Cascode amplifier
A common-emitter stage has a hidden capacitor from collector to base (C_μ, drawn here as a real 100 pF). Its gain multiplies that capacitor (the Miller effect), killing the high frequencies. Stacking a common-base transistor on top holds Q1’s collector still, so C_μ is no longer multiplied: same gain, far more bandwidth.
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How it works
The base divider (Thevenin 1.96 V behind 13 kΩ, β ≈ 180) sets Q1 at I_C ≈ 1.2 mA, g_m = 46.5 mS; voltage gain ≈ g_m·R_C = 219, and with the 10 kΩ source the overall gain is 50.
Common-emitter: C_μ looks like (1 + A)·C_μ ≈ 22 nF at the base. Cascode: Q1’s gain is only about 1, so it looks like 2·C_μ.
Try it on the bench
- Raise V_in’s frequency to 50 kHz on the source: the cascode still amplifies; the plain CE has fallen away.
Parts on the bench
- Bench power supply
- AC source
- 7 × Resistor
- Film capacitor
- 2 × NPN transistor
- 2 × Electrolytic capacitor
- Ceramic capacitor
In the course
Electronics Engineering, module “Transistor amplifiers”.