Lecture 12

Single Stage FET Amplifiers: Common Gate Amplifier Common Drain Amplifier

The Building Blocks of Analog Circuits - II

In this lecture you will learn:

• Common Gate (CG) and Common Drain (CD) Amplifiers • Small signal models of amplifiers

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

The Common Source Amplifier

VDD

R IOUT + iout VOUT  vout

D ID + id RS G + + I + i R v G g L VOUT  vout s - - S + VBIAS -

An attribute of the common source amplifier:

The input resistance is very large:

Rin   Not suitable for current amplifiers or transimpedance amplifiers

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

1 The Common Gate Amplifier VDD

R Assume an AC short I + i I + i IS + is D d OUT out

R S + I + i + G g RL VOUT  vout IBIAS + v s - - VBIAS -

The gate terminal is “common” between the input and the output The common gate amplifiers are useful when small input resistances and large output resistances are desired in amplifiers (they also have good high frequency performance – but that will come later in the course) But the current gain is unity! Note: The bulk is not tied to the source

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

The Common Gate Amplifier: Same Topology, Different Look VDD

R IOUT + iout I + i D D d

IG + ig G + Assume an S RL VOUT  vout IS + is - + AC short V BIAS RS - IBIAS + v s -

The gate terminal is “common” between the input and the output The common gate amplifiers are useful when small input resistances and large output resistances are desired in amplifiers But the current gain is unity! Note: The bulk is not tied to the source

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

2 Choice of Substrate Potentials

D S B D S B

N+ P+ NN P+ P P N+

N well

P Substrate

In order to keep the source and drain PN-junctions with the substrate (or bulk) reverse biased at all times, i) The P-substrate (for NFETs) is generally tied to the most negative voltage in the circuit ii) The N-substrate or N-well (for PFETs) is tied to the most positive voltage in the circuit

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

The Common Gate Amplifier: Small Signal Model i ig Gate Drain id out + v gs r g v o + - m gs gmbvbs R vout RL + Source - - vbs RS + vin + v s - - Base

Note:

vbs  v gs

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

3 The Common Gate Amplifier: Short Circuit Current Gain

ig Gate Drain id + v gs r g v o - m gs gmbvbs R Source - iout vbs + i test Base

Short circuit current gain:

itest  id  iout iout Ai   1 Short circuit current gain is unity! itest

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

The Common Gate Amplifier: Input Resistance i ig Gate Drain id out + v gs r g v o + - m gs gmbvbs R vout RL Source vs  vgs - - vbs i test + + Base vtest -

Input resistance: v  v  gm  gmb ro  1 out gs itest  id    vgs id  gm  gmb vgs   ro  R || RL  ro v vtest vgs ro  R || RL gs R || RL Rin      gm  gmb vgs   id itest itest gm  gmb ro  1 ro ro v R can be small if: g  g v  gs in m mb gs r i  o ggrmmbo1 d R || R 1 L ro

rRRoL ||1  RR || L gm  gmb ro  1 R 1   vgs in Small r  R || R ggrmmbo1  gg mmb r o o L

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

4 The Common Gate Amplifier: Open Circuit Voltage Gain ig Gate Drain id + v gs r g v o - m gs gmbvbs R + Source - vout vbs - + itest + Base vtest -

Open circuit voltage gain:  g  g r  1  1  v  v v  i R  R m mb o  v   g  g   R || r v out gs out d   gs  m mb o gs id  gm  gmb vgs   ro  R   ro  ro v v  1  vgs R out out g g v i  Av      gm  gmb  R || ro  m  mb gs   d vtest vgs  ro  ro ro gm  gmb ro  1 id  vgs If: ro  R gm  gmb ro  1  1   Av   gm  gmb  R || ro ~ gm  gmb R || ro Large if R is large  ro 

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

The Common Gate Amplifier: Output Resistance

ig Gate Drain id + v gs r g v o itest - m gs gmbvbs R Source - + vs  vgs v bs vtest - + RS Base RD

Output resistance:

vtest  v gs v gs id  gm  gmb v gs    ro Rs  1  ro || || Rs   gm  gmb   vgs  vtest ro vtest roRs RD    ro  Rs 1 ro gm  gmb id  1  ro || || Rs   gm  gmb 

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

5 The Common Gate Amplifier: Output Resistance

ig Gate Drain id + v gs r g v o itest - m gs gmbvbs R Source - + v bs vtest - + RS Base RD

Output resistance:

