Small Signal Model PNP Transistor Section ,4.6

Schedule 92/11TuesdayPhysics of a BJT L2/11TuesdayMeasure Beta of a transistor 102/13ThursdayPNP /18TuesdayBJT in saturation mode4.5 L2/18Tuesday BJT in saturation/BJT implementation of an NAND gate 122/20ThursdaySmall Signal model [homework: small eq. circuit, (PNP)] 4.4,4.6

Overview

Review

Small Signal Model Section 4.4

Schematic of an Audio Amplifier Microphone produces a small signal. How does the amplifier circuit respond to a small change in the input signal? How is the analysis performed? Small signal model

Small Signal Analysis (For a Circuit You have not Seen Before) 1.Replace each ideal DC voltage source with a small signal ground. 2.Replace each ideal DC current source with an open circuit. 3.Replace each transistor by its small signal model 4.Analyze the small signal equivalent circuit.

Small Signal Analysis (For a Circuit You have not Seen Before) 1.Analyze the Circuit by Inspection

Voltage Source DC Voltage Source in Small Signal Analysis R S should be 0 for a good battery!

Current Source R S should be infinity for a good battery!

Small Signal Model (NPN)(PNP) Statements that are always true for both NPN and PNP. 1.r π is between B and E. 2.the direction of the dependent current source always points from the collector to emitter. 3. r o is always between B and C.

Question Replace Q1 and Q2 by their small equivalent circuit.

Answer

Question

Answer

Derivation of the Small Signal Model

Change in the Collector Current Due to a Small Change in Base-Emitter Voltage If a signal changes the base-emitter voltage by a small amount, how much change is produced in the collector current ?

Derivation of Transconductance If a signal changes the base-emitter voltage by a small amount, how much change is produced in the collector current ? Small signal model of Q1

But there is something else…. A change in V BE creates a change in base current! Small signal model

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 Small Signal Model

Summary

Output Resistance Due to 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

Slides to Cover During the Lab

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