1 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Figure 6.32 Biasing the BJT amplifier at a point

1Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Figure 6.32 Biasing the BJT amplifier at a point Q located on the active-mode segment of the VTC.

A.1 Large Signal Operation-Transfer Charact.

2

A.1 Large Signal Operation-Transfer Charact.

V O CE CC C CR i

/ V / VBE T I TC S Si I e I e

/ VV I TO CC C SR I e sat

sat

V V CC CE

CC

IR

3

/ V (5.53)BE TC SI I e

V V (5.54)CE CC C CR I

I V

ABE

O

I

d

d

V / V1A

VBE T

S CT

I e R

VA (5.56)

V VC C RC

T T

I R

V V V (5.57)RC CC CE

A.2 Amplifier Gain

BJT is biased at a point in active region called Quiescent point

4

VCE CC C Ci R V 1CC

C CEC C

iR R

A.3 Graphical Analysis

5

IB must be defined previously.

Q is quiescent bias point



7


Small signal analysis around the bias Q point

8

A.3 Operation as a Switch

V I BE

BB

iR

C Bi i

V C CC C CR i

(EOS)

V 0.3 CC

CC

IR

(EOS)(EOS)

CB

II

(EOS) (EOS)V V I B B BEI R Utilize the cutoff and

saturation modes.

Edge of saturation (EOS)

satsat

V V CC CE

CC

IR

9

A.4 Small Signal Operation and Models

V / V BE TC SI I e / E CI I

/ B CI I V V VC CE CC C CI R

DC bias conditions are set by these equations.

10

VBE BE be

/ VV / V(V )/ V/ V be TBE TBE be TBE TC S S Si I e I e I e e

/ V be TC Ci I e 1 for small

Vbe

C C beT

i I

VC

C C beT

Ii I

A.4.1 collector current and transconductance

Small signal approximation

Small signal component

VC

c beT

Ii c m bei g

VC

mT

Ig

C C

Cm

BE i I

ig

whereor or

gm is called

transconductance !

11

A.4.1 collector current and transconductance

Small signal approximation is restricted to an almost

linear segment of i-v curve.

12

1

VC C C

B beT

i I Ii

B B bi I i 1

VC

b beT

Ii

m

b be

gi

m

rg

VT

B

rI

be

b

ri

A.4.2 base current and input resistance at base

or

Small signal r is defined for small signal ib

orTherefore,

is called small signal base resistance

13

C C CE

i I ii

E E ei I i

V V

C C Ee be be

T T

i I Ii

bee

e

ri

V T

eE

rI

A.4.3 emitter current and input resistance

For small signal vbe

Small signal reis defined for small signal ie

Therefore, is called small signal emitter resistance

1 e

m m

rg g

( 1) er r r and re relationship


Figure 6.38 Illustrating the definition of rπ and re.


Figure 6.39 The amplifier circuit of Fig. 6.36(a) with the dc sources (VBE and VCC) eliminated (short-circuited). Thus only the signal components are present. Note that this is a representation of the signal operation of the BJT and not an actual amplifier circuit.

16

V V ( )

(V ) VC CC C C CC C c C

CC C C c C C c C

i R I i R

I R i R i R

( ) c c C m be C

m C be

i R g R

g R

A cm C

be

g R

A VC C

T

I R

A.4.4 Voltage Gain

or

Voltage gain of amplifier is

Voltage gain is directly proportional to collector current Ic.

17

A.4.5 Separating Signal and DC quantities

Amplifier circuit with DC sourcesEliminated (short circuited)

=> We will make equivalent small signal circuit using equivalent small signal transistor model

• Voltage and current are composed of DC and signal

components.

• since ideal dc supply voltage does not change, the

signal voltage across it will be zero.

18

(1 )

(1 )1

/

be bee m be m

bebe

be e

i g g rr r

r

r

r

( )

( )m be m b

m b b

g g i r

g r i i

A.4.6 The Hybrid- Model

• the equivalent small signal circuit model

19

A.4.7 The T Model

(1 )

(1 ) 11

( 1)

be beb m be m e

e e

be be

e e

be be

e

i g g rr r

r r

r r

( )

( )m be m e e

m e e e

g g i r

g r i i

20

A.4.8 Application of small signal equivalent circuits 1. Determine DC operating point of BJT (particularly Ic)

2. Calculate values of small signal model parameters

such as

gmm = Ic/VT, r = /gm, and re = VT/IE.

3. eliminate DC sources by replacing DC voltage with

short circuit and DC current with open circuit.

4. Replace BJT with one of small signal equivalent

circuit models.

5. Analyze the resulting circuit !

21

V V0.023 mABB BE

BBB

IR

2.3 mAC BI I

V V 3.1 VC CC C Ci R

V10.8 T

eE

rI

92 mA/VVC

mT

Ig

1.09 m

rg

Small signal model parameters

DC operating point

A.4.8 Application of small signal equivalent circuits

- model used !

