BJT amplifier & small-signal concept

Lecture 8

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Preview

• Basic characteristics of BJT amplifiers• DC and ac analysis & equivalent circuits• Small-signal hybrid-π equivalent circuits• Small-signal voltage gain, input resistance &

output resistance• Common-emitter amplifiers• dc & ac load lines

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Linear analog amplifier

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Notation

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Basic characteristics of an amplifier

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Basic characteristics of an amplifier (cont’d)

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Basic BJT amplifier

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Graphical analysis of BJT amplifiers

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Graphical analysis of BJT amplifiers (Cont’d)

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Analysis of BJT amplifiers

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dc analysis and equivalent circuit

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ac analysis and equivalent circuit

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Small-signal hybrid-π equivalent circuit

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Small-signal hybrid-π equivalent circuit (Cont’d)

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Small-signal voltage gain

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Input and output resistances

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Example: dc circuit analysis

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Example: dc circuit analysis (Cont’d)

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Example: dc circuit analysis (Cont’d)

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Small-signal hybrid-π equivalent circuit with Early effect

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Example: determine BJT amplifier parameters

• Determine the small-signal voltage gain, input resistance, and output resistance of the BJT amplifier circuit in previous example with the early effect. Assume that the early voltage is 50V.

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Common-emitter amplifiers (with voltage-divider biasing & coupling capacitor)

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Common-emitter amplifiers (with voltage-divider biasing & coupling capacitor)- Cont’d

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Common-emitter amplifiers (with voltage-divider biasing & coupling capacitor & emitter resistor)

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dc & ac load lines

• Dc load line is used to find Q-point• Ac load line is used to determine graphically

the operation of a BJT amplifier• Dc and ac load lines are essentially different

since capacitors appear as an open circuit for a de operation but a short circuit for an ac operation

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dc load line

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ac load line

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Maximum output symmetrical swing

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Saturation

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Cutoff

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Example: Calculate BJT circuit parameters

• Determine the small-signal voltage gain, input resistance, and output resistance of the circuit shown in attached Figure. Assume the transistor parameters are: β=100, VBE(on) =0.7V, and VA=100V

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END

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• TQ8.1• In Figure 8.1, let β=150,VA=200V, VCC=7.5V,

VBE(on)=0.7V, RC=15KΩ, RB=100kΩ, and VBB=0.92V.

• (a) Determine the small-signal hybrid- parameters rπ, gm, and r0

• (b) Find the small-signal voltage gain Av

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Fig. 8.1