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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 IB 0 VB 0 V VE 0.5 V IC I E 0.5 15 RE 0.8 mA VC 15 I C RC 8.6 V Voltage gain IC g m VT 0 .8 25 32 mS vout g m RC || RL vin 32 4.4 140 (43 dB) Input impedance 1 re 31 gm 350 rin 100 || 100 || re 100 || 100 || 10 8 k Lower Cut-Off Frequency vin i 1 RS rin j 2fC Cut - off when : 1 RS rin 2fC 1 f 19 kHz 2 RS rin C NB. At 19 kHz: 1 XC2 2fC2 1 2 19000 100 10 9 84 1 X C3 2fC3 1 2 19000 10 10 6 0.8 Frequency Response High Frequency Transistor Model 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. 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. Simplified Hybrid- Model Simplest possible small signal transistor model. Suitable for low frequency analysis only. r re gm IC gm VT Using the Hybrid- Model I C 0.8 mA IC g m VT 0.8 25 32 mS 350 r g m 0.032 10 k vout vout g m v 4400 140 vin v v High Frequency Hybrid- Model 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 . 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 vbe and ib. VT r re gm IC 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. 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. 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.
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