comparison of amplifier configurations

1: Ch. 7 Frequency Response Part 5 1 Comparison of Amplifier Configurations

2: Ch. 7 Frequency Response Part 5 2

3: Ch. 7 Frequency Response Part 5 3

4: Ch. 7 Frequency Response Part 5 4 Comparison of CB to CE Amplifier (with same Rs = 5 O)

5: Ch. 7 Frequency Response Part 5 5 Comparison of EF to CE Amplifier (For RS = 5O )

6: Ch. 7 Frequency Response Part 5 6 Comparison of Amplifier Configurations

7: Ch. 7 Frequency Response Part 5 7 Cascade Amplifier Emitter Follower + Common Emitter (EF+CE) Voltage gain from CE stage, gain of one for EF. Low output resistance from EF provides a low source resistance for CE amplifier so good matching of output of EF to input of CE amplifier High frequency response (3dB frequency) for Cascade Amplifier is improved over CE amplifier.

8: Ch. 7 Frequency Response Part 5 8 Cascade Amplifier - DC analysis

9: Ch. 7 Frequency Response Part 5 9 Cascade Amplifier - Midband Gain Analysis

10: Ch. 7 Frequency Response Part 5 10 Cascade Amplifier - Low Frequency Poles and Zeroes Use Gray-Searle (Short Circuit) Technique to find the poles. Three low frequency poles Equivalent resistance may depend on rp for both transistors. Find three low frequency zeroes.

11: Ch. 7 Frequency Response Part 5 11 Cascade Amplifier - Analysis of Low Frequency Poles Gray-Searle (Short Circuit) Technique

12: Ch. 7 Frequency Response Part 5 12 Cascade Amplifier - Analysis of Low Frequency Poles Gray-Searle (Short Circuit) Technique

13: Ch. 7 Frequency Response Part 5 13 Cascade Amplifier - Analysis of Low Frequency Poles Gray-Searle (Short Circuit) Technique

14: Ch. 7 Frequency Response Part 5 14 Cascade Amplifier - Low Frequency Zeros

15: Ch. 7 Frequency Response Part 5 15 Cascade Amplifier - High Frequency Poles and Zeroes Use Gray-Searle (Open Circuit) Technique to find the poles. Four high frequency poles Equivalent resistance may depend on rp for both transistors. Find four high frequency zeroes.

16: Ch. 7 Frequency Response Part 5 16 Cascade Amplifier - High Frequency Poles

17: Ch. 7 Frequency Response Part 5 17 Cascade Amplifier - High Frequency Poles

18: Ch. 7 Frequency Response Part 5 18 Cascade Amplifier - High Frequency Poles and Zeroes

19: Ch. 7 Frequency Response Part 5 19 Cascade Amplifier - High Frequency Poles

20: Ch. 7 Frequency Response Part 5 20 When does Vo = 0? When ? ? 8, ZCµ1? 0, so signal shorted to ground. ?ZH1= 8. When ? ? 8, ZCp2? 0, so rp2 shorted, so Vp2 = 0. ?ZH2= 8. For Cp1 , we get a zero when Ie1 = 0.

21: Ch. 7 Frequency Response Part 5 21 When does Cµ2 produce a zero, i.e. make Vo = 0? For Cµ2 , we get a zero when IRL’ = 0, or ICµ2 = gm2Vp2 , i.e. the output load resistance RL’ is starved of any current.

22: Ch. 7 Frequency Response Part 5 22 Cascade Amplifier - High Frequency Poles and Zeroes

23: Ch. 7 Frequency Response Part 5 23 Comparison of Cascade to CE Amplifier

24: Ch. 7 Frequency Response Part 5 24 Comparison of Cascade to CE Amplifier Why the better voltage gain for the cascade? Emitter follower gives no voltage gain! Cascade has better matching with source than CE. Cascade amplifier has an input resistance that is higher due to EF first stage. Versus Ri2 = rp2 = 2.5 K for CE So less loss in voltage divider term (Vi / Vs ) with the source resistance. 0.91 for cascade vs 0.37 for CE. Why better bandwidth? Low output resistance re1 of EF stage gives smaller effective source resistance for CE stage and higher frequency for dominant pole due to CT (including Cµ2)

25: Ch. 7 Frequency Response Part 5 25 Another Useful Amplifier – Cascode (CE+CB) Amplifier

26: Ch. 7 Frequency Response Part 5 26 Example of Cascode (CE +CB) Amplifier

27: Ch. 7 Frequency Response Part 5 27 Other Examples of Multistage Amplifiers

28: Ch. 7 Frequency Response Part 5 28 Other Examples of Multistage Amplifiers

29: Ch. 7 Frequency Response Part 5 29 Differential Amplifier Similar to CE amplifier, but two CE’s operated in parallel Signal applied between two equivalent inputs instead of between one input and ground Common emitter resistor or current source used Current shared or switched between two transistors (they compete) Analyze using equivalent half-circuit 1/2 of signal at input 1/2 of signal at output 1/2 of source resistance Gain and frequency response similar to CE amplifier for high frequencies Advantage: Rejects common noise pickup on input No coupling capacitors so can operate down to zero frequency.

30: Ch. 7 Frequency Response Part 5 30 Differential Amplifier Analysis

31: Ch. 7 Frequency Response Part 5 31 Summary In this chapter we have shown how to analyze the high and low frequency dependence of the gain for an amplifier. Analyzed the effects of the coupling capacitors on the low frequency response Found the expressions for the corresponding poles and zeros. Demonstrated Bode plots of magnitude and phase. Analyzed the effects of the capacitances within the transistor on the high frequency response. Found the expressions for the corresponding poles and zeros. Demonstrated Bode plots of the magnitude and phase. Analyzed the high and low frequency performance of the three bipolar transistor amplifiers: common emitter, common base and emitter follower. Found the expressions for the corresponding poles and zeros. Demonstrated Bode plots of the magnitude and phase. Demonstrated how to find the expressions for the gain and the high and low frequency poles and zeros for multistage amplifiers.