An operational amplifier or op amp is a circuit that takes an input voltage and amplifies it. The symbol used to represent an op amp in a circuit diagram is shown in Fig. 9-1.

An op amp is defined by two simple equations. The first thing to note is that the voltage across the input terminals is zero. Hence Va = Vb

The second relation that is essential for analyzing op amp circuits is that the currents drawn at a and b in Fig. 9-1 are zero Ia = Ib = 0

Despite this, we will see that the op amp will result in voltage gains at the output terminal c. How does this work? Two voltages are input to terminals a and b.

Their difference is then amplified and output at c, which is taken with referenc to ground. Although we won’t worry about the internal construction of an op amp, note that it consists of a set of resistors and dependent voltage source. The internal voltage source is related to the input voltages by A(V+ − V−)

The constant A is known as the open-loop voltage gain. To see how op amp circuits work, it’s best to examine some popular example circuits.

When analyzing op amp circuits, remember to take the input voltage across the op amp terminals to be zero and that the op amp draws zero current. The analysis is then reduced to applying KVL and KCL to the circuit elements connected to the op amp.

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