An operational amplifier generally has 5 pins, 2 input terminals, and 1 output terminal. The “+” is the non-inverting input terminal, the “-” is the inverting input terminal,
and the two terminals are used to power the device.
1. Basic functional description of operational amplifier
The operational amplifier has many functions, the simplest function is to perform voltage comparison. When the voltage value of the positive-phase terminal is higher than the voltage of the inverting terminal,
the output terminal outputs a high level; when the voltage value of the positive-phase terminal is lower than the voltage of the inverting terminal, the output terminal outputs a low level.
The output high and low-level values of the output terminal are determined by its power supply. When the power supply voltage is 5V, the maximum voltage of the output terminal is 5V;
when the common voltage is 3.3V, the maximum voltage of the output terminal is 3.3V. Likewise, the minimum voltage at the output is determined by the negative voltage of the supply voltage.
In addition to being used as comparators, operational amplifiers can also be used as analog signal processing, signal generators, voltage followers, voltage regulators, and other functions.
2. Knowledge points of ideal operational amplifier
An operational amplifier can amplify the voltage at the input terminal tens of thousands of times. The specific theoretical amplification factor is one hundred thousand times to one million times. For example: if the positive terminal is 0.6V and the negative terminal is 0.4V. Then it is (0.6 – 0.4) x 100000 = 20000V. In fact, this is impossible. The output voltage of the op-amp is limited. Generally, the maximum output voltage of the op-amp is 10~14V. Because of its high gain, it is suitable for analog operation circuits such as addition, subtraction, differentiation, and integration.
Because the voltage amplification factor of an ideal operational amplifier is very large, and the operational amplifier works in the linear region, it is a linear amplifier circuit, and the output voltage does not exceed the linear range. Therefore, the potentials of the non-inverting input terminal and the inverting input terminal of the operational amplifier are very close to the same. If the difference is below 1mV, the two input terminals are approximately short-circuited. This characteristic is called a virtual short circuit. Obviously, this is not a real short circuit, but a reasonable approximation within the allowable error range when analyzing the circuit.
Since the input resistance of an op-amp is generally above a few hundred kiloohms, the current flowing into the non-inverting input terminal and the inverting input terminal of the op-amp is very small, several orders of magnitude smaller than the current in the external circuit. The current flowing into the op-amp can often be Ignored which is equivalent to an open circuit at the input end of the op-amp. This feature is called a virtual break. Obviously, the input terminal of the op-amp cannot be truly an open circuit.
Virtual short means that the voltages at the positive and negative input terminals of the op-amp are equal; virtual open means that the current flowing into the positive and negative input terminals is 0. As long as you master this point and use Ohm’s law, you can easily analyze common op-amp amplifier circuits such as in-phase proportional amplification circuits and inverse proportional amplification circuits.