Working principle of the operational amplifier

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Operational amplifiers are abbreviated as operational amplifiers. Because they were used in analog computers in the early days to realize mathematical operations, they were named "operational amplifiers." It is mainly used in analog circuits, such as amplifiers, comparators, and analog operation units, which are devices often used by electronic engineers. An operational amplifier is a circuit unit with a very high amplification factor. In actual circuits, a certain functional module is usually combined with a feedback network. It is an amplifier with a special coupling circuit and feedback. The output signal can be the result of mathematical operations such as addition, subtraction, differentiation, and integration of the input signal. An op-amp is a circuit unit named from the point of view of function, which can be realized by a discrete device or in a semiconductor chip.

With the development of semiconductor technology, most operational amplifiers exist in the form of a single chip. There are many types of operational amplifiers, which are widely used in the electronics industry. To make better use of op-amps, a thorough understanding of the working principles of op-amps is necessary.

The operational amplifier (OperaTIonal Amplifier, referred to as OP, OPA, OPAMP) is a DC-coupled, differential mode (differential mode) input, usually, a single-ended output (DifferenTIal-in, single-ended output) high gain (gain) voltage Amplifier, because it was mainly used in arithmetic circuits such as addition and multiplication at first, so it got its name. An ideal operational amplifier must have the following characteristics: infinite input impedance, equal to zero output impedance, infinite open-loop gain, infinite common-mode rejection ratio part, infinite bandwidth. The most basic operational amplifier is shown in Figure 1-1. An operational amplifier module generally includes a positive input terminal (OP_P), a negative input terminal (OP_N), and an output terminal (OP_O).

The open-loop operational amplifier is shown in Figure 1. When an ideal operational amplifier works in an open loop, the relationship between its output and input voltage is as follows:

Vout = (V+ -V-) * Aog

open-loop operational amplifier


Among them, Aog represents the open-loop differential gain of the operational amplifier (open-loop differential gain, because the open-loop differential gain of the operational amplifier is very high, so even if the differential signal at the input end is small, it will still make the output signal "saturation" (saturation), Resulting in nonlinear distortion. Therefore, operational amplifiers rarely appear in circuit systems as open-loop loops. A few exceptions are the use of operational amplifiers as comparators. The output of the comparator is usually logic level "0". "And "1".

Connect the inverting input terminal and the output terminal of the operational amplifier, and the amplifier circuit is in a negative feedback configuration. At this time, the circuit can usually be simply referred to as a closed-loop amplifier. The closed-loop amplifier enters the endpoint of the amplifier according to the input signal and can be divided into two types: inverting amplifier and non-inverting amplifier.

The inverting closed-loop amplifier is shown in Figure 2. Assuming that this closed-loop amplifier uses an ideal operational amplifier because its open-loop gain is infinite, the two input terminals of the operational amplifier are virtual ground. The relationship between its output and input voltage is as follows:

Vout = -(Rf / Rin) * Vin


inverting closed-loop amplifier


The non-inverting closed-loop amplifier is shown in Figure 3. Assuming that this closed-loop amplifier uses an ideal operational amplifier because its open-loop gain is infinite, the voltage difference between the two input terminals of the operational amplifier is almost zero, and the relationship between its output and input voltage is as follows:
Vout = ((R2 / R1) + 1) * Vin

non-inverting closed-loop amplifier

Connect the positive input end of the operational amplifier to the output end, and the amplifier circuit is in a positive feedback state. Since the positive feedback configuration works in an extremely unstable state, it is mostly used in applications that need to generate an oscillating signal.

The TL074ID is a high-speed JFET input single operational amplifier, don't miss the chance to have a deep learning of it.



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