1. Inverse proportional operational amplifier circuit
2. Integral operational amplifier circuit
The integral circuit is a circuit that makes the output signal proportional to the time integral value of the input signal. The integral circuit is mainly used for waveform transformation, elimination of the offset voltage of the amplifier circuit, and integral compensation in feedback control. The integrator circuit is a widely used analog signal operation circuit. It is the basic unit of an analog computer to simulate differential equations. At the same time, the integrating circuit is also an important unit commonly used in the control and measurement system. Using its charging and discharging process, the delay, timing and the generation of various waveforms can be realized.
3. Add a small capacitor to the feedback resistor of the integral operational amplifier circuit
First of all, we must understand the two most essential functions of capacitors:
Signals of different frequencies will have different degrees of phase shift and attenuation when they pass through the capacitor.
If you're using its attenuation, it's filtering.
If you're taking advantage of its phase shift, then it's compensation.
A circuit using a first-order RC is called a first-order filter. The stopband region of the first-order low-pass filter decays slowly, and the decay slope is -20Db/10 octave. First-order filtering is divided into forward and reverse operation filtering.
3.1 Non-inverting input-----first-order low-pass filter design
3.1. Inverting input-----first-order low-pass filter design
At first glance, it is an integrating circuit. A classical model is given below. It can be seen that the capacitance value of the capacitor CF is very small. It can be said that this is not an integrating circuit, but a filtering circuit.
The role of CF : phase compensation, preventing oscillation, suppressing high-frequency noise.
Why? Generally speaking, because of the parasitic capacitance of the wiring, the input end of the op amp will have a parasitic capacitance of 10~20pF , as shown in CIN (we will call it input capacitance for the time being). It is this input capacitance that will make the high frequency noise gain of the op amp very large, which may make the system unstable.
C1 and R1 form a negative feedback circuit with a certain frequency characteristic. If there is no C1, the frequency characteristic of the circuit is flat. After C1 is connected in parallel, the negative feedback of the high frequency band increases, and at the same time, it can also suppress the high frequency self-excitation.
Our input signal is generally a DC signal or a low-frequency signal, and this capacitor will not work at this time.
This is called differential negative feedback. Usually connected in parallel on the feedback path in the voltage feedback, mainly for the sudden fluctuation of the input voltage or the sudden fluctuation of the output load, when the output voltage changes, increase the feedback amount (that is, the feedback strength) and adjust the output in time to achieve stability. The dynamic response process of the power supply is accelerated, so that the output fluctuation range is reduced when the input voltage is abruptly changed or the load is abruptly changed, avoiding overshoot and increasing stability. In actual use, if the parameter configuration is appropriate, the switching power supply can work stably even under no-load state without oscillation (usually the switching power supply will have a dummy load to increase the stability at no-load). The dummy load can be eliminated or the power consumption of the dummy load can be reduced.
This is used to track the change feedback of the output voltage. When the voltage at the output load terminal changes, it can be quickly fed back to the ic to adjust the output voltage in time.
Speaking of voltage differential negative feedback, first of all, we must understand the meaning of differential, which is a small amount of change;
voltage differential negative feedback Its essence is the feedback of the small change of voltage. It uses the leading effect of the capacitor current to reflect the small change of the voltage to the operational amplifier in advance, which makes the system respond faster to the small change and avoids the overshoot at the end of the system adjustment. appearance.
The C1 filter above, plus C1 can reduce the high frequency gain. That is to say, a part of the high frequency signal at the output will be returned to the input through C1. However, the returned signal is out of phase with the input signal. Therefore, a part of the high frequency signal will be cancelled out. The last phenomenon reflected to the output is that the low-frequency gain becomes larger.
电阻在电路中的作用就是限制放大倍数。因为现在所用的运放其有效增益都很大。如果，不对增益进行限制的话，运放很容易就进入锁死状态。就是，运放内部的某些晶体管因为工艺的原因饱和。然后，饱和导通的晶体管就不会再对输入的信号有反应。当然，实际使用中，我们要坚决的杜绝掉这个问题。所以就引入了负反馈电阻。限制放大倍数的工作原理就是，输出端的任意电压都会被反馈电阻以一定的返回系数送回输入端与输入信号进行相减处理。这样，小信号输入的时候，反馈回来的信号也是小的。那么，对输入信号的影响就要小些。这时，信号基本以最大放大倍数输出。当输入信号较大时，返回的信号也会变大。然后，就会对输入的信号进行相减处理。这样，就不至于使运放进入锁死状态。同三极管放大电路中的反馈电路作用相同。主要就是稳定工作点，防止运放进入截止或是饱和导通的非法工作状态。