sign in sign up
<<
Home , Cascode

ECE 342 Electronic Circuits

Lecture 26 Frequency Response of Cascaded

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

ECE 342 –Jose Schutt‐Aine 1 CE Cascade

 Exact analysis too tedious  use computer  CE cascade has low upper‐cutoff frequency

ECE 342 –Jose Schutt‐Aine 2 MOS Cascode Amplifier

Common source amplifier, followed by

stage – G2 is an incremental ground 1 Define go1  ro1 1 go2  ro2 1 GL  RL RL  impedance

ECE 342 –Jose Schutt‐Aine 3 MOS Cascode Amplifier • CS cacaded with CG  Cascode  Very popular configuration  Often considered as a single stage amplifier • Combine high and large in CS with current buffering and superior high frequency response of CG • Can be used to achieve equal gain but wider bandwidth than CS • Can be used to achieve higher gain with same GBW as CS

ECE 342 –Jose Schutt‐Aine 4 MOS Cascode Incremental Model

iL vo  GL

igvgvvvgL mb22 s m 22 s s 2 o o 2

iL ivgLsmbm22 g 2 vg so 22 g o 2 GL

ECE 342 –Jose Schutt‐Aine 5 MOS Cascode Analysis

go2 ivgggLsmbmo122  22 GL

igGLoL1/ 2  vs2  gggmb222 m o

KCL at vs2 gvmgs121222222 v s g o g m v s  g mbs v  g o()0 v s  v o

ECE 342 –Jose Schutt‐Aine 6 MOS Cascode Analysis

ggGooL121/  gvmgs1110 i L  ggommb22 g 2

gvmgs11 iL  ggGooL121/ 1 ggommb22 g 2 vg om 1 vin  gGoL12 g o GL    ggommb22 g 2 Two cases

ECE 342 –Jose Schutt‐Aine 7 MOS Cascode Analysis CASE 1

Case1: If GL  0 The voltage gain becomes

vo ggmo12  g m 2  g mboo 212 rr vin

AggggggrrMBmommmmboo12  1 2  1 2 12

AgrgrMBmomo  11 2 2

ECE 342 –Jose Schutt‐Aine 8 MOS Cascode Analysis

CASE 2

gGoL12  g o Case2: If GL  ggommb22 g 2

The voltage gain becomes

vgom 1 AMB vGin L

ECE 342 –Jose Schutt‐Aine 9 MOS Cascode at High Frequency

The upper corner frequency of the cascode can be approximated as: 1 f2o  where CC22233 db  Cgddb C Cgd 2 rCds32

ECE 342 –Jose Schutt‐Aine 10 MOS Cascode at High Frequency

• Capacitance Cgs1 sees a resistance Rsig

• Capacitance Cgd1 sees a resistance Rgd1

• Capacitance (Cdb1+Cgs2) sees resistance Rd1

• Capacitance (CL+Cgd2) sees resistance (RL||Rout)

 1 RL  R1gRRRgd1 m1 d1 sig  d1 Rrd1 o1     ggm2 mb2 A vo2 

  HCR gs1 sig C gd1 1gR m1 d1 R sig  R d1

CCRCCRRdb1 gs2 d 1  L gds2 L out  1 fH  2 H

ECE 342 –Jose Schutt‐Aine 11 MOS Cascode at High Frequency

If Rsig is large, to extend the bandwidth we must lower RL. This lowers Rd1 and makes the insignificant

If Rsig is small, there is no Miller effect. A large value of RL will give high gain

ECE 342 –Jose Schutt‐Aine 12 Effect of Cascoding

(when Rsig=0)

ECE 342 –Jose Schutt‐Aine 13 BJT Cascode Amplifier

Common emitter amplifier, followed by

stage – Base of Q2 is an incremental ground

ECE 342 –Jose Schutt‐Aine 14 Cascode Amplifier

First stage is CE and second stage is CB If Rs << r1, the voltage gain can be approximated by

AvmL gR12

ECE 342 –Jose Schutt‐Aine 15 BJT Cascode Incremental Model

vomLg 22Rv 

vrxx 22 r vvv 22 x 1   rrx22 r 2

ECE 342 –Jose Schutt‐Aine 16 BJT Cascode Analysis

vx gvmm11 gv 2 2 rr 22 x

Ignoring rx2

v 2 gvmm11  gv 2 2 r 2

1 gvmm11 v 2 g 2 r 2

 ECE 342 –Jose Schutt‐Aine 17 BJT Cascode Analysis

 r1 vv1  in Rsxrr11

 grvmin11 1 v 2  Rrsx11 r1  gm2  r 2

gRgrvmLm211 in 1 vo  Rr r 1 sx11 gm2  r 2

ECE 342 –Jose Schutt‐Aine 18  BJT Cascode Analysis

vgrgrR ommL 22 11 vRrrgr1  in s x11 m 22

v   R o  12L vRrr 1 in s x112 

If Rs << rx1+r1, the voltage gain can be approximated by

AvmL gR12

ECE 342 –Jose Schutt‐Aine 19 BJT Cascode at High Frequency

Define rcs as the internal impedance of the current source. R includes generator resistance and

base resistance rx1 base of Q1

ECE 342 –Jose Schutt‐Aine 20 BJT Cascode at High Frequency

There is no Miller multiplication from the input of Q2 (emitter) to the output (collector).

ECE 342 –Jose Schutt‐Aine 21 BJT Cascode at High Frequency

Define R32 rout r cs 111 AA MB 111jC11  r R jrCeout 22 jRC 3

1 1 fin high  f2  2 rRC11  2 rCe22 11 AA MB f f 11jj ffin high out high

ECE 342 –Jose Schutt‐Aine 22 BJT Cascode at High Frequency 1 fout high  2CRout 3

CCout out2  C outcs

Coutcs  current source output capacitance

ECE 342 –Jose Schutt‐Aine 23 Cascode Amplifier – High Frequency

High‐frequency incremental model

ECE 342 –Jose Schutt‐Aine 24 Cascode Amplifier – High Frequency Applying Kirchoff’s current law to each node:

GV'' G g s C  C V  sC V s is1 11a1b 

0g sCVg  g sCC  V  g g sCV m11a 2m22  1b   2m22d 

0gsCVggsCCVsCV       22bx22    22d2o

0 gm2 V b  g m2  sC 2 V d  G L  sC 2 V o

Find solution using a computer

ECE 342 –Jose Schutt‐Aine 25 Cascode Amplifier – High Frequency As an example use:

gm=0.4 mhos =100 r=250 ohms rx=20 ohms C=100 pF C=5 pf GL=5 mmhos GS’=4.5 mmhos

ZEROS (nsec‐1) POLES (nsec‐1)

sa=8.0 sd=‐0.0806 sb=‐2.02 + j5.99 se=‐0.644 sc=‐2.02 – j5.99 sf=‐4.05 sg=‐16.45

ECE 342 –Jose Schutt‐Aine 26 Cascode Amplifier – High Frequency If one pole is at a much lower frequency than the zeros and the other poles, (dominant

pole) we can approximate 3dB

9 3dB  0.0806 10 rad / sec

f3dB  12.9 MHz

For the same gain, a single stage amplifier would yield:

9 3dB  0.0169 10 rad / sec

f3dB  2.7 MHz Second stage in cascode increases bandwidth

ECE 342 –Jose Schutt‐Aine 27