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. β = 100
VBE = 0.7V
VA = 80

5. Perform DC analysis to obtain the value of IC
BE loop: 25IB IE – 2.5 = 0
25IB (1+ β)IB = 2.5
IC = βIB = mA
Calculate the small-signal parameters
r = 3.28 k and ro = k

6. Output at emitter terminal
β = 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 x
RTH = 25 k
RS = 0.5 k
r = 3.28 k
RE = 2 k
ro = k 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. 2. At node x, use KCL and get io in terms of ib vb
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 = ib

10. 4. Find vb in terms of ib using supermesh: vb = ibr + io(Rout) x vb
4. Find vb in terms of ib using supermesh:
vb = ibr + io(Rout)
vb = ib (r +101 (1.96)) = ib
5. Calculate Rib Rib = vb / ib  k
6. Calculate Ri  Ri = RTH||Rib = 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
vb = vs
vb = 0.978vs Vb
vo = ib but ib = vb / Rib
vo = (vb / Rib) = ( vs) = vs
201.24
AV = vo / vs =

12. Output Resistance

13. Output Resistance
Assume that an imaginary voltage source VX is connected into the output terminal and there is an imaginary current IX flowing through the circuit
The output resistance,
1. vbe in terms of vx + Vx - + Vx -
vbe = vx
vbe = vx
0.49 k
1.96 k

14. r + RS + Vx - 2. Use nodal analysis - Vx + gmvbe - Vx + Ix = 0
3.77 k
1.96 k
2. Use nodal analysis
- Vx gmvbe Vx Ix = 0
Vx – Vx Vx + Ix = 0
Vx – Vx Vx + Ix = 0
Vx + Ix = 0
Ix = Vx
= Vx Ix
The output resistance, k

15. Output Resistance
The input signal source is short circuited and assume it is an ideal source so RS = 0
The output resistance,
1. vbe in terms of vx + Vx - + Vx -
vbe = - vx
1.96 k

16. + Vx - r 2. Use nodal analysis - Vx + gmvbe - Vx + Ix = 0 3.28 1.96
1.96 k r
2. Use nodal analysis
- Vx gmvbe Vx Ix = 0
Vx – 30.5 Vx Vx + Ix = 0
Vx – 30.5 Vx Vx + Ix = 0
Vx + Ix = 0
Ix = Vx
= Vx Ix
The output resistance, k