RD  ro  Rs  roRs gm  gmb 

Rout  R || RD  Rout depends on Rs  R || ro  Rs  roRs gm  gmb

Rout can be large if R is large

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

The Common Gate Amplifier: Transimpedance Gain ig Gate Drain id + v gs r g v o - m gs gmbvbs R + Source - vout vbs - + itest + Base vtest -

Transimpedance gain:

itest  id

vout idR Rm     R itest itest

Rm can be large if R is large

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

6 The Common Gate Amplifier: A Current Buffer ig Gate Drain id

+ iout v gs r g v o + - m gs gmbvbs R vout RL Source - - vbs + is RS Base

Compare with the standard current amplifier model:

Rout can be large i i in out R is small + + in i R Ai iin R v R s S Rin out out L A is unity - - i It is a good current buffer!!

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

The Common Gate Amplifier: A Transimpedance Amplifier ig Gate Drain id

+ iout v gs r g v o + - m gs gmbvbs R vout RL Source - - vbs + is RS Base

Compare with the standard transimpedance amplifier model: Rout can be large R iin out Rin is small + + + i R R i v R can be large s S Rin - m in out RL m - - It is a decent transimpedance amplifier!!

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

7 The Common Drain Amplifier (or the Source Follower) VDD

R

I + i D D d

IG + ig G

RS S IOUT + iout + IS + is v s - + + Ro RL VOUT  vout - VBIAS -

The drain terminal is “common” between the input and the output The common drain amplifiers are useful when large input resistances and small output resistances are desired in voltage amplifiers The voltage gain is less than unity! Note: The bulk is not tied to the source

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

The Common Drain Amplifier: Small Signal Model

ig Gate Drain id + v gs ro R Rs - gmvgs gmbvbs i + s v s Source - + - v R bs vout R o + L Base -

Note: vbs  vout

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

8 The Common Drain Amplifier: Open Circuit Voltage Gain i Drain i g Gate d vd + v gs ro R - gmvgs gmbvbs + vtest is - Source - + v R bs v o + out Base -

Open circuit voltage gain:

vtest  vgs  vout  vgs  idRo vd  vout id  gmv gs  gmbvout  ro vout gmrovgs  1 gmbro vout   id   i R  v Ro ro  R  g v  g v  d out m gs mb out r  1 1 g r  g r g r o  v   mb o   v m o  v  v m o out  R r  R  gs r  R test out r  R gmrov gs  1 gmbro vout  o o  o o  id  ro  R  1 1 gmbro gmro  gmro  vout      vtest  Ro ro  R ro  R  ro  R gmroRo v r  R  A  out  o  1 v g  g r R Less than unity – but can be very vtest 1 m mb o o ro  R close to unity

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

The Common Drain Amplifier: Input Resistance

ig Gate Drain id + itest v gs ro R - gmvgs gmbvbs i + s vtest Source - + v - R bs v R o + out L Base -

Input resistance:

vtest Rin    Large itest

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

9 The Common Drain Amplifier: Output Resistance i Drain i g Gate d vd + v gs ro R - gmvgs gmbvbs R i s is test Source - v + R bs v o + test Base - Output resistance: vtest vd  vtest itest   id id  gmv gs  gmbvtest  Ro ro v g r v  1 g r v  id R  vtest  test  m o gs mb o test  gmv gs  gmbvtest  ro Ro ro  R gmrov gs  1 gmbro vtest vtest gmro  1 gmbro  id    vtest ro  R Ro ro  R 1 itest 1 gm  gmb ro  1     Rout can be small if: Rout vtest Ro ro  R ro >> R (gm+gmb) ro >>1 and/or if Ro is small

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

The Common Drain Amplifier: A Voltage Buffer

ig Gate Drain id + v gs ro R Rs - gmvgs gmbvbs i + s v s Source - + v - R bs v R o + out L Base -

Compare with the standard voltage amplifier model: Rout can be small

Rin is large Rout RS A can be almost + + + v v R A v vout R unity + in in - v in L v s - - - It is a good voltage buffer!!

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

10 The CS, CG, and CD Topologies

The common source (CS) topology: 1) Large input resistance 2) Large output resistance 3) Decent voltage gain 4) A decent voltage amplifier 5) A decent transconductance amplifier

The common gate (CG) topology: 1) Can have small input resistance 2) Large output resistance 3) Decent voltage gain 4) Unity current gain 5) A good current buffer 6) A decent voltage amplifier 7) A decent transimpedance amplifier

The common drain (CD) topology: 1) Large input resistance 2) Small output resistance 3) Less than unity voltage gain 4) A good voltage buffer

ECE 315 – Spring 2007 – Farhan Rana – Cornell University

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