In the circuit design process, bootstrap circuits composed of bootstrap capacitors can often be used to improve some performance characteristics of the circuit, such as increasing the input impedance of the circuit, increasing the gain of the circuit, and expanding the dynamic range of the circuit. Here, I An example of a bootstrap circuit is used to detail how it increases the input impedance of the circuit.
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A very common schematic is as follows,
In the above figure, in order to make the operation work normally when static, it is necessary to add a resistance with a certain resistance between the non-inverting input and ground. After a simple analysis, it is known that a voltage series negative feedback is introduced here. Those who are familiar with the working principle of the operational amplifier can see at a glance that the input resistance of this circuit is:
Obviously, such input resistance is relatively too small. In the figure, the amplification circuit will accordingly require a large amount of current from the signal source, and the voltage drop of the internal resistance of the signal source will increase, and the signal voltage loss will naturally increase. . Therefore, we have to find a way to improve its input resistance. At this time, we can set up a bootstrap circuit to effectively solve this problem. The solution is as follows:
With just one more capacitor added, the input resistance of this circuit is "not the same as before." Using the instantaneous polarity method, it can be judged that in addition to introducing a negative feedback through the reverse input terminal of R4, a positive feedback is introduced through R1 connected to the non-inverting input terminal. At this time, two resistors R2 and R3 are connected in parallel. Together. It should be noted that the values ​​of the capacitors (C1, C2) are relatively large, and they are equivalent to short circuits with respect to the AC signal. The result of the positive feedback causes the dynamic potential at the input to rise, that is, the circuit that causes the dynamic potential of the input to rise by feedback, which is called a "bootstrap circuit."
Due to the good "crossing straight" nature of capacitor C2, the voltage drop across R1 is (uP-uN), at which point the current through resistor R1 is:
Let's take a look at the input resistance of this circuit, and we can get the following equation:
Obviously, for the op amp, since the deep negative feedback is introduced in the circuit, uP and uN are almost equal, then Ri will tend to a maximum value, and the input resistance is greatly improved. The performance of the circuit has also been improved.
Conclusions of this paper: It can be seen from this analysis that in the RC coupling amplifying circuit, it is often possible to introduce a suitable positive feedback while introducing negative feedback, so as to improve the input impedance of the circuit and effectively improve the performance index of the circuit.
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