CE CB Cascade Amplifier , Cascade Connection.

2. The cascade configuration
consists of a common emitter
configuration and a common
base configuration.
The CB stage provides a good
high frequency operation.

3. A CE-CB cascade
connection is as shown in
the fig. :

4. Features of a Cascade
Amplifier :
 The input resistance and current gain of the
cascade configuration are equal to the
corresponding value of a single stage CE
amplifier.
 The output resistance is equal to that of a CB
configuration.
 The millers capacitor shunting the CE input
stage is very small.

5. Applications of Cascade
Amplifier :
• In the tuned RF amplifiers in T.V.
circuits.
• It is also used as wideband amplifier.
• The isolation provided between input
and output by the cascade amplifiers
is very high.

6. Step 1 : Analysis of stage 2 :
1. Current gain A12 = – hfb / (1 + hobRL )
Neglecting hob we get A12 ~ – hib
2. Input resistance Ri2 = hib+hrbA12RL
Neglecting hrb we get, Ri2 = hib
3. Voltage gain Av2 = A12 x ( RL2 / Ri2 )
= – hfb x (RL2 / hib )
But hfb ~ – hfe /(1+hfe) and hib = hie / (1+ hfe)
So,
Av = hfe / (1+hfe) X (RL(1+hfe) / hie)
= hfeRL / hie

7. Step 2 : Analysis of stage 1:
1.Load resistance of stage 1, Rl1 = Ri2= hie/(1+hfe)
2.Current gain A11 = – hfe
3.Input resistance Ril = hie
4.Voltage gain, Av1=A11 X (RL1/Ril)
= -hfe X (hie / (1+hfe) hie)
So,
Av1 = -hfe/(1+hfe)

8. Step 3 : Overall voltage gain :
Av = Av1 X Av2
= -[ hfe / (1+hfe) ] X [ (hfeRL ) / hie ]
Since hfe ~ (1+hfe)
So,
Av ~ -(hfeRL) / hie
Step 4 : Overall input resistance :
Ri = Ril = hie
and R’i = hie || Ri3

9. Step 5 : Overall output resistance :
Ro = Ro2 = Rc
Step 6 : Overall current gain :
The overall current gain,
AI = AI1 X AI2
= - hfe X – hfb
= -hfe X (hfe / (1+hfe))
~ -hfe

10. Conclusions :
The cascade configuration has :
 Low input resistance.
 Moderate to high voltage and current
gains.
 High output resistance.

11. Thank you
- Rakesh Makhija.