Small Signal ModelPNP Transistor

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 modelbecause this model is applicable even if VBE

changes from 300 mV to 800 mV

Experiments

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 change in VBE creates a change in base current!

Example 4.10

Signal Generated By a microphone

Small Signal Equivalent CircuitVBE=800 mVβ=100IS,Q1=3 x 10-16 A 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

A Simple Cadence Example

Assumption

Assume that1.The DC at Vout is 0.9 V2.gm=1 mS

Gain is approximately equal to –gmRC.Bias current is IC=gmVt

R=(1.8V-0.9V)/26uA=34.6 KohmsGain is -34.6.

DC Bias of the Amplifier

Sweep the Base Voltage to Get the IC=26 uA

Display the Transconductance

Display Transconductance

Verify Transconductance (1)

Verify Transconductance (2)

Transconductance

VB IC

562.5 mV 25.64 uA

563.5 mV 26.64 uA

∆VBE 1 mV

∆IC 1 uA

gm=∆VBE/∆IC 1 mS

Introduce a Small Signal

Calculate Peak to Peak Voltage

Peak to Peak Voltage=67.78 mV

67.78 mV/2=33.9

PNP Transistors

Structures of BJT Transistors

(NPN transistor) (PNP transistor)

Voltage and Current Polarities of NPN and PNP transistors

A “fat” voltage between collector and emitter voltage places a transistor in the active region!

A “skinny” voltage between collector and emitter voltage places a transistor in the active region!

Large-Signal Model of BJT Transistors

(NPN) (PNP)

Equations

Small Signal Model of BJT Transistors

(NPN) (PNP)