Steps for D.C Analysis of MOSFET Circuits

Transcription

1 10/22/2004 Seps for DC Analysis of MOSFET Circuis.doc 1/7 Seps for D.C Analysis of MOSFET Circuis To analyze MOSFET circui wih D.C. sources, we mus follow hese five seps: 1. ASSUME an operaing mode 2. ENFORCE he equaliy condiions of ha mode. 3. ANALYZE he circui wih he enforced condiions. 4. CHECK he inequaliy condiions of he mode for consisency wih original assumpion. If consisen, he analysis is complee; if inconsisen, go o sep MODIFY your original assumpion and repea all seps. Le s specifically look a each sep in deail. 1. ASSUME Here we have hree choices cuoff, riode, or sauraion. You can make an educaed guess here, bu remember, unil you CHECK, i s jus a guess!

2 10/22/2004 Seps for DC Analysis of MOSFET Circuis.doc 2/7 2. ENFORCE For all hree operaing regions, we mus ENFORCE jus one equaliy. Cuoff Since no channel is induced, we ENFORCE he equaliy: I D = 0 Triode Since he conducing channel is induced bu no in pinch-off, we ENFORCE he equaliy: Sauraion ( ) 2 I K D = 2 DS DS Since he conducing channel is induced and is in pinch-off, we ENFORCE he equaliy: ( ) 2 I = K D

3 10/22/2004 Seps for DC Analysis of MOSFET Circuis.doc 3/7 Noe for all cases he consan K is: 1 W K k 2 L and is he MOSFET hreshold volage. 3. ANALYZE The ask in D.C. analysis of a MOSFET circui is o find one curren and wo volages! a) Since he gae curren I is zero ( I = 0) for all G G MOSFETS in all modes, we need only o find he drain curren I D --his curren value mus be posiive (or zero). b) We also need o find wo of he hree volages associaed wih he MOSFET. Typically, hese wo volages are and DS, bu given any wo volages, we can find he hird using KL: = + DS DG Some hins for MOSFET DC analysis: 1) Gae curren I = 0 always!!! G 2) Equaions someimes have wo soluions! Choose soluion ha is consisen wih he original ASSUMPTION.

4 10/22/2004 Seps for DC Analysis of MOSFET Circuis.doc 4/7 4. CHECK You do no know if your D.C. analysis is correc unless you CHECK o see if i is consisen wih your original assumpion! WARNING!-Failure o CHECK he original assumpion will resul in a SIGNIFICANT REDUCTION in credi on exams, regardless of he accuracy of he analysis!!! Q: Wha exacly do we CHECK? A: We ENFORCED he mode equaliies, we CHECK he mode inequaliies. We mus CHECK wo separae inequaliies afer analyzing a MOSFET circui. Essenially, we check if we have/have no induced a conducing channel, and hen we check if we have/have no pinched-off he channel (if i is conducing). Cuoff We mus only CHECK o see if he MOSFET has a conducing channel. If no, he MOSFET is indeed in cuoff. We herefore CHECK o see if: < >

5 10/22/2004 Seps for DC Analysis of MOSFET Circuis.doc 5/7 Triode Here we mus firs CHECK o see if a channel has been induced, i.e.: > < Likewise, we mus CHECK o see if he channel has reached pinchoff. If no, he MOSFET is indeed in he riode region. We herefore CHECK o see if: < DS > DS

6 10/22/2004 Seps for DC Analysis of MOSFET Circuis.doc 6/7 Sauraion Here we mus firs CHECK o see if a channel has been induced, i.e.: > < Likewise, we mus CHECK o see if he channel has reached pinchoff. If i has, he MOSFET is indeed in he sauraion region. We herefore CHECK o see if: > DS < DS

7 10/22/2004 Seps for DC Analysis of MOSFET Circuis.doc 7/7 If he resuls of our analysis are consisen wih each of hese inequaliies, hen we have made he correc assumpion! The numeric resuls of our analysis are hen likewise correc. We can sop working! However, if even one of he resuls of our analysis is inconsisen wih our ASSUMPTION, hen we have made he wrong assumpion! Time o move o sep MODIFY If one or more of he circui MOSFETSs are no in heir ASSUMED mode, we mus change our assumpions and sar compleely over! In general, all of he resuls of our previous analysis are incorrec, and hus mus be compleely scraped!