Do you know how to eliminate self-excitation in op amps?

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Do you know how to eliminate self-excitation in op amps? [Copy link]

From the self-oscillation conditions, we can know that the oscillation can be eliminated from two aspects:

(I) Reduce loop gain (but this method increases the gain error of the op amp)

For the op amp, it means reducing the feedback factor F. In other words, the larger the F, the greater the possibility of self-oscillation. For the resistor feedback network, the maximum value of F is 1, and the typical circuit of F=1 is the voltage follower circuit. This is why the voltage follower op amp is prone to oscillation (this is also the reason why we often see the op amp manual marked with a unit gain stability description, but the gain error of the voltage follower is smaller).

This is also the case for voltage feedback, where the capacitive load driving capability increases proportionally with the closed-loop gain. So, if the VFA can stably drive a 100pF capacitive load when the closed-loop gain is 1, then it can drive a 1000pF capacitive load when the closed-loop gain is 10.

However, due to design reasons, the size of the closed-loop gain is usually not able to be changed casually, so this method is not very applicable. (2) Increase the phase margin

Phase compensation:

According to the compensation principle, it is divided into lag compensation, lead compensation and lag-lead compensation

Lag compensation: Any compensation that causes the phase shift to lag is called lag compensation. Lag compensation reduces the frequency of the main pole, that is, the amplifier bandwidth becomes narrower. (Similar to RC low-pass filtering)----------Curve ②

Lead compensation: Any compensation that makes the phase shift advance is called lead compensation. Lead compensation makes the amplitude-frequency characteristic curve appear zero, that is, the amplifier bandwidth becomes wider (similar to RC high-pass filtering)-----Curve ③

For the capacitance (CL) of the load, the loop gain decreases under the action of the output resistance and CL. At the same time, there is no longer a proportional relationship between phase and gain, and phase lag becomes the decisive factor.

1. Out-of-loop compensation--advanced compensation (applicable to small capacitive load <1500pf or constant load impedance)

A resistor RX is connected in series between the output of the op amp and the load capacitor. Generally speaking, its resistance is 10-100 ohms.

2. In-loop compensation - lead compensation (applicable to large capacitive load>1500pf or uncertain load impedance)

Rx is within the feedback loop of the op amp, and a feedback capacitor is connected in parallel to the feedback resistor (the capacitor can eliminate the pole formed by the op amp input capacitance and stray capacitance). Generally speaking: Rx=50～200Ω, Cf is about 3～10pF