An operational amplifier (op-amp) is an integrated circuit (IC) that amplifies the difference in voltage between two inputs.
It is so named because it was developed for perform arithmetic operations. Amplifiers, buffers, comparators, filters, etc. can be implemented with simple external circuits.
An op-amp has five terminals: positive power supply, negative power supply (GND), noninverting input, inverting input, and output. Generally, these terminals are named as shown below. (Positive and GND terminals may be omitted from the symbol of single-supply op-amps.)
An op-amp amplifies the difference in voltage between the noninverting (IN(+)) and inverting (IN(-)) inputs. Its output voltage is given by Equation 1, which indicates that the output is in the same phase as VIN(+) and in opposite phase to VIN(-).
VOUT = A * ( VIN(+) – VIN(-) ) (1)
In the basic form of usage, an op-amp acts as a voltage amplifier or a comparator. It can also be configured as a filter, phase shifter, buffer (voltage follower), etc. Nowadays, op-amps are commonly used to amplify weak analog signals from sensors in a wide range of IoT devices and home appliances.
Op-amps are generally used with negative feedback to reduce product variations in gain and expand the bandwidth. Typical applications of op-amps include noninverting amplifiers, inverting amplifiers, and voltage followers, which are configured as shown below:
*: Closed-loop gain
There are several ways to categorize op-amps. For example, they are categorized based on: 1) manufacturing process (bipolar and CMOS), 2) power supply type (single-supply and dual-supply), 3) input circuit (differential P-channel MOSFET pair, differential N-channel MOSFET pair, rail-to-rail input, etc.), and 4) electrical characteristics (low input offset, low current consumption, low noise, etc.). Op-amps may also be categorized based on their applications.
Basic Characteristics of op-amp：
High Gain: The operational amplifier has a very high voltage gain, which enables the input signal to be effectively amplified. The gain of an ideal operational amplifier is infinite, but the gain of an actual operational amplifier is finite but still very high.
High Input Impedance: The input impedance of the operational amplifier is very high, which means that it does not have a significant loading effect on the input signal, thus ensuring the accuracy of the input signal.
Low Output Impedance: The output impedance of the operational amplifier is very low, which means that it can effectively drive the load without significantly affecting the amplitude of the signal.
Differential Input: The operational amplifier has two input terminals - the inverting input terminal (-) and the non-inverting input terminal (+). It amplifies the voltage difference between these two input signals.
Negative Feedback: In order to stabilize the gain and improve linearity, the operational amplifier is often used in conjunction with a negative feedback network. Negative feedback is the return of part of the output signal to the input terminal to adjust the gain and performance.
Application of op-amp：
Signal amplification: op-amp are used to amplify weak signals to make them suitable for subsequent processing. They are often used in audio amplification, sensor signal processing, etc.
Filters: op-amp can be configured as different types of filters, such as low-pass, high-pass, band-pass, and band-stop filters, for frequency-selective signal processing.
Comparator: An operational amplifier can be configured as a comparator to compare two voltage signals and output a high or low level, determining the output state based on the relative size of the input signal.
Integral and differential circuits: Integral and differential operations used to process signals are widely used in control systems and signal processing.
Analog computing: In some analog computing circuits, op-amp are used to perform operations such as addition, subtraction, and multiplication, which are applied to analog computing and signal processing.
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