Using op amps to form the stability problem of voltage followers

Using an op amp to form a circuit for a voltage follower , traditional textbooks simply connect the output to the inverting input, but the actual circuit is much more complicated, and the stability problem cannot be ignored. I hope it will be helpful for practical applications. .

What issues should I pay attention to to stabilize the operational amplifier with a voltage follower?

A: As for the amplification circuit using negative feedback, how to reduce the oscillation to maintain stability is currently inconclusive. Voltage followers are no exception.

The ideal operating state of an operational amplifier is that the output voltage and the input voltage are in phase, that is, when the impressed voltage at the negative input causes the output to increase, the operational amplifier can reduce the increased voltage accordingly. However, there is always a difference in phase between the input and output of the op amp. When the phase difference between the output and the output is 180 °, the negative input is exactly the same as the positive input, but the output that should have been reduced is enhanced. (It becomes the state of positive and negative rupture.) If you fall into this state in a specific frequency band and still maintain the original amplitude, then the output frequency and oscillation state will continue.

FIg1. Voltage follower and feedback loop

2. The main reason for the phase difference between the input and output terminals

The reasons can be roughly divided into two types:

1. Due to the inherent characteristics of operational amplifiers

2. Due to the characteristics of the feedback loop other than the operational amplifier

2.1. Characteristics of operational amplifiers

Fig2a and Fig2b respectively represent the voltage gain-frequency characteristic and phase-frequency characteristic of the operational amplifier. These two graphs are also in the data sheet.

As shown in the figure, the voltage gain and phase of the operational amplifier vary with frequency. The difference between the gain of the operational amplifier and the feedback gain (0dB when using a voltage follower) is the gain (feedback gain) around the feedback loop for one week. If the feedback gain is less than 1 times (0dB), even if the phase changes by 180o and returns to the positive feedback state, the negative gain will gradually attenuate in the circuit, and theoretically will not cause oscillation.

Conversely, when the phase changes by 180o, if the loop gain corresponding to the frequency is 1 times, the original amplitude will be maintained; if the loop gain corresponding to the frequency is greater than 1 times, the amplitude will gradually diverge. In most cases, in the process of amplitude divergence, the amplitude is limited due to the influence of nonlinear factors such as the maximum output voltage, and the oscillation state will be maintained.

For this reason, the difference between the phase corresponding to the frequency when the loop gain is 0 dB and 180o is an important factor in judging the stability of the negative feedback loop. This parameter is called the phase margin. (Fig2b.)

Unless otherwise specified, when a single amplifier is used as a voltage follower, sufficient phase margin must be maintained.

Note: Amplifiers that are indicated in the data sheet as "recommended to use gains above 6dB" cannot be used as voltage followers.