op-amp achieves a low-frequency gain of 144 dB, a phase margin of 58 ∘ , and a unity-gain frequency of 1.38 MHz while consuming a total bias current of 31 μA from a 1.8-V supply voltage. Comparisons with the published amplifiers show that our op-amp achieves the figure of merits comparable to those of the state of the art. outer feedback path between the output nodes of the third and the first gain stages, NMC utilizes 'Miller effect' to split the poles associated with the two high-impedance nodes twice. Because of the presence of right-half-plane (RHP) zeros, the bandwidth limitation, and the high power required to achieve stability in NMC, other NMC-based compensation schemes have been proposed [9][10][11][12][13][14][15][16]. These techniques employ nulling resistors, active feedback, and feedforward transconductance to eliminate the RHP zeros, extend the unity-gain bandwidth, and enhance slew rate of the op-amp. To further extend the bandwidth without extra power consumption, reversed nested-Miller frequency compensation (RNMC) [5,[17][18][19][20][21][22] has been proposed. Instead of forming the inner feedback path between the output nodes of the third and the second gain stages, the inner feedback path is formed between the output nodes of the second and the first gain stages. As will be explained later in this paper, compared with the conventional NMC scheme, this choice of the inner feedback path allows for a higher unity-gain bandwidth in the forward-path transfer function before forming the outer loop. As a result, the overall op-amp can achieve a higher unity-gain bandwidth. When the active feedback technique is used to remove the RHP zeros such as in [18][19][20][21], RNMC is sometimes called reversed active-feedback frequency compensation (RAFFC). Because RAFFC allows for a high unity-gain bandwidth without extra passive resistors, it is the most efficient in terms of power consumption and chip area, thus will be the focus of this paper. Analyses of R...
