ECE 342 Electronic Circuits

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

Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois [email protected]

ECE 342 – Jose Schutt‐Aine 1

Substrate is not connected to the source must account for body effect Drain signal current becomes

igvgvD m gs mb bs

And since vvgsbs

Body effect is fully accounted for by using gggmmmb

ECE 342 – Jose Schutt‐Aine 2 Common Gate Amplifier

with vvis iggviimmbiro  1 ggmmbi v vvio viR iiL ro iiro  i rroo R 1 L ro

ECE 342 – Jose Schutt‐Aine 3 IC - Common Gate Amplifier

vrRioL Rin  i1ggrimmbo

1 As r,Roin  ggmmb

If R,RLi 

vggrvvommboii 

Avo1g m  gr mb o

ECE 342 – Jose Schutt‐Aine 4 IC - Common Gate Amplifier

rRoL 1 RL Rin ,Aggrwhere o m mb o AggAvo m mb o

Taking ro into account adds a component (RL/Ao) to the input resistance. The open‐circuit voltage is:

Avo1g m  gr mb o The voltage gain of the loaded CG amplifier is:

RL GAvvo RLorAR vos

ECE 342 – Jose Schutt‐Aine 5 CG Output Resistance

viggvrv   xx mmbo with viR x s

Rr1ggrRout o m  mb o sor RrAR out  o vo s

ECE 342 – Jose Schutt‐Aine 6 CG Amplifier as Current Buffer

Rs GGis vo  1 Rout

Gis is the short‐circuit current gain

ECE 342 – Jose Schutt‐Aine 7 High-Frequency Response of CG

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

ECE 342 – Jose Schutt‐Aine 8 High-Frequency Response of CG 2 poles:

1 f  P1 1 2C gs R s  ggmmb 1 fP2  2C gdLL CR

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

ECE 342 – Jose Schutt‐Aine 9 Common Base (CB) Amplifier

ECE 342 – Jose Schutt‐Aine 10 CB Amplifier

rR R  oL in r R 1o L r1ree  ' Rr1grRout o m o e Avo1gr m o r  ' re  RRr    1 ee

ECE 342 – Jose Schutt‐Aine 11 High-Frequency Analysis of CB Amplifier Exact analysis is too tedious  approximate

From current gain analysis The amplifier’s 1 in 3dB  upper cutoff rrx  CRSE R frequency will 11  be the lower of 1 out 3dB  these two poles. CR L

ECE 342 – Jose Schutt‐Aine 12 Source Follower

' 1 ' RRrLLo  vomgsL g vR gmb

ECE 342 – Jose Schutt‐Aine 13 Source Follower

vvvgs i o

' vgRomL Av  ' vgRimL1

grmo Avo  1ggrmmbo

gm 1 Avo  ggmmb1 

ECE 342 – Jose Schutt‐Aine 14 Source Follower – Output Resistance

1 Roo  r Rom1/ 1  g ggmmb 

ECE 342 – Jose Schutt‐Aine 15 Frequency Response of Source Follower

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

ECE 342 – Jose Schutt‐Aine 16 Determination of Zeros

• Three capacitances form a continuous loop • Two transmission zeros

gm Z   Z  fZ  fT Cgs

ECE 342 – Jose Schutt‐Aine 17 Determination of Poles

Rgs R sig

' Rsig R L Rgd  ' 1 gmLR

R  RR CLoL 1 f 1/2 CR  CR  CR HgdgdgsgsLCL 2 H The source follower has excellent high-frequency response

ECE 342 – Jose Schutt‐Aine 18 Emitter Follower

ECE 342 – Jose Schutt‐Aine 19 Emitter Follower

ECE 342 – Jose Schutt‐Aine 20 Emitter Follower High-Frequency Exact analysis is too tedious  approximate

sC 1  ' gR g 1gR g A(s) mL m  mE m  v 1gR sC R 3dB T mL1 E RCE  C C 1gR mE

ECE 342 – Jose Schutt‐Aine 21 High-Frequency Analysis of Emitter Follower

V gVmZ sC 0 r 1 grm  (1 / ) 1 leads to:  Z   Cr CCr  e  e

ECE 342 – Jose Schutt‐Aine 22 High-Frequency Analysis of Emitter Follower

''  RRrsig 1 R L

R'' R R  sig L  R' R' 1sig L fH 1/2 CR  CR rre 

ECE 342 – Jose Schutt‐Aine 23