22

1.09 0.011 (5.105)

101.09

be iBB

i i

r

r R

92 0.011 3 3.04o m be C

i i

g R

A 3.04 V/V (5.106)o

i


23


10 V0.93 mAE

EE

IR

0.92 mACI

V 10 5.4 VC C CI R

0.99

V 25 mV27

0.93 mAT

eE

rI

A 183.3 V/Vo

i

DC operating point

Small signal model parameters

24

A.4.10 Small signal model to account for Early effect.

+VA CE Ao

C C

V Vr

I I ( // )o m be C og R r

Early effect

In most cases, since ro >> RC, reduction in gain is not

critical.

Furthermore, we can neglect ro in our analysis for

simplifying the circuit analysis.

25

A.4.10 Small signal model to account for Early effect.

26

A.5 Single Stage BJT Amplifier

27

Table 5.5

A.5 Single Stage BJT Amplifier

28

A.5.1 The common emitter (CE) amplifier- AC ground at

emitter

- CE is bypass

capacitor

- CC1 is coupling

capacitor

iin B ib

i

R R Ri

ibR r

inR r

Small signal model for circuit

29

A.5.2 CE Amplifier with emitter resistance

in B ibR R R

iib

b

R

(1 )1

eb e

ii i

i

ee e

ir R

( 1)( ) ib e eR r R


and

- It says that input resistance looking into base is +1 times

total resistance in emitter (resistance reflection rule)

30

(with included) ( 1)( )1 1

(without ) ( 1)ib e e e e

m eib e e e

R R r R Rg R

R R r r

- Inclusion of RE in emitter can substantially increase the input

resistance.

- Therefore, designer can control Rin by controlling value of RE.

( )

( )o c C L

e C L

i R R

i R R

( )

A o C L

i e e

R R

r R

( )A C L

e e

R R

r R A C

oe e

R

r R

Now we determine the voltage gain

and


~ 1

- voltage gain from base to collector is equal to ratio of collector

resistance to emitter resistance.

31

A1 /

Co

e e e

R

r R r

A 1 / 1

m C m Co

e e m e

g R g R

R r g R

out CR R

os ei i in/i ii R inA eis

i

R i


Avo can be expressed in other form.

There is trade between increase in input resistance and decrease in

voltage gain by factor of 1+gmRe

Output resistance :

and

( )A B ibis

e e

R R

r R

( 1)( )

A e eis

e e

r R

r R

if RB >> Rib

Rib=(+1)(re+Re)

32


Summary of CE amplifier with emitter resistance

- Input resistance is increased by factor of 1+gmRe.

- The voltage gain from base to collector is reduced by

factor of 1+gmRe.

- For the same nonlinear distortion, input signal can be

increased by factor of 1+gmRe.

- The overall voltage gain is less dependant on .

- The high frequency response is significantly improved.

33

A.5.3 The Common Base (CB) Amplifier

in eR r ( )o e C Li R R

ie

e

ir

A ( ) ( )o

C L m C Li e

R R g R Rr


and

34

A.5.3 The Common Base (CB) Amplifier

Summary of CB amplifier with emitter resistance

- Input resistance is very low (re).

- Short circuit current gain is nearly unity ().

- Like CE amplifier, it has high output resistance RC.

- A very importance application of CB amplifier is current

buffer.

35

A.5.4 The Common Collector (CC) Amplifier

CC amplifier is commonly used and known by name of emitter follower.

Redrawn for rO parallel with RL.

Unlike CE and CB, CC amp. is notunilateral because Rin depends onoutput RL !



Figure 5.2 The enhancement-type NMOS transistor with a positive voltage applied to the gate. An n channel is induced at the top of the substrate beneath the gate.









Figure 5.10 Cross-section of a CMOS integrated circuit. Note that the PMOS transistor is formed in a separate n-type region, known as an n well. Another arrangement is also possible in which an n-type body is used and the n device is formed in a p well. Not shown are the

connections made to the p-type body and to the n well; the latter functions as the body terminal for the p-channel device.





Figure 5.20 The relative levels of the terminal voltages of the enhancement-type PMOS transistor for operation in the triode region and in the saturation region.



Figure 5.28 Biasing the MOSFET amplifier at a point Q located on the segment AB of the VTC.




Figure 5.31 Graphical construction to determine the voltage transfer characteristic of the amplifier in Fig. 5.29(a).


Figure 5.33 Two load lines and corresponding bias points. Bias point Q1 does not leave sufficient room for positive signal swing at the drain (too close to VDD). Bias point Q2 is too close to the boundary of the triode region and might not allow for sufficient negative signal swing.


Figure 5.43 The three basic MOSFET amplifier configurations.



Figure 5.49 Illustrating the need for a unity-gain buffer amplifier.


Figure 5.57 (a) Common-source amplifier based on the circuit of Fig. 5.56. (b) Equivalent circuit of the amplifier for small-signal analysis.

Documents

1 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Figure 6.32 Biasing the BJT amplifier at a point