BJT amplifier smallsignal concept Lecture 8 1 Preview
BJT amplifier & small-signal concept Lecture 8 1
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 • • 2
Linear analog amplifier 3
Notation 4
Basic characteristics of an amplifier 5
Basic characteristics of an amplifier (cont’d) 6
Basic BJT amplifier 7
Graphical analysis of BJT amplifiers 8
Graphical analysis of BJT amplifiers (Cont’d) 9
Analysis of BJT amplifiers 10
dc analysis and equivalent circuit 11
ac analysis and equivalent circuit 12
Small-signal hybrid-π equivalent circuit 13
Small-signal hybrid-π equivalent circuit (Cont’d) 14
Small-signal voltage gain 15
Input and output resistances 16
Example: dc circuit analysis 17
Example: dc circuit analysis (Cont’d) 18
Example: dc circuit analysis (Cont’d) - 19
Small-signal hybrid-π equivalent circuit with Early effect 20
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 50 V. 21
Common-emitter amplifiers (with voltagedivider biasing & coupling capacitor) 22
Common-emitter amplifiers (with voltagedivider biasing & coupling capacitor)- Cont’d 23
Common-emitter amplifiers (with voltage-divider biasing & coupling capacitor & emitter resistor) 24
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 25
dc load line 26
ac load line 27
Maximum output symmetrical swing 28
Saturation 29
Cutoff 30
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. 7 V, and VA=100 V 31
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• TQ 8. 1 • In Figure 8. 1, let β=150, VA=200 V, VCC=7. 5 V, VBE(on)=0. 7 V, RC=15 KΩ, RB=100 kΩ, and VBB=0. 92 V. • (a) Determine the small-signal hybridparameters rπ, gm, and r 0 • (b) Find the small-signal voltage gain Av Fig. 8. 1 35
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