Recall Last Lecture

1
Recall Last Lecture

• Introduction to BJT Amplifier
• Small signal or AC equivalent circuit parameters
• Have to calculate the DC collector current by
  performing DC analysis first
• Common Emitter-Emitter Grounded
• Voltage gain
• Current gain

2
Common-Collector (Emitter-Follower) Amplifier
3

• Remember that for Common Collector Amplifier,
• the output is measured at the emitter terminal.
• the gain is a positive value

4
ß 100 VBE 0.7V VA 80
5

• Perform DC analysis to obtain the value of IC
• BE loop 25IB 0.7 2IE 2.5 0
• 25IB 0.7 2(1 ß)IB 2.5
• IC ßIB 0.793 mA
• Calculate the small-signal parameters
• r? 3.28 k? and ro 100.88 k?

6
ß 100 VBE 0.7V VA 80
Small-signal equivalent circuit of the
emitter-follower amplifier
Output at emitter terminal
7

• Redraw the small signal equivalent circuit so
  that all signal grounds connected together.

x
Vb
Vb
8
STEPS

• OUTPUT SIDE
• Get the equivalent resistance at the output side,
  ROUT
• At node x, use KCL and get io in terms of ib
  where io ib ? ib
• Get the vo equation where vo io ROUT
• INPUT SIDE
• Find vb in terms of ib using supermesh
• Calculate Rib input resistance seen from base
  Rib vb / ib
• Calculate Ri
• Get vb in terms of vs.
• Go back to vo equation and replace where necessary

9
1. Get the equivalent resistance at the output
terminal, ROUT ? ROUT ro RE 1.96 k?
2. At node x, use KCL and get io in terms of ib
? io ib?ib ( 1? )ib 101 ib
3. Get the vo equation where vo io ROUT ? vo
Rout ( 1? )ib 197.96 ib
10
x
vb
4. Find vb in terms of ib using supermesh vb
ibr? io(Rout) vb ib (r? 101 (1.96))
201.24 ib
5. Calculate Rib? Rib vb / ib ? 201.24 k?
6. Calculate Ri ? Ri RTHRib 22.24 k?
11
Small-Signal Voltage Gain

• 7. Get vb in terms of vs using voltage divider
• 8. Go back to vo equation and replace where
  necessary

AV vo / vs 0.9621
12
Output Resistance
13
1. vbe in terms of vx
Vx -
Vx -
0.49 k?
1.96 k?
14
r? RS
Vx -
Vx -
3.77 k?
1.96 k?
2. Use nodal analysis
- 0.2653 Vx 26.535 Vx - 0.5102 Vx Ix 0
- 0.2653 Vx 26.535 Vx - 0.5102 Vx Ix 0
- 27.3105 Vx Ix 0
Ix 27.3105 Vx
0.0366 k?
15
Output Resistance
The input signal source is short circuited and
assume it is an ideal source so RS 0
1. vbe in terms of vx
Vx -
Vx -
vbe - vx
1.96 k?
16
Vx -
Vx -
1.96 k?
r?
2. Use nodal analysis
- 0.3049 Vx 30.5 Vx - 0.5102 Vx Ix 0
- 0.3049 Vx 30.5 Vx - 0.5102 Vx Ix 0
- 31.3151 Vx Ix 0
Ix 31.3151 Vx
0.0319 k?