The term operational amplifier, abbreviated op amp, was coined in the 1940s to refer to a special kind of amplifier that, by proper selection of external components, can be configured to perform a variety of mathematical operations. Early op amps were made from vacuum tubes consuming lots of space and energy. Later op amps were made smaller by implementing them with discrete transistors. Today, op amps are monolithic integrated circuits, highly efficient and cost effective.

Before jumping into op amps, lets take a minute to review some amplifier fundamentals. An amplifier has an input port and an output port. In a linear amplifier, output signal = A × input signal, where A is the amplification factor or gain.

Depending on the nature of input and output signals, we can have four types of amplifier gain:

• Voltage (voltage out/voltage in)
• Current (current out/current in)
• Transresistance (voltage out/current in)
• Transconductance (current out/voltage in)

Since most op amps are voltage amplifiers, we will limit our discussion to voltage amplifiers.

Thevenin’s theorem can be used to derive a model of an amplifier, reducing it to the appropriate voltage sources and series resistances. The input port plays a passive role, producing no voltage of its own, and its Thevenin equivalent is a resistive element, R_i. The output port can be modeled by a dependent voltage source, AV_i, with output resistance, R_o. To complete a simple amplifier circuit, we will include an input source and impedance, V_s and R_s, and output load, R_L. Figure 1-1 shows the Thevenin equivalent of a simple amplifier circuit.

Figure 1-1 Thevenin Model of Amplifier with Source Load

It can be seen that we have voltage divider circuits at both the input port and the output port of the amplifier. This requires us to re-calculate whenever a different source and/or load is used and complicates circuit calculations.