ho8.l05_gain_and_ro

ho8.l05_gain_and_ro - 1 Lecture 5 Gain and Biasing...

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Unformatted text preview: 1 Lecture 5 Gain and Biasing Considerations Finite Output Resistance R. Dutton, B. Murmann R. Dutton, B. Murmann 1 EE114 (HO #8) Stanford University Common Source Amplifier Revisited Interesting question How much voltage gain can we get from this circuit? I D +i d V O +v o V DD v R R g A m v = R. Dutton, B. Murmann 2 EE114 (HO #8) v i V I 2 Upper Bound on Gain (1) Plug in expression for g m L OV D L m v R V I R g A 2 = = OV For operation in the saturation region, we definitely require DD D V R I < Therefore, we have OV DD v V V A 2 < R. Dutton, B. Murmann 3 EE114 (HO #8) This is an upper bound only, useful for back of the envelope calculations (and job interviews) Important to note that this expression does not hold with equality Upper Bound on Gain (2) The above derived upper bound comes for the fact that both gain and bias point depend on R Want large R for large gain Want small R to prevent device from entering triode region The upper bound may not be a serious issue in some circuits Nonetheless it is interesting and insightful to think about ways around the issue Well do this graphically using load line plots R. Dutton, B. Murmann 4 EE114 (HO #8) 3 Construction of Load Line Plot V DD I r I d V o R I r V i R V V I o DD r = V o I D I d i d = g m v i R. Dutton, B. Murmann 5 EE114 (HO #8) V o Putting Things Together I r , I d Operating point V DD /R i d = g m v i v R. Dutton, B. Murmann 6 EE114 (HO #8) V o V DD v o 4 Larger Resistance I r , I d V DD /R small i d = g m v i v V DD /R large R. Dutton, B. Murmann 7 EE114 (HO #8) V o V DD v o Discussion The issue with this configuration is that the load line is pinned to the point (V DD , 0) Larger V DD allows use of larger R and therefore larger gain DD This is also evident from the equation on slide 3 It is possible to overcome this limitation by adding additional degrees of freedom; consider e.g. the following alternative load network I x B o B x I R V V I + = R. Dutton, B. Murmann 8 EE114 (HO #8) V o I B V B R 5 Plot for I B = I D I x , I d i d = g m v i v o R. Dutton, B. Murmann 9 Can increase R (and gain) without changing operating point EE114 (HO #8) V o V DD Infinite Gain?...
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ho8.l05_gain_and_ro - 1 Lecture 5 Gain and Biasing...

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