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Thought of the DayThought of the Day

When you meet other people,When you meet other people,

look them in the eye,look them in the eye,

state your name clearly,state your name clearly,

and shake hands firmly.and shake hands firmly.

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Small-Signal Amplifier Small-Signal Amplifier D.C. KulshreshthaD.C. Kulshreshtha

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2. Equivalent Circuit Method2. Equivalent Circuit Method The transistor is replaced by its ac The transistor is replaced by its ac

equivalent circuit model.equivalent circuit model. Modelling of FET is much simpler Modelling of FET is much simpler

than that of BJT, because of two than that of BJT, because of two reasons :reasons :

1.1. BJT is current operated devicesBJT is current operated devices

2.2. The output affects the input.The output affects the input. For BJT, we have either For BJT, we have either hh-parameter -parameter

model or hybrid-model or hybrid-ππ model. model.

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Hybrid ParametersHybrid Parameters

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Complete h-parameter circuit model of a BJT.

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Simplified h-parameter model of a BJT.

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Hybrid-Hybrid-ππ Circuit ModelCircuit Model

; ( 100 ) is base spreading resistance.

is incremental resistance of the junction.i b br r r r

r

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Vce = 0 means output shorted. Whole current flows through the short.

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From the input of the circuit :

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Equivalent circuit method of Equivalent circuit method of AC Analysis AC Analysis

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Input ResistanceInput Resistance

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Output resistance

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How Small is Small

Peak-to-peak current should be less than 10 % of dc current.

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Example : Analyze the CE amplifier circuit using h-parameter model of BJT.

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Note : Since 1/hoe can be ignored, the h – parameter model is further simplified.

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Solution

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FET AmplifierFET Amplifier

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AC AnalysisAC Analysis

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FET ParametersTypical Values

500 kΩ

0.5 mS to 4 mS

80 to 200

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Determining gm

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Example 7.8 Calculate gm for a JFET with IDSS = 8 mA and Vp = 4 V, for dc bias point of (a) -0.5 V, and (b) -1.5 V

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AC Circuit Model of an FETAC Circuit Model of an FET

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AC Equivalent of FET AC Equivalent of FET AmplifierAmplifier

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Negative sign means that the output is 180° out of phase.

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Maximum Voltage Gain :

As Rac

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This is much smaller than RS , hence it is completely bypassed.

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ReviewReviewEquivalent Circuit Method.Equivalent Circuit Method.Hybrid Parameters.Hybrid Parameters.Hybrid-Hybrid-ππ Circuit Model.Circuit Model.

How Small is Small.

FET Parameters.

Determining gm.

AC Equivalent of FET Amplifier.AC Equivalent of FET Amplifier.

Maximum Voltage Gain