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In the most basic circuit, op-amps are used as voltage amplifiers, which can be divided into noninverting and inverting amplifiers. In addition, op-amps are also commonly used as voltage followers (also called buffer amplifiers or simply buffers). Op-amps are also used in many other configurations, including differential amplifiers and integrator circuits. Figure 1 shows an example of a noninverting amplifier. Both R1 and R2 are external resistors. In this circuit, negative feedback is used to apply a portion of the output voltage to the inverting input via R1 and R2. The closed-loop gain*1 (GV) of this circuit is expressed only with R1 and R2. The ease of gain setting is one of the advantages of an op-amp.
Figure 2 shows a voltage follower. In the voltage follower, R1 is infinite and R2 is equal to zero. As a result, all of the output voltage is applied to the inverting input. Because V+ and V- are virtually shorted*2, the output voltage is equal to the input voltage.
A voltage follower is commonly used as a buffer since it is useful for impedance conversion because of low output impedance.
Figure 3 shows an inverting amplifier. R1 and R2 are external resistors. As is the case with a noninverting amplifier, an inverting amplifier uses negative feedback. Therefore, the closed-loop gain of the inverting amplifier can be calculated with a simple equation shown in Figure 3.
*1 See the FAQ entry “What are open-loop and closed-loop gains of an op-amp?”
What are open-loop and closed-loop gains of an op-amp?
*2 See the FAQ entry “What is the virtual short of an op-amp?”
What is the virtual short of an op-amp?
The following documents also have related explanations.
Basics of Operational Amplifiers and Comparators
Low-Noise CMOS Operational Amplifier Ideal for Sensor Signal Amplification