About op amp compensation

光芒。

About op amp compensation [Copy link]

 

I would like to ask, what is a fully compensated op amp? What is a non-fully compensated op amp? What are the differences in their uses in practical applications? Or what should I pay attention to when selecting and using them?

maychang

Not sure what a "fully compensated op amp" is.

Hypothesis: Does “fully compensated op amp” refer to an op amp that is still stable at unity gain?

maychang

Some types of op amps cannot operate at unity gain. The gain must be greater than a certain value, otherwise they will become unstable.

gmchen

The concepts of full compensation and incomplete compensation apply to so-called "voltage feedback op amps." Due to the influence of various distributed parameters, the open-loop frequency characteristics of this type of op amp have multiple poles, which may cause the op amp to be unstable after the loop is closed.

In order to make the op amp stable after the loop is closed, a compensation capacitor is inserted inside the op amp. The pole separation effect of the capacitor causes the first pole of the op amp to shift to the left (the frequency is reduced) and the higher-order pole to shift to the right (the frequency is increased).

If the compensation capacitor is large enough to move all high-order poles of the op amp below 0dB of the open-loop frequency characteristic, the op amp is called fully compensated. If the compensation capacitor is not large enough and the high-order poles of the op amp are still above 0dB, the op amp is called incompletely compensated.

For the same structure of op amps, the gain bandwidth product (GBP) of fully compensated op amps is smaller than that of incompletely compensated op amps, so incompletely compensated op amps are mostly used in high-frequency applications. However, due to incomplete compensation, in order to avoid self-oscillation, it is usually required that the closed-loop gain of the circuit cannot be less than a certain specified value, that is, a unit gain circuit cannot be formed using incompletely compensated op amps.

A typical example of these two types of op amps is OP27 and OP37. The circuit structures of the two op amps are exactly the same, but the former is fully compensated and the latter is incompletely compensated. The frequency response of the latter is significantly higher than that of the former, but the former can work at a gain of 1, while the minimum stable gain of the latter is 5.

光芒。

maychang posted on 2020-6-29 19:01 I don't know what a "fully compensated op amp" is. Guess: Does "fully compensated op amp" refer to unity gain...

Haha, yeah.

光芒。

gmchen posted on 2020-6-29 19:26 The concepts of full compensation and incomplete compensation apply to the so-called "voltage feedback op amps". Due to the influence of various distributed parameters, this type of op amp...

I think I understand. Thank you, teacher.

光芒。

maychang posted on 2020-6-29 19:02 Some types of op amps cannot operate at unity gain. The gain must be greater than a certain value, otherwise it will be unstable.

Thank you

Maksim

Thank you for your answer

光芒。

Maksim posted on 2020-6-30 01:11 Thank you for your answer

Um,... Isn't this a post I made?

guojian047

The 4th post has the correct answer. In English, this thing is called decompensated. The disadvantage of decompensated amp is that it is unstable at a gain of 1, and is generally stable only when the gain is greater than 1. The advantage is that the gain-bandwidth product is wider at this gain. There are generally two models, one that is stable at a gain of 1, and the other that is stable at a gain greater than 1.

In addition to OP27 and OP37, let's take a few more examples, the latter is a decompensated amp.

• ADA4627-1 and ADA4637-1
• ADA4896 and ADA4895
• LT6233 and LT6233-10
• LT6200 and LT6200-5/-10
• LTC6269 and LTC6269-10
• LTC6090 and LTC090-5

xiaxingxing

gmchen posted on 2020-6-29 19:26 The concepts of full compensation and incomplete compensation apply to the so-called "voltage feedback op amps". Due to the influence of various distributed parameters, this type of op amp...

"The pole-splitting effect of this capacitor causes the op amp's first pole to shift left (lower frequency) and higher-order poles to shift right (increase frequency)."

I would like to ask the teacher, are the second-order and above poles called high-order poles?

gmchen

xiaxingxing posted on 2020-8-3 22:19 "The pole-splitting effect of the capacitor causes the first pole of the op amp to shift to the left (lower frequency) and the higher-order poles to shift to the right (increase frequency)." ...

The high-order pole shifting to the right after adding the compensation capacitor mentioned on the 4th floor mainly refers to the pole closest to the first pole shifting to the right. The remaining poles are usually at higher frequencies, and the compensation capacitor has little effect on them.