1. Radio Frequency Amplifiers In this section of the course: Why do common emitter amplifiers often have a disappointingly low upper cut-off frequency ? Where are the hidden capacitances which cause the effect ? How can we design high frequency amplifiers ?

2. Frequency Response of Amplifiers

3. Voltage gain

4. Input impedance

5. Lower Cut-Off Frequency

6. NB. At 19 kHz:

7. Frequency Response

8. High Frequency Transistor Model

9. Values of Internal Capacitances Both capacitances vary depending on the bias conditions of the junctions. C BC is particularly variable due to wide range of possible V CE values. For simplicity, we will assume both capacitances are fixed – usually at a few picofarads.

10. Small Signal Transistor Models A popular technique for analysing transistor amplifiers involves small signal transistor models. A small signal (a.c.) equivalent circuit of the transistor is placed in the circuit. Circuit analysis now only involves primitive circuit elements.

11. Simplified Hybrid-  Model  Simplest possible small signal transistor model.  Suitable for low frequency analysis only.

12. Using the Hybrid-  Model

14. High Frequency Hybrid-  Model

15. Resistance, r bb r bb is the base spreading resistance. It is a real physical resistance. It represents the imperfect electrical contact between the base material and the terminal. Typical value < 100 .

16. Resistance, r  r  is the same small signal base-emitter resistance used in the simplified model. It is not a real resistance, but the small signal relationship between v be and i b.

17. Resistance, r o In the simplified model, the collector appears to be an ideal current source. Of course, it isn’t ideal; a resistance, r o, appears in parallel. Usually, r o is much larger than the collector resistor and can be ignored.

18. Capacitances The two capacitances C BC and C BE correspond to the junction capacitances. Both vary with d.c. bias conditions. For simplicity, we’ll assume they are constant.

19. Summary Frequency response of amplifiers depends chiefly on the capacitances within the circuit. This includes internal capacitances of the transistor. Simplified hybrid-  transistor model can help analyse a circuit. High frequency hybrid-  model is a more accurate approximation. Next time : applying the high frequency hybrid-  model.