BJTs: AC Analysis of Large Signals

The “large” in the title of this chapter refers to the size of the input signal. Size is relative in this case. We apply this type of analysis if the input signal is large enough to move the transitor between regions of operation. This is more common when using the transistor as a switch and/or for logical operations. Transistors in many amplifers remain in the active region during operation but there are some classes of amplifiers that don’t. Analysis for large signals focus on relating the input signal to the output signal through a transfer characteristic much like we developed for diode circuits in an earlier chapter.

Voltage Transfer Characteristic

The voltage transfer characteristic is the relationship between the input signal and output signal. We can represent it as a plot or a piecewise function. Finding this relationship is the first step towards AC analysis. The characteristic is also useful in defining what constitutes “large” signal analysis and what is “small” signal analysis.

1. Assume active region. Using this assumption find:

   1. The output as a function of the input

   2. The collector-emitter voltage as a function of the input

   3. The value of the input that moves the transistor from the active region to cut-off

   4. The value of the input that moves the transistor from the active region to saturation

2. Assume saturation region. Using this assumption find:

   1. The output as a function of the input

3. Assume cut-off region. Using this assumption find:

   1. The output as a function of the input

If you are building an amplifier, you know how much Vin can move while staying in the active region. If you are building a switch/logic circuit, you know how much Vin needs to swing to turn the transitor on and off.