1. Transistor equivalent circuits and models

2. Equal vs. Equivalent – To analyze transistor circuits easily n rapidly – Discuss here – small signal: in which the AC input signal voltage and currents are in the order of +/- 10% of Q-point V or I.

3. 4 h-parameters or constants h – hybrid

4. DC eqv. CB circuit Ideal trans.  alpha = 1  Ic = Ie Emitter diode  F.Biased ideal diode Collector diode  acts as a current source Draw: 56.1/2: draw for NPN and PNP --- CB.

5. AC eqv. CB ckt. For small i/p AC signals, Emitter diode does not rectify, it offers resistance called ac resistance

6. T-model – ~not used now F. 56.33

7. h-parameters? 4 h parameters or constants for 2-port network. h 11, h 12, h 21, h 22 hybrid? – mixture of different items; different units or just ratio.

10. Freq. response

11. Frequency response of amplifiers  Midband: The frequency range of interest for amplifiers Large capacitors can be treated as short circuit and small capacitors can be treated as open circuit Gain is constant and can be obtained by small-signal analysis

12. Low-frequency band: Gain drops at frequencies lower than fL Large capacitors can no longer be treated as short circuit The gain roll-off is mainly due to coupling and by-pass capacitors

13. LOW FREQUENCY At low frequency range, the gain falloff due to coupling capacitors and bypass capacitors. As signal frequency , the reactance of the coupling capacitor, X C  - no longer behave as short circuits. 13

14. High-frequency band: Gain drops at frequencies higher than fH Small capacitors can no longer treated as open circuit The gain roll-off is mainly due to parasitic capacitances of the MOSFETs and BJTs

16. Amplifier gain vs frequency Midband range Gain falls of due to the effects of C C and C E Gain falls of due to the effects of stray capacitance and transistor capacitance effects 16

18. Gain & frequencies Gain-bandwidth product : constant value of the product of the voltage gain and the bandwidth. Unity-gain frequency : the frequency at which the amplifier’s gain is 1 18

19. CH 11 Frequency Response 19 High Frequency Roll-off of Amplifier As frequency of operation increases, the gain of amplifier decreases.

20. Example: Human Voice I Natural human voice spans a frequency range from 20Hz to 20KHz, however conventional telephone system passes frequencies from 400Hz to 3.5KHz. Therefore phone conversation differs from face-to-face conversation. CH 11 Frequency Response 20 Natural Voice Telephone System

21. Example: Video Signal Video signals without sufficient bandwidth become fuzzy as they fail to abruptly change the contrast of pictures from complete white into complete black. CH 11 Frequency Response 21 High BandwidthLow Bandwidth

22. Typical Frequency Response Lower Corner Upper Corner CH 11 Frequency Response22