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 Common Gate Amplifier
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
Avo1g 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 gain is:
Avo1g m gr mb o The voltage gain of the loaded CG amplifier is:
RL GAvvo RLorAR 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 Miller effect
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 1o L r1ree ' Rr1grRout o m o e Avo1gr 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 CRSE 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 1ggrmmbo
gm 1 Avo ggmmb1
ECE 342 – Jose Schutt‐Aine 14 Source Follower – Output Resistance
1 Roo r Rom1/ 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 HgdgdgsgsLCL 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
'' RRrsig 1 R L
R'' R R sig L R' R' 1sig L fH 1/2 CR CR rre
ECE 342 – Jose Schutt‐Aine 23