After completing this chapter you will learn

Transcription

1 CHAPTER 7 Trasistor Amplifiers Microelectroic Circuits, Seeth Editio Sedra/Smith Copyright 015 by Oxford Uiersity Press After completig this chapter you will lear 1. How to use MOSFET as amplifier. How to model the liear operatio of the trasistor aroud the Q poit usig a equialet circuit (small sigal model) 3. The three basic amplifier cofiguratio Microelectroic Circuits, Seeth Editio Sedra/Smith Copyright 015 by Oxford Uiersity Press 1

2 I saturatio, the MOSFET acts as a oltage cotrolled curret source i 1 ' W o k L (1 No-liear If the curret (i ) flows i a resistie load, output oltage is proportioal to i. S ) Microelectroic Circuits, Seeth Editio Sedra/Smith Copyright 015 by Oxford Uiersity Press MOSFET as a amplifier o = S = -i R Later, we will discuss small sigal equialet circuit Sedra/Smith Copyright 010 by Oxford Uiersity Press, Ic.

3 < t cut off icreases whe S = t Triode icreases whe reaches t MOSFET is ON Sice iitially S is high (o drop o R ) S > t Saturatio 3

4 Microelectroic Circuits, Sixth Editio Sedra/Smith Copyright 010 by Oxford Uiersity Press, Ic. A A S k d d S R 1/ k O R ( ) t Saturatio I > t, o I - t Cutoff I < t o =, I =0 Triode I > t, o i - t Sedra/Smith Copyright 010 by Oxford Uiersity Press, Ic. 4

5 S ( t) 1/ k R ( t) ( ) t esi for a gai of Chagig R while keepig O costat. Chagig O while keepig R costat t =0.4, =1.8, =0.6, K =0.4mA/, W/L=10, R = 17.5K 5

6 a) For =0 fid o, I, ad A b) What is the max symmetrical sigal swig allowed at the drai, ad t =0.4, =1.8, =0.6, K =0.4mA/, W/L=10, R = 17.5K Microelectroic Circuits, Seeth Editio Sedra/Smith Copyright 015 by Oxford Uiersity Press TC by Graphical Aalysis Not used i circuit aalysis, used oly to illustrate for gaiig a greater isight ito circuit operatio. From elemetary circuit theory we hae = i R + S That represets a lie with a slope of - 1/R The trasistor operates o a poit alog that lie. 6

7 Figure 5.31 Graphical costructio to determie the oltage trasfer characteristic of the amplifier i Fig. 5.9(a). Microelectroic Circuits, Sixth Editio Sedra/Smith Copyright 010 by Oxford Uiersity Press, Ic. Microelectroic Circuits, Sixth Editio Sedra/Smith Copyright 010 by Oxford Uiersity Press, Ic. 7

8 Small Sigal Operatios i 1 k O k O 1 k Small Sigal Operatio To miimize the oliear part 1 k k O O i I +i d S S S S i i ( I d R R i ) R 8

9 i Small Sigal Operatio 1 k i d g O m k k i d O O k O 1 k g m i Assumig small Trascoductace: relates i d ad ds A g m R Trascoductace The slope of the i S - characteristics at the Q poit (C bias poit) As show, almost liear. g m i k ' W L ( t ) Microelectroic Circuits, Sixth Editio Sedra/Smith Copyright 010 by Oxford Uiersity Press, Ic. 9

10 R i R ( I i ) R i d Separatig C Aalysis ad Sigal Aalysis Sigal quatities are superimposed o C quatities. We ca separate C ad AC Aalysis. The C Aalysis determie the Q Poit (Bypass) Capacitors are added to preet disturbig the C bias (Q poit). WHY? raw the circuit from the sigal poit of iew C oltages (curret) are short (ope) Capacitors are short MOSFET replaced by small sigal equialet Circuit 10

11 Addig the load Why Capacitors R L Small Sigal Equialet Circuit Represets oly time aryig compoet (C oly determie the bias poit) What is the differece betwee (a) ad (b). ro I 11

12 Example Fid the small sigal oltage gai, iput resistace, ad the largest allowable iput sigal. t =1.5, k =0.5mA/, A=50. Example cot. 1

13 Example cot. T-Equialet-Circuit Model 13

14 Icorporatig r 0 Example Assume saturatio, fid gai ad iput resistace 14