1. Solid-State Devices & Circuits 11. CE Amplifiers at High Frequencies
ECE 342
Solid-State Devices & Circuits
11. CE Amplifiers at High Frequencies
Jose E. Schutt-Aine
Electrical & Computer Engineering
University of Illinois

2. BJT Capacitances Base: Diffusion Capacitance: Cde (small signal)
where Qn is minority carrier charge in base
where tF is the forward transit time (time spent crossing base)

3. BJT Capacitances Base-emitter junction capacitance:
Cjeo is Cje at 0 V. Voe is EBJ built in voltage ~ 0.9 V

4. BJT Capacitances In hybrid pi model, Cde+Cje=Cp
Collector-base junction capacitance
Cmo is Cm at 0 V. Voc is CBJ built in voltage ~ 0.9 V
Cp is around a few tens of pF
Cm is around a few pF

5. High-Frequency Hybrid-p Model

6. CE - Three Frequency Bands

7. CE High-Frequency Model

8. CE High-Frequency Model

9. CE High-Frequency Model

10. CE High-Frequency Model

11. Bipolar Miller Effect
The left hand side of the circuit at XX’ knows the existence of Cm only through the current Im  replace Cm with Ceq from base to ground

12. Bipolar Miller Effect

13. Bipolar Miller Effect

14. Bipolar Miller Effect (cont’)

15. Short-Circuit Current Gain

16. Short-Circuit Current Gain (Cont’)
Define hfe as short-circuit current gain

17. Short-Circuit Current Gain (con’t)
Define hfe has a single pole (or STC) response. Unity gain bandwidth is for:
In some cases, if Cm is known, then

18. Short-Circuit Current Gain (con’t)
From which we get

19. BJT-CE – Miller Effect – Exact Analysis

20. BJT-CE – Miller Effect – Exact Analysis

21. BJT-CE – Miller Effect – Exact Analysis
We neglect the terms in s2 since
Miller
If we multiply through by

22. BJT-CE – Miller Effect – Exact Analysis
If Ri = 0