2.4. Basic op-amp applications

In the most basic form, op-amps are used as noninverting amplifiers (Figure 2-10) and inverting amplifiers (Figure 2-11). Both noninverting and inverting amplifiers have negative feedback (with the output connected to V_IN(-)) as described in the previous section.
The closed-loop gain (A_CL) is shown in the following figures. The gain can be calculated easily by using the concept of a virtual short-circuit (also known as a virtual short, virtual ground or imaginary short) described in the next section.
The noninverting amplifiers have a very high input impedance since their input is directly connected to an op-amp. In contrast, the input impedance of the inverting amplifier is lower than that of the noninverting amplifier because V_IN(-) and V_IN(+) have the same potential as they are virtually short-circuited and because R₁ acts as input impedance.

Figure 2-12 shows a voltage follower. The voltage follower can be regarded as a noninverting amplifier with an R₁ of infinite resistance and an R₂ of zero. Since the voltage follower has a low gain (unity gain, A_V=1), it has a wide bandwidth. Therefore, care should be taken since it is susceptible to the effect of a second pole as discussed in Section 2.3, “Oscillation.” Most op-amps can be used as unity gain amplifiers since they have a second pole at a frequency sufficiently higher than the unity gain cross frequency (f_T). However, they might go into oscillation because of wire or load capacitance. If the datasheet for a given op-amp states that it can be used at a unity gain, it can be used as a voltage follower. Contact Toshiba’s sales representative if you want to use any other op-amp as a voltage follower.

In addition, op-amps have various applications, including a differential amplifier (subtraction circuit) as well as an adder and integrator circuits.