Small Signal Amplifiers 2n1l6y

Small-Signal Amplifier

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2. Equivalent Circuit Method The transistor is replaced by its ac equivalent circuit model.  Modelling of FET is much simpler than that of BJT, because of two reasons : 

1. 2.



BJT is current operated devices The output affects the input.

For BJT, we have either h-parameter model or hybrid-π model.

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Hybrid Parameters

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Complete h-parameter circuit model of a BJT. 18-11-16 08:42 AM

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Simplified h-parameter model of a BJT.

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Hybrid-π Circuit Model

ri  rb  r ; rb (  100 ) is base spreading resistance. r is incremental resistance of the junction. 18-11-16 08:42 AM

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Vce = 0 means output shorted. Whole current flows through the short. 18-11-16 08:42 AM

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From the input of the circuit :

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Equivalent circuit method of AC Analysis

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Input 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 hparameter 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 Amplifier

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AC Analysis

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FET Parameters Typical 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 FET

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AC Equivalent of FET Amplifier

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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 byed.

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Review  Equivalent

Circuit Method.  Hybrid Parameters.  Hybrid-π  How  FET

Circuit Model.

Small is Small.

Parameters.

 Determining  AC

gm.

Equivalent of FET Amplifier.

 Maximum 18-11-16 08:42 AM

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