MOSFET Amplifier Configuration. MOSFET Amplifier Configuration

Size: px

Start display at page:

Download "MOSFET Amplifier Configuration. MOSFET Amplifier Configuration"

Oscar Blankenship
6 years ago
Views:

1 MOSFET Amplifier Configuration Single stage The signal is fed to the amplifier represented as sig with an internal resistance sig. MOSFET is represented by its small signal model. Generally interested in gain, input and output resistance (oerall amplifier circuit not only the small signal model). MOSFET Amplifier Configuration Considering only the small signal not the bias 1

2 Characterizing Amplifiers Find gain, input and output resistance i in ii o Ao, A i L o i G o sig Oerall oltage gain Amplifier Configuration Common Source Common Source with a source resistance Common gate Common drain or oltage follower 2

3 Amplifiers o i A o sig i in L in L sig o A G A o o sig L L in o in sig A o L L o Common Source Most widely used configuration In multistage amplifiers, the bulk of the gain is from common source. The source is grounded, making it common between input and output. We can use hybrid model. 3

4 Common Source i A g o For o, set i =0 r o 0 D A V =? G =? r m ( 0 D ) Example CS at I D =0.25 ma, V OV =0.25V, D =20k, Signal source has a resistance of 100K, and 20K load Find: in, A o, o, A, and G Microelectronic Circuits, Seenth Edition Sedra/Smith Copyright 2015 by Oxford Uniersity Press 4

5 Common Source with Source For simplicity, r 0 is not included. No effect on discrete implementation, not so for IC s s proides a negatie feedback to control the magnitude of the signal to preent nonlinear distortion. Also reduces the oltage gain and extends the useful bandwidth. Common Source with Source gs gs i 1/ g 1 g 1/ g i m m S m S 5

6 Example In the preious example, in=50 mv, the output was 1V Now, we hae sig = 0.2V, we want to modify the circuit to keep gs unchanged, what alue we should use for s Microelectronic Circuits, Seenth Edition Sedra/Smith Copyright 2015 by Oxford Uniersity Press Common Gate Amplifier 6

7 Common Drain Amplifier Voltage Follower Since there is a resistance L connected to the source, it is easier to use the T-model Common Drain Voltage Follower Parallel i 7

Comparison CS CS+S CG CD in out D ro D G -g

8 Comparison CS CS+S CG CD in out D ro D G -g m ( D L r o ) V DD =V SS =10V, I=0.5 ma, G =4.7M, D =15K, V t =1.5V k n =1mA/V 2 V A =75V Find V OV, V GS, V G, V S, V D, g m, r o What is the max. possible oltage swing at drain and the MOSFET remains in saturation? 8

9 Find in, A o, o, G, with and D without r o = 15 K sig =100K and L Discrete CMOS Amplifiers MOS are mostly used in IC s Howeer, we present practical circuits for discrete implementation. Good if you are building a circuit on a breadboard (in the lab). 9

10 Common Source DC Analysis Can use any biasing technique we studied Calculate the bias (Q) point AC Point of iew Coupling capacitors: To connect the input and output without disturbing the bias. C is large enough to hae ery low (zero) impedance at signal frequency. Signal ground: Bypass capacitor. Current source is an open circuit, proide a path to ground (source is ground for signal). 10

11 Time arying (signal) 11

12 Common Gate Voltage Follower Figure 5.60 (a) A source-follower amplifier. (b) Small-signal, equialent-circuit model. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright 2010 by Oxford Uniersity Press, Inc. 12

13 Biasing in MOS Amplifiers How to choose the operating point? Want a stable Q-point (known I D and V DS ) to ensure operation in the saturation region. Biasing -- Fixing V GS I D depends on µ, C ox, W/L and V t, and V GS C ox, V GS ( een W/L) can ary across deices of the same type. Constant V GS Not a good idea. µ,c ox are a f(t) Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright 2010 by Oxford Uniersity Press, Inc. 13

14 Biasing Fixing V G and S S proides a negatie feedback to stabilize I D Biasing Fixing V G and S Uses one power supply What is the effect on input resistance when you add gs signal? 2 Power supplies 14

15 Figure 5.53 Circuit for Example Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright 2010 by Oxford Uniersity Press, Inc. Biasing D-to-G esistor Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright 2010 by Oxford Uniersity Press, Inc. 15

16 Biasing Constant Current Source Figure 5.55 (a) Biasing the MOSFET using a constant-current source I. (b) Implementation of the constant-current source I using a current mirror. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright 2010 by Oxford Uniersity Press, Inc. Frequency esponse 16

4.5 Biasing in MOS Amplifier Circuits

4.5 Biasing in MOS Amplifier Circuits Biasing: establishing an appropriate DC operating point for the MOSFET - A fundamental step in the design of a MOSFET amplifier circuit An appropriate DC operating