How to avoid instability in rail-to-rail CMOS amplifiers

　　MOS transistors have been shrunk dramatically since their invention decades ago. Reductions in gate oxide thickness, channel length, and width have driven significant reductions in overall circuit size and power consumption. The maximum allowable supply voltage is reduced due to the reduction in gate oxide thickness, while the reduction in channel length and width has resulted in smaller form factors and faster speed performance. These improvements have driven the performance of high frequency CMOS rail-to-rail input/output amplifiers to meet the increasing demands of today’s system designers for a new type of analog circuit that must operate at the same low supply voltages as digital circuits.

　　This application note answers some of the unique questions surrounding the latest generation of CMOS rail-to-rail amplifiers. The article begins with an overview and discussion of the traditional voltage feedback and current feedback amplifier circuit topologies, as well as the most common causes of oscillation in feedback amplifiers. For ease of analysis and discussion, the CMOS rail-to-rail amplifier circuit is divided into four major blocks: input, intermediate gain, output, and feedback network stages. The gain and phase shift as a function of frequency for each stage is shown, followed by a complete system simulation that includes all four basic circuit blocks. Part II presents and discusses the mechanisms, tradeoffs, and advantages of three commonly used approaches to address amplifier oscillation problems.

　　Voltage Feedback Amplifiers

　　Figure 1 shows a simplified implementation of the EL5157, a very popular high-bandwidth voltage feedback amplifier. This implementation uses a classic differential input stage to drive a folded cascode second stage, which converts the differential voltage at the input stage into a current at a high impedance gain node that is implemented with the amplifier’s high voltage gain. Essentially, the second stage current source output impedance becomes an output signal at a high impedance node, increasing any current differences created in the signal path transistors. The output stage is a push-pull class AB buffer that buffers the high voltage gain to the amplifier’s single-ended output.

Figure 1: Voltage Feedback Amplifier

　　Output Inductance

　　An inductor is an electronic component whose impedance is affected by frequency: it is low at low frequencies and increases at high frequencies. The "ideal" op amp output impedance is zero, but in reality the amplifier's output impedance is inductive and, like an inductor, increases with frequency. Figure 2 shows the output impedance of the EL5157. A common challenge in applications that utilize op amps is driving a capacitive load. This is challenging because the op amp's inductive output combines with the capacitive load to create an LC tank topology where the capacitive load, along with the inductive drive impedance, creates additional phase lag as feedback closes around the loop. This reduction in phase margin can cause the amplifier to oscillate. When oscillating, the amplifier can get very hot and may even self-destruct. There are several well-known solutions to this problem.