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promotedNovember 11th 2019 693 reads

editor at Kynix Electronic

The voltage follower circuit of the op amp, as shown in Figure 1, uses virtual short and virtual break. At first glance, it seems simple and clear. If you think that there is not much content to pay attention to, then you may be wrong. Understanding the op amp's voltage-following circuit is a great help for understanding the op amp's in-phase, inverting, differential, and various op amp circuits.

Figure 1 op amp voltage follower circuit

If we connect the output of the op amp to its inverting input and then apply a voltage signal to the noninverting input, we will find that the output voltage of the op amp will follow the input voltage very well.

Assume that the input and output voltages of the initial state op amp are both

`0V`

, and then when Vin increases from `0V`

, `Vout`

will also increase in the direction of positive voltage. This is because it is assumed that Vin suddenly increases, `Vout`

still has no response and is still `0V`

. `Ve=Vin-Vout`

is greater than 0, so multiply the open-loop gain of the op amp. `Vout=Ve*A`

, so that the output of the op amp `Vout`

begins to increase in the direction of the positive voltage.
When

`Vout`

increases, the output voltage is fed back to the inverting input, which then reduces the voltage difference between the two inputs of the op amp, ie, `Ve`

will decrease, in the same open-loop gain case. Next, `Vout`

will naturally decrease. The end result is that no matter how large the input voltage is (of course, within the op amp's input voltage range), the op amp will always output a voltage that is very close to

`Vin`

, but this output voltage `Vout`

is just below `Vin`

. There is a sufficient voltage difference `Ve`

between the two inputs of the guaranteed op amp to maintain the output of the op amp, which is `Vout=Ve*A`

.
This circuit will soon reach a steady state, the amplitude of the output voltage will accurately maintain the voltage difference between the two input terminals of the op amp, which in turn will produce an accurate amplitude of the op amp output voltage. Connecting the output of the op amp to the inverting input of the op amp is called negative feedback, which is the key to self-stabilizing the system.

This applies not only to op amps, but also to any common dynamic system. This stability allows the op amp to have the ability to operate in a linear mode, rather than just being saturated, fully "on" or "off", just as it is used for comparators without any negative feedback.

Due to the high gain of the op amp, the voltage maintained at the inverting input of the op amp is almost equal to

`Vin`

. For example, an op amp has an open loop gain of 200,000. If `Vin`

is equal to 6V, then the output voltage will be 5.999 970 000 149 999V. This produces a sufficient voltage difference at the input of the op amp, `Ve`

=6V-5.999 970 000 149 999V=29.999 85uV, which is amplified and then produces a voltage of 5.999 970 000 149 999V at the output, thus the system Will be stable here. As you can see, 29.999 85uV is a small voltage, so for practical calculations, we can think that the voltage difference between the two inputs of the op amp maintained by negative feedback is

`Ve=0V`

. The whole process is shown in Figure 2. This is also the “virtual short” we are familiar with, and since the impedance between the two inputs of the op amp is very large, there is naturally a “virtual break”. The circuit below has a stable double-loop closed-loop gain, and the output voltage simply follows the input voltage.Figure 2 The role of negative feedback

One big advantage of using negative feedback is that we don't have to worry about the actual voltage gain of the op amp, as long as it is large enough. If the op amp's voltage gain is not 200 0000 but 250 000, this will cause the op amp's output voltage to be closer to

`Vin`

, and the voltage difference between the smaller inputs will be used to produce the desired output voltage. In the circuit illustrated in Figure 2, the output voltage is also equal to the input voltage at the inverting input of the op amp. Therefore, for the circuit design engineer, in order to achieve a stable closed-loop gain of the amplifier circuit, the open-loop gain of the op amp does not have to be an accurate value, and the negative feedback causes the system to self-adjust.
Using negative feedback improves linearity, gain stability, output impedance, and gain accuracy, but using negative feedback also poses a serious problem, which is to reduce system stability for voltage-following circuits with unity gain. This is the worst case scenario, especially when driving capacitive loads, interested students can check the relevant information themselves.

Regarding the op amp circuit, many times we are infused with the inverting end to follow the in-phase end. As mentioned above, can't we follow the inverting end of the phase?

For the voltage follower circuit mentioned today, only the inverting terminal follows the non-inverting terminal. Here, if a positive input voltage is applied to the inverting terminal and the output is connected to the non-inverting terminal, and the output is assumed to be 0.

`Ve`

will be a negative voltage, multiplied by the open-loop gain of the op amp, and the output will be a A negative voltage, returning to the non-inverting input of the op amp, will further result in a larger negative absolute voltage difference. Soon the output of the op amp will be saturated, and naturally the in-phase will not follow the inverting end.
However, for an op amp, if a reference voltage is applied to the inverting terminal, and other electronic components, such as a triode, MOS, etc., the overall loop of the op amp forms a negative feedback, and the in-phase end can also follow the inverting end. And this naturally breaks the rule that the inverting end of the familiar op amp follows the in-phase end.

The voltage of the op amp follows the circuit, "virtual short", "virtual break" is the surface, and negative feedback is the root. Based on this root, it can help us understand the ever-changing op amp circuit.