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.

Active Matrix LCD

Signal
Response time refers to the response speed of the Liquid Crystal Display to the input signal, that is, the response time of the liquid crystal from dark to bright or from bright to dark (the time for the brightness from 10%-->90% or 90%-->10%) , Usually in milliseconds (ms). To make this clear, we have to start with the human eye's perception of dynamic images. There is a phenomenon of "visual residue" in the human eye, and the high-speed motion picture will form a short-term impression in the human brain. Animations, movies, etc. to the latest games have applied the principle of visual residue, allowing a series of gradual images to be displayed in rapid succession in front of people's eyes to form dynamic images. The acceptable display speed of the picture is generally 24 frames per second, which is the origin of the movie playback speed of 24 frames per second. If the display speed is lower than this standard, people will obviously feel the picture pause and discomfort. Calculated according to this index, the display time of each picture needs to be less than 40ms. In this way, for the liquid crystal display, the response time of 40ms becomes a hurdle, and the display above 40ms will have obvious picture flicker, which makes people feel dizzy. If you want the image screen to reach the level of non-flicker, it is best to achieve a speed of 60 frames per second.
I used a very simple formula to calculate the number of frames per second under the corresponding response time as follows:
Response time 30ms=1/0.030=approximately 33 frames per second
Response time 25ms=1/0.025=approximately 40 frames per second
Response time 16ms=1/0.016=approximately 63 frames per second
Response time 12ms=1/0.012=approximately 83 frames of pictures displayed per second
Response time 8ms=1/0.008=approximately 125 frames per second
Response time 4ms=1/0.004=approximately 250 frames per second
Response time 3ms=1/0.003=approximately display 333 frames per second
Response time 2ms=1/0.002=approximately 500 frames per second
Response time 1ms=1/0.001=approximately 1000 frames per second
Tip: Through the above content, we understand the relationship between response time and the number of frames. From this, the response time is as short as possible. At that time, when the LCD market first started, the lowest acceptable range of response time was 35ms, mainly products represented by EIZO. Later, BenQ's FP series came out to 25ms. From 33 to 40 frames, it was basically undetectable, and it was really quality. The change is 16ms, displaying 63 frames per second to meet the requirements of movies and general games, so 16ms is not obsolete. With the improvement of panel technology, BenQ and ViewSonic started a speed battle. ViewSonic started from 8ms to 4ms. Released to 1ms, it can be said that 1ms is the final controversy of LCD speed. For game enthusiasts, 1ms faster means that CS's marksmanship will be more accurate, at least psychologically, such customers should recommend the VX series of monitors. But everyone should pay attention to the grayscale response when selling. The text difference in full-color response may sometimes mean the same thing as gray-scale 8ms and full-color 5ms. It is the same as when we sold CRTs before, we said that the dot pitch is .28, LG just I have to say that he is .21, but the horizontal dot pitch is ignored. In fact, the two are talking about the same thing. LG has come up with a sharpness of 1600:1. This is also a conceptual hype. Everyone uses the basic screen. There are only a few companies on the list, and how can only the LG family achieve 1600:1, and everyone stays at the level of 450:1? When it comes to consumers, the meaning of sharpness and contrast is obvious, just like AMD's PR value, which has no real meaning.

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