ECE 342 Electronic Circuits Lecture 25 Frequency Response

ECE 342 – Jose Schutt‐Aine 1

Jose E. Schutt-AineElectrical & Computer Engineering

University of [email protected]

ECE 342Electronic Circuits

Lecture 25Frequency Response of CG, CB,SF and EF

2ECE 342 – Jose Schutt‐Aine

Common Gate Amplifier

Substrate is not connected to the sourcemust account for body effect

Drain signal current becomes

D m gs mb bsi g v g v

And since gs bsv v

Body effect is fully accounted for by using m m mbg g g


i m mb i roi g g v i

m mb ioi o i i L

ro io o L

o

1g g vrv v v i Ri i

r r R1r

with i sv v

Common Gate Amplifier


i o L

ini m mb o

v r RRi 1 g g r

As o inm mb

1r , Rg g

If L iR , R

o m mb o i iv g g r v v

vo m mb oA 1 g g r

IC - Common Gate Amplifier


whereo L Lin o m mb o

vo m mb o

r R R1R , A g g rA g g A

Taking ro into account adds a component (RL/Ao) to the input resistance.

vo m mb oA 1 g g r

The open‐circuit voltage gain is:

The voltage gain of the loaded CG amplifier is:

Lv vo

L o vo s

RG AR r A R

IC - Common Gate Amplifier


x x m mb ov i g g v r v with x sv i R

orout o m mb o s out o vo sR r 1 g g r R R r A R

CG Output Resistance


CG Amplifier as Current Buffer

sis vo

out

RG G 1R

Gis is the short‐circuit current gain


- Include CL to represent capacitance of load‐ Cgd is grounded‐ No Miller effect

High-Frequency Response of CG


P1

gs sm mb

1f12 C R

g g

P2gd L L

1f2 C C R

• fP2 is usually lower than fp1• fP2 can be dominant• Both fP1 and fP2 are usually much higher than fP in CS case

2 poles:

High-Frequency Response of CG


Common Base (CB) Amplifier


o Lin

o L

e e

r RR r R1r 1 r

vo m oA 1 g r 'out o m o eR r 1 g r R

'e eR R r e

rr1

CB Amplifier


High-Frequency Analysis of CB Amplifier

in 3dBx

S E

1r rC R R

1 1

out 3dBL

1C R

The amplifier’s upper cutoff frequency will be the lower of these two poles.

Exact analysis is too tedious approximate

From current gain analysis


Source Follower

' 1L L o

mb

R R rg

'o m gs Lv g v R


Source Follower

gs i ov v v

'

'1o m L

vi m L

v g RAv g R

1m o

vom mb o

g rAg g r

11

mvo

m mb

gAg g


Source Follower – Output Resistance

1o o

m mb

R rg g

1/ 1o mR g


Frequency Response of Source Follower

•Determine location of transmission zeros• Use method of open‐circuit time constants to estimate 3‐dB frequency


• Three capacitances form a continuous loop• Two transmission zeros

Z mZ

gs

gC

Z Tf f

Determination of Zeros


Determination of Poles

gs sigR R

'

'1sig L

gdm L

R RR

g R

LC L oR R R

1 1/ 22 LH gd gd gs gs L C

H

f C R C R C R

The source follower has excellent high-frequency response


Emitter Follower


Emitter Follower


Emitter Follower High-Frequency Exact analysis is too tedious approximate

' m L mv

Em L

m E

sC1g R gA ( s ) sC R1 g R 1

1 g R

m E m3dB T

E

1 g R gR C C C


High-Frequency Analysis of Emitter Follower

0m ZVg V s Cr

leads to:(1/ ) 1m

e

g rC C r

1Z

eC r


' '1sig LR R r R

' '

' '

1

sig L

sig L

e

R RR

R Rr r

1/ 2Hf C R C R

High-Frequency Analysis of Emitter Follower

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ECE 342 Electronic Circuits Lecture 25 Frequency Response