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 x 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, Ri. The output port can be modeled by a dependent voltage source, AVi, with output resistance, RO. To complete a simple amplifier circuit, we will include an input source and impedance, VS and RS, and output load, RL. Figure shows the Thevenin equivalent of a simple amplifier circuit.
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.