1. EMT 112 / 4 ANALOGUE ELECTRONICS
Lecture II
Multistage Amplifiers
[Cascode Connections]
March 18, 2008 (Week XIII) – 1400 DKQ 1

2. CASCADED BJT AMPLIFIER
Cascode connection
A cascode connection has one transistor on top of (in series with) another
The i/p applied to a C-E amp. (Q1) whose output is used to drive a C-B amp. (Q2)

3. CASCADED BJT AMPLIFIER
Cascode connection
The o/p signal of Q1 becomes the i/p signal of Q2

4. CASCADED BJT AMPLIFIER
Cascode connection
The advantage: provides a high i/p impedance with low voltage gain to ensure the i/p Miller capacitance is at a min. with the C-B stage providing good high freq. operation

5. CASCADED BJT AMPLIFIER
Cascode connection
DC analysis
May be performed using the following figure;

6. CASCADED BJT AMPLIFIER
Cascode connection
The equations are (assuming VBE = 0.7 V for both BJT’s);

7. CASCADED BJT AMPLIFIER
Cascode connection
And;
The above equations may solved for the two unknown currents namely I1 and IB1.

8. CASCADED BJT AMPLIFIER
Cascode connection
AC analysis
The equivalent circuit under AC condition

9. CASCADED BJT AMPLIFIER
Cascode connection
The ac equivalent circuit using hybrid- model for BJT

10. CASCADED BJT AMPLIFIER
Cascode connection

11. CASCADED BJT AMPLIFIER
Cascode connection
At node E2;
Or;

12. CASCADED BJT AMPLIFIER
Cascode connection
Substituting in (1);

13. CASCADED BJT AMPLIFIER
Cascode connection
The small-signal voltage gain;
When 2 >> 1

14. CASCADED BJT AMPLIFIER
Cascode connection
Compute the approximate small-signal voltage gain
Example 4

15. CASCADED BJT AMPLIFIER
Example 4 – Solution
DC analysis
The circuit under DC condition

16. CASCADED BJT AMPLIFIER
Example 4 – Solution (cont’d)

17. CASCADED BJT AMPLIFIER
Example 4 – Solution (cont’d)
Substituting values;

18. CASCADED BJT AMPLIFIER
Example 4 – Solution (cont’d)
Or;

19. CASCADED BJT AMPLIFIER
Example 4 – Solution (cont’d)
Substituting values;
Therefore;

20. CASCADED BJT AMPLIFIER
Example 4 – Solution (cont’d)
Substituting for I1 in (1)

21. CASCADED BJT AMPLIFIER
Example 4 – Solution (cont’d)

22. CASCADED BJT AMPLIFIER
Example 4 – Solution (cont’d)
AC analysis
Small-signal equivalent circuit using hybrid- model

23. CASCADED BJT AMPLIFIER
Example 4 – Solution (cont’d)
At node E2;

24. CASCADED BJT AMPLIFIER
Example 4 – Solution (cont’d)
Hence;
Substituting for v2 in (1);
Or;

25. CASCADED BJT AMPLIFIER
Example 4 – Solution (cont’d)
Or;
The voltage gain;
When 2   1;

26. CASCADED BJT AMPLIFIER
Example 4 – Solution (cont’d)
Substituting values;