10-DiffAmpDesign

8/2/2019 10-DiffAmpDesign

1/15

Practical Differential Amplifier DesignWeve discussed

Large signal behaviour

Small signal voltage gainToday:

Input impedance

Output impedance

Coupling & biasing D.C. effects

Comparisons with the common-emitter amplifier


2/15

Input and Output Impedances

An equivalent small signal circuit of a differential amplifiercan bedrawn as:


3/15

Input ImpedanceDuring the small signal analysis, itwas shown that:

( )m

C

m

CCC

m

BBg

i

g

iiigvv

21

2121

221

===

BxCx ii =But,

m

BBB

givv 1

212=

mB

BBin

gi

vvr

2

1

21 =

=


4/15

Output Impedance00Set == CIN iv

Applying Kirchoffs current law:

RCOUTOUTRCC iiiii ==+= 0

RC

OUT

C

RC

C

CRCC

i

vR

i

vRIV ===15

By Ohms law:

( ) CCRC

OUT

OUT

OUTOUT RR

i

v

i

vr ====

NB. Same result as common emitter