Example: MOSFET Amplifier Distortion

Transcription

1 4/25/2011 Example MSFET Amplfer Dsoron 1/9 Example: MSFET Amplfer Dsoron Recall hs crcu from a prevous handou: ( ) = I ( ) D D d 15.0 V RD = 5K v ( ) = V v ( ) D o v( ) - K = ma/v V = 2.0 V 40V. We found ha he small-sgnal volage gan s: A vo vo( ) = = 5.0 v ( )

2 4/25/2011 Example MSFET Amplfer Dsoron 2/9 Say he npu volage o hs amplfer s: v ( ) = V cosω Q: Wha s he larges value ha V can ake whou producng a dsored oupu? A: Well, we know ha he small-sgnal oupu s: v ( ) = A v ( ) o vo = 5.0V cosω BUT, hs s no he oupu volage! The oal oupu volage s he sum of he small-sgnal oupu volage and he DC oupu volage! Noe for hs example, he DC oupu volage s he DC dran volage, and we recall we deermned n an earler handou ha s value s: V = V = 10 V D Thus, he oal oupu volage s : v ( ) = V v ( ) D o = V cosω

3 4/25/2011 Example MSFET Amplfer Dsoron 3/9 I s very mporan ha you realze here s a lm on boh how hgh and how low he oal oupu volage v ( ) can go! Tha s rgh! If he oal oupu volage v ( ) res o exceed hese lms even for a momen hemsfet wll leave sauraon mode. And leavng sauraon mode resuls n sgnal dsoron!

4 4/25/2011 Example MSFET Amplfer Dsoron 4/9 Le s break he problem down no wo separae problems: 1) If oal oupu volage v ( ) becomes oo small, he MSFET wll ener he rode mode. 2) If oal oupu volage v ( ) becomes oo large, he MSFET wll ener cuoff. We ll frs consder problem 1. For a MSFET o reman n sauraon, v ( ) mus reman greaer han he excess gae volage ( V V ) v ( ) > V V DS GS DS GS for all me. Snce he source ermnal of he MSFET n hs crcu s conneced o ground, we know ha V = 0V. Therefore: vds ( ) = vd( ) = v ( ) and V = V GS G And so he MSFET wll reman n sauraon only f he oal oupu volage remans larger han V GS V = V G V! S v ( ) > V V G

5 4/25/2011 Example MSFET Amplfer Dsoron 5/9 Thus, we conclude for hs amplfer ha he oupu floor s: L V V = G And snce V = 4.0V and V = 2.0V, we fnd: G L = V V = 4 2 = 2.0V G Thus, o reman n sauraon, he oal oupu volage mus reman larger han he floor volage L for all me : Snce hs oal volage s: v ( ) > L = 2.0V v ( ) = V cosω we can deermne he maxmum value of small-sgnal npu magnude V : V cosω > > 5.0V cosω V cosω < 1.6

6 4/25/2011 Example MSFET Amplfer Dsoron 6/9 Snce cosω can be as large as 1.0, we fnd ha he magnude of he npu volage can be no larger han 1.6 V,.e., V < 1.6 V If he npu magnude exceeds hs value, he MSFET wll (momenarly) leave he sauraon regon and ener he dreaded rode mode! Now le s consder problem 2 For he MSFET o reman n sauraon, he dran curren mus be greaer han zero (.e., D > 0 ). herwse, he MSFET wll ener cuoff mode. Applyng hm s Law o he dran ressor, we fnd he dran curren s: VDD v 15 v D = = R 5 C s evden ha dran curren s posve only f v < 15 V. In oher words, he upper lm (.e., he celng ) on he oal oupu volage s: L = V = 15.0V DD Snce: v ( ) = V cosω

7 4/25/2011 Example MSFET Amplfer Dsoron 7/9 we can conclude ha n order for he MSFET o reman n sauraon mode: V cosω > 15.0 Therefore, we fnd: 5.0 Vs cosω > = Snce cosω 1, he above equaon means ha he npu sgnal magnude V can be no larger han: V < 1.0 V If he npu magnude exceeds 1.0 V, he MSFET wll (momenarly) leave he sauraon and ener he cuoff regon! In summary: 1) If V > 1.6 V, he MSFET wll a mes ener rode, and dsoron wll occur! 2) If 1.0 V V >, he MSFET wll a mes ener cuoff, and even more dsoron wll occur!

8 4/25/2011 Example MSFET Amplfer Dsoron 8/9 To demonsrae hs, le s consder hree examples: 1. V < 1.0 V The oupu sgnal n hs case remans beween V = 15.0 and DD V V = 2.0 V for all me. Therefore, he oupu sgnal s no G dsored. L = V = DD 15 v ( ) V = 10 L = V V = 2 G V > V > 1.0 V The oupu sgnal n hs case remans greaer han L = V V 2 G = for all me. However, he small-sgnal oupu s now large enough so ha he oal oupu volage a mes res o exceed L = V DD = 15. For hese mes, he MSFET wll ener cuoff, and he oupu sgnal wll be dsored.

9 4/25/2011 Example MSFET Amplfer Dsoron 9/9 L = V = DD 15 v ( ) V = 10 L = V V = 2 G 3. V > 1.6V In hs case, he small-sgnal npu sgnal s suffcenly large so ha he oal oupu wll aemp o exceed boh lms (.e., V DD = 15.0V and VG V = 2.0V ). Therefore, here are perods of me when he MSFE wll be n cuoff, and perods when he MSFET wll be n sauraon. L = V = DD 15 v ( ) V = 10 L = V V = 2 G