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