Small Signal Model PNP 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 model because this model is applicable even if V BE 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 V BE creates a change in base current!

Example 4.10 Signal Generated By a microphone Small Signal Equivalent Circuit V BE =800 mV β=100 I S,Q1 =3 x 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. I C 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 that 1.The DC at Vout is 0.9 V 2.g m =1 mS Gain is approximately equal to –g m R C. Bias current is I C =g m V t R=(1.8V-0.9V)/26uA=34.6 Kohms Gain is

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 VBIC mV25.64 uA mV26.64 uA ∆VBE1 mV ∆IC1 uA gm=∆VBE/∆IC1 mS

Introduce a Small Signal

Calculate Peak to Peak Voltage

Peak to Peak Voltage=67.78 mV 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)