Small Signal Model Section 4.4

BJT in the active region Electrons cross the forward biased BE junction and are swept reverse biased BC junction.

Large Signal Model of a BJT Called “large” signal model because this model is applicable even if VBE changes from 300 mV to 800 mV

Experiments

Problem IS,Q1=5 x 10-17 A VBE=800 mV β=100 What is the voltage at VX?

Problem What value of RC will take Q1 out of the active region?

Transconductance If a signal changes the base-emitter voltage by a small amount, how much change is produced in the collector current?

Illustration of Transconductance

But there is something else…. A in VBE creates a change in base current!

Example 4.10 Small Signal Equivalent Circuit VBE=800 mV β=100 Signal Generated By a microphone VBE=800 mV β=100 IS,Q1=3 x 10-16 A Small Signal Equivalent Circuit Question: If a microphone generates a 1 mV signal, how much change is observed in the collector and base current?

A Simple Amplifier Determine the output signal level if the microphone produces a 1 mV signal.

AC Ground The voltage produced by a voltage source is constant. The small signal model is concerned only with changes in quantities. Therefore, a DC voltage source must be replaced with a ground in small signal analysis.

Example

Early Effect A larger reverse bias voltage leads to a larger BC depletion region. The effective base width (WB) is reduced. The slope of the electron profile increases. IC increases as VCE is increased.

Early Effect

James M. Early

Modeling of Early Effect

What Doesn’t Change with Early Effect?

Modification of the Small Signal Model

Summary