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Radio Frequency Amplifiers

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 ?. Frequency Response of Amplifiers.

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Radio Frequency Amplifiers

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  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. • CBC is particularly variable due to wide range of possible VCE 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-p Model • Simplest possible small signal transistor model. • Suitable for low frequency analysis only.

  12. Using the Hybrid-p Model

  13. High Frequency Hybrid-p Model

  14. Resistance, rbb • rbb 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 W.

  15. Resistance, rp • rp 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 vbe and ib.

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

  17. Capacitances • The two capacitances CBC and CBE correspond to the junction capacitances. • Both vary with d.c. bias conditions. • For simplicity, we’ll assume they are constant.

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

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