A new interpretation of the principle of integral circuits: the transformation of amplifiers and capacitors

Jacktang

A new interpretation of the principle of integral circuits: the transformation of amplifiers and capacitors [Copy link]

Replacing the feedback resistor in the inverting amplifier with a capacitor will result in an integrating amplifier circuit as shown in Figure 1. Resistors seem to be more tangible, and the circuit output state can be seen at a glance. However, capacitors are more "virtual" objects due to the uncertainty of charging and discharging, and their circuit output state is a little more difficult to understand. Figure 1 Structure and signal waveform of the integrating circuit To understand its output state, you must first understand the nature of the capacitor. The basic function of a capacitor is to charge and discharge, and it is an energy storage element. It is sensitive to changing voltages (strongly reacts), insensitive to direct current (even indifferent), and has the characteristics of passing alternating current and blocking direct current. For those who think that everything in the world presents resistance characteristics, capacitors can also be regarded as changing resistors, which can solve the mystery of the output of the integral circuit. According to the law of conservation of energy, energy cannot be generated or disappeared without reason, and the theorem that the voltage across the capacitor cannot change suddenly is derived from it. At the moment of charging, the charge has not yet accumulated between the two plates of the capacitor, so the original state of zero voltage at both ends can be maintained, but the charging current at this moment is the maximum, which can be equivalent to a very small resistor or even a wire. If the capacitor is short-circuited at the moment of charging, it is also possible. For example, in the main circuit of the inverter, there should be current-limited charging measures for the loop capacitor. This is the reason; during the charging period of the capacitor, as time goes by, the charging voltage gradually increases, while the charging current gradually decreases. It can also be considered that the equivalent resistance of the capacitor changes from the minimum to the maximum at this time; after the capacitor is fully charged, the voltage across the two ends is the highest, but the charging current is basically zero. At this time, the capacitor is equivalent to the maximum value resistance. For direct current, it can even be equivalent to an open circuit, an infinite resistance. To summarize, during the charging process of the capacitor, there are three states: equivalent to the minimum resistance or wire, equivalent to the resistance changing from small to large, and equivalent to the maximum resistance or open circuit. It is this changing characteristic of the capacitor that can make the integrating amplifier circuit transform into the three identities shown in Figure 2. Figure 2 The "three transformations" of the integrating circuit during operation See Figure 2. 1. Voltage follower. At the t0 (positive jump) moment of the input signal, the capacitor charging current is the largest and the equivalent resistance is the smallest (or regarded as a wire). The circuit immediately transforms into a voltage follower circuit. From the virtual ground characteristics of the circuit, it can be seen that the output is still 0V. 2. Inverting amplifier. During the flat-top period after the input signal t0, the capacitor is in a relatively gentle charging process, and its equivalent RP goes through three stages: less than R, equal to R, and greater than R. Therefore, in the amplification process, under the action of the amplification characteristics, it actually goes through three small processes: inverting attenuation, inverting, and inverting amplification. Whether it is attenuation, inverting or inverting amplification, it shows that at this stage, the integration circuit actually plays the role of a linear amplifier. 3. In the second half of the input signal flat period, the charging process of the capacitor has ended, the charging current is zero, the capacitor is equivalent to an open circuit, the integration amplifier is amplified from a closed loop to an open loop comparison state, and the circuit is transformed into a voltage comparator. At this time, the output value is a negative power supply value. It is said that people can change their faces, but in fact, circuits can also change. Under the control of the capacitor, the amplifier instantly changes into three identities. If you can see through the three identities of the integrating amplifier, the “true identity” of the integrating amplifier will be exposed. The amplifier is actually playing in the circle of “amplification is inseparable from comparison, and comparison is inseparable from amplification”.