CE CB (cascade) amplifier

CE CB Cascade Amplifier , Cascade Connection.

1. CE-CB (Cascade) Amplifier

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 A 12 = – h fb / (1 + h ob R L ) Neglecting h ob we get A 12 ~ – h ib 2. Input resistance R i2 = h ib +h rb A 12 R L Neglecting h rb we get, R i2 = h ib 3. Voltage gain A v2 = A 12 x ( R L2 / R i2 ) = – h fb x (R L2 / h ib ) But h fb ~ – h fe /(1+h fe ) and h ib = h ie / (1+ h fe )   So, Av = h fe / (1+h fe ) X ( R L (1+h fe ) / h ie ) = h fe R L / h ie

7. Step 2 : Analysis of stage 1: 1. Load resistance of stage 1, R l1 = R i2 = h ie /(1+h fe ) 2. Current gain A 11 = – h fe 3. Input resistance R il = h ie 4. Voltage gain, A v1 =A 11 X (R L1 /R il ) = -h fe X (h ie / (1+h fe ) h ie ) So, A v1 = -h fe /(1+h fe )

8. Step 3 : Overall voltage gain : A v = A v1 X A v2 = -[ h fe / (1+h fe ) ] X [ (h fe R L ) / h ie ] Since h fe ~ (1+h fe ) So, A v ~ -(h fe R L ) / h ie   Step 4 : Overall input resistance : R i = R il = h ie and R’ i = h ie || R i3

9. Step 5 : Overall output resistance : R o = R o2 = R c   Step 6 : Overall current gain : The overall current gain, A I = A I1 X A I2 = - h fe X – h fb = -h fe X (h fe / (1+h fe )) ~ -h fe  

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

11. Thank you - Rakesh Makhija.