Microelectronic Circuit Design, 3E McGraw-Hill Chapter 14 Single-Transistors Amplifiers Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock.

Microelectronic Circuit Design, 3EMcGraw-Hill

Chapter 14Single-Transistors Amplifiers

Microelectronic Circuit Design

Richard C. Jaeger

Travis N. Blalock

Chap 14 - 1

Microelectronic Circuit Design, 3EMcGraw-Hill

Signal Injection and Extraction: BJT

• In forward-active region,

• To cause change in current, vBE = vB - vE must be changed. Base or emitter terminals are used to inject signal because even if Early voltage is considered, collector voltage has negligible effect on terminal currents.

• Substantial changes in collector or emitter currents can create large voltage drops across collector and emitter resistors and collector or emitter can be used to extract output. Since iB is a factor of F smaller than iC or iE currents, base terminal is not used to extract output.

TVBEv

F

SI

Ei

TVBEv

SI

Ci

exp

exp

TV

BEv

FO

SI

Bi exp

Chap 14 - 2

Microelectronic Circuit Design, 3EMcGraw-Hill

Signal Injection and Extraction: FET

• In pinch-off region,

• To cause change in current, vGS = vG - vS must be changed. Gate or source terminals are used to inject signal because even with channel-length modulation, drain voltage has negligible effect on terminal currents.

• Substantial changes in drain or source currents can create large voltage drops across drain and source resistors and drain or source can be used to extract output. Since iG is always zero, gate terminal is not used to extract output.

2

2

TNVGS

vnKDiS

i

Chap 14 - 3

Microelectronic Circuit Design, 3EMcGraw-Hill

Amplifier Families

• Constraints for signal injection and extraction yield three families of amplifiers

– Common-Emitter (C-E)/Common- Source (C-S)

– Common-Base (C-B)/Common- Gate (C-G)

– Common-Collector (C-C)/Common- Drain (C-D)

• All circuit examples here use the four-resistor bias circuits to establish Q-point of the various amplifiers

• Coupling and bypass capacitors are used to change the ac equivalent circuits.

Chap 14 - 4

Microelectronic Circuit Design, 3EMcGraw-Hill

Inverting Amplifiers: Common-Emitter (C-E) and Common-Source (C-S) Circuits

AC equivalent for C-E Amplifier AC equivalent for C-S Amplifier

Chap 14 - 5

Microelectronic Circuit Design, 3EMcGraw-Hill

Followers: Common-Collector (C-C) and Common-Drain (C-D) Circuits

AC equivalent for C-C Amplifier AC equivalent for C-D Amplifier

Chap 14 - 6

Microelectronic Circuit Design, 3EMcGraw-Hill

Inverting Amplifiers: Common-Base (C-B) and Common-Gate (C-G) Circuits

AC equivalent for C-B Amplifier AC equivalent for C-G Amplifier

Chap 14 - 7

Microelectronic Circuit Design, 3EMcGraw-Hill

Inverting Amplifiers: Summary

• C-E and C-S amplifiers have similar voltage gains.

• C-S amplifier provides extremely high input resistance but that of C-E is also substantial due to the f RE term.

• Output resistance of C-E amplifier is much higher than that of C-S amplifier as f is much larger for BJT than for FET.

• Input signal range of C-E amplifier is also higher than that of C-S amplifier.

• Current gains of both are identical to those of individual transistors.

• Following transformation is used to simplify circuit analysis by absorbing RE (or RS ) into the transistor (For FET, current gain and input resistance are infinite).

ERmg

mgmg

1' )1(' ERmgrr )1(' ERmgoror

ormgo ''' formgf

'''

Chap 14 - 8

Microelectronic Circuit Design, 3EMcGraw-Hill

Follower Circuits: Common-Collector and Common-Drain Amplifiers

AC equivalent for C-C Amplifier AC equivalent for C-D Amplifier

Chap 14 - 9

Microelectronic Circuit Design, 3EMcGraw-Hill

Follower Circuits: Terminal Voltage Gain

Neglecting ro,

LRmgL

RmgACC

vt

LRorL

Ro

bvov

vtA

1

)1(

)1(

1oAssuming

For C-S Amplifier, take limit of voltage gain of C-E amplifier asand

In most C-C and C-D amplifiers,

Output voltage follows input voltage, hence theses circuits are called followers. BJT gain is closer to unity than FET. Mostly,ro can be neglected as gain<< f

r

rmgo

LRmgL

RmgACD

vt

1

1 ACDvtACC

vt

175.0 vtA

1LRmg

Chap 14 - 10

Microelectronic Circuit Design, 3EMcGraw-Hill

Follower Circuits: Input Signal Range

For small-signal operation, magnitude of vbe developed across r in small-signal model must be less than 5 mV.

If , vb can be increased beyond 5 mV limit.Since only small portion of input signal appears across base-emitter or gate-source terminals, followers can be used with relatively large input signals without violating small-signal limits.

In case of FET, magnitude of vgs must be less than 0.2(VGS - VTN).

rL

R

LRmg

r

1

bv

ibe

vV)1(005.01005.0 LRmg

o

LR

LRmg

bv

1LRmg

)(2.01 TNV

GSV

LRmg

gv

gsv

)1)((2.0 LRmgTNVGS

Vgv

Chap 14 - 11

Microelectronic Circuit Design, 3EMcGraw-Hill

Overall voltage gain is

For C-S Amplifier,

Follower Circuits: Input Resistance and Overall Voltage Gain

Input resistance looking into the base terminal is given by

For C-S Amplifier,

iBCCR

vb

ib

r (o1)RL

r

iGCDR

vCCA

vovi

vovb

vb

vi

vtCCA

vb

vi

vtCCA

RB iB

CCR

RI R

B iBCCR

vCDA vt

CDAR

GR

IR

G

Chap 14 - 12

Microelectronic Circuit Design, 3EMcGraw-Hill

Follower Circuits: Voltage Gain Calculations (Example)

• Problem: Find overall voltage gain.

• Given data: Q-point values and values for RI, R1, R2, R4, R7 ,for both BJT and FET.

• Assumptions: Small-signal operating conditions.

• Analysis: For C-C Amplifier,

RB

R1

R2104k

RLR

4R

711.5k

iBCCR r (1gmR

L)10.2k[19.8mS(11.5k)]1.16M

vtCCA Avt

CC gmR

L1 gmRL

9.80mS(11.5k)

1+ 9.80mS(11.5k)0.991

vCCA vtCCAR

B iBCCR

RI R

B iBCCR

0.956

Chap 14 - 13

Microelectronic Circuit Design, 3EMcGraw-Hill

Follower Circuits: Voltage Gain Calculations (Example cont.)

• Analysis: For C-D Amplifier,

RG

R1

R2892k

RLR

6R

310.7k

vtCDA

gmRL

1gmRL

(0.491mS)(10.7k)1(0.491mS)(10.7k)

0.840

838.0

GR

IR

GR

ACDvtACD

v

Chap 14 - 14