1. Common-Base vs. Common-Emitter
Identical quiescent conditions
Identical voltage gain (except C-E inverts)
Identical output resistance
Common-Base input impedance is very low
Common-Emitter suffers Miller effect

2. Cascode Configuration
Common-Emitter Amp
High Rin, Low Gain
Common-Base Amp
Low Rin, High Gain

3. Cascode Quiescent Conditions
VB1
VB2
VB1-0.5
VB2-0.5
(VB1-0.5+VS)/RE
Note, IC1 = IC2

4. Cascode (I) Common-Base Stage
From common-emitter input stage

5. Cascode (II) Common-Emitter Stage
To common-base stage, rin = re

6. Upper Cut-Off Frequency
Cascode configuration
Common-emitter amplifier

7. Practical Cascode Configuration
To keep Q1 and Q2 out of saturation:
To ensure this, biasing networks for Q1 and Q2 are often combined.

8. Analysis Example Quiescent conditions
Assume base currents are negligible.

9. -5.5 V
+4.5 V
+8.6 V
+5 V
IC = 0.8 mA
-5 V

10. Voltage Gain :
+8.6 V
+5 V
+4.5 V
-5 V
-5.5 V
IC = 0.8 mA

11. Cut-off frequencies Lower: Upper: -5 V +8.6 V +5 V +4.5 V -5.5 V
IC = 0.8 mA

12. Measured Response

13. Summary – RF Design
Upper cut-off frequency of a common-emitter amplifier is set mostly by CBC due to the Miller effect.
Increase bandwidth by:
Reducing gain
Reducing RS
Reducing CBC (different transistor)
Suppressing Miller effect – Cascode configuration