How to design the following voltage circuit of the op amp?
Most designers know the voltage-following circuit design of op amps, so what are the precautions? For the voltage-following circuit of op amps, it has always been a difficult point, and it is a bottleneck for beginners to learn. Understanding the voltage following circuit of an op amp is very helpful for understanding the in-phase, inverted, differential, and various op amp circuits. This article brings a detailed analysis process of the voltage circuit of the op amp. We can slowly understand it and find a breakthrough to grasp the key content!
The voltage-following circuit of the op amp, as shown in Figure 1, uses virtual shortness and virtual disconnection. It looks simple and clear at a glance. There is not much content to pay attention to, then you may be wrong.
Analysis of Voltage Following Circuit
If we connect the output of the op amp to its inverting input, and then apply a voltage signal to the non-inverting input, we will find that the output voltage of the op amp will follow the input voltage well. Assuming that the input and output voltages of the op amp in the initial state are both 0V, then when Vin increases from 0V, Vout will also increase, and it will increase in the direction of positive voltage. This is because assuming that Vin suddenly increases and Vout has no response, when Vout 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 starts to increase in the direction of positive voltage.
When Vout increases, the output voltage is fed back to the inverting input terminal, and then the voltage difference between the two input terminals of the op amp will be reduced, that is, Ve will decrease, in the same open-loop gain situation , Vout will naturally decrease. The final result is that no matter how large the input voltage is (of course within the input voltage range of the op amp), the op amp will always output a voltage very close to Vin, but the output voltage Vout is just lower than Vin. It is guaranteed that there is enough voltage difference Ve between the two input terminals of the op amp to maintain the output of the op amp, that is, Vout=Ve*A.
Negative feedback in op amp circuits
Then, this circuit will soon reach a stable state, and the amplitude of the output voltage will accurately maintain the voltage difference between the two input terminals of the op amp. This voltage difference Ve in turn will produce an accurate op amp output voltage. Amplitude. Connecting the output of the op amp with the inverting input of the op amp is called negative feedback, which is the key to making the system self-stabilizing. This applies not only to op amps, but also to any common dynamic system. This stability allows the op amp to work in linear mode instead of just being in a saturated state, fully "on" or fully "off", just like it is used in a comparator 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, the open loop gain of an op amp is 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 end of the op amp. Ve=6V-5.999 970 000 149 999V=29.999 85uV. This voltage will be amplified and then generate a voltage of 5.999 970 000 149 999V at the output end, thus this system Will be stable here. As you can see, 29.999 85uV is a very small voltage, so for actual calculations, we can think that the voltage difference between the two input terminals of the op amp maintained by negative feedback is Ve=0V. The whole process is shown in Figure 2. Show. This is the "virtual short" that we are familiar with, and because the impedance between the two input terminals of the op amp is very large, there will naturally be "virtual disconnection". The following circuit has a stable closed-loop gain of 1 times, and the output voltage will simply follow the input voltage.
A big advantage of using negative feedback is that we don't need to care about the actual voltage gain of the op amp, as long as it is large enough. If the voltage gain of the op amp is not 200 0000 but 250,000, this will make the output voltage of the op amp closer to Vin, and a smaller voltage difference between the input terminals is used to generate the required output voltage. In the circuit shown in Figure 2, the output voltage will also be equal to the input voltage on the inverting input of the op amp. Therefore, for circuit design engineers, 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 negative feedback will make the system self-adjust.
Using negative feedback will improve linearity, gain stability, output impedance, and gain accuracy, but using negative feedback will also bring a serious problem, which is to reduce the stability of the system, and for unity gain voltage follower circuits This is the worst case, especially in the case of driving capacitive loads, interested students can check relevant information by themselves. Regarding the operational amplifier circuit, many times we are instilled that the inverting terminal follows the non-inverting terminal. As mentioned earlier, can't the non-inverting terminal follow the inverting terminal?
For the voltage-following circuit discussed today, the inverting terminal can only follow the non-inverting terminal. This is because if a positive input voltage is applied to the inverting terminal and the output is connected to the non-inverting terminal. Also assuming that the output is 0, then Ve will be a negative voltage, multiplied by the open loop gain of the op amp, and the output will be a The negative voltage, returning to the non-inverting input of the op amp, will further get a negative voltage difference with a greater absolute value. Soon the output of the op amp will reach saturation, and naturally it will not be possible for the non-inverting end to follow the inverting end.
But for the op amp, if a reference voltage is applied to the inverting terminal, and other electronic components, such as transistors, MOS, etc., make the overall loop of the op amp form negative feedback, and the non-inverting terminal can also follow the inverting terminal. And this naturally breaks the familiar rule that the inverting end of the op amp follows the non-inverting end. The voltage of the op amp follows the circuit, "virtual shortness" and "virtual disconnection" are the surface, and negative feedback is the root. Based on this root, it can help us understand the ever-changing operational amplifier circuits. The above is the detailed analysis process of the voltage-following circuit of the op amp.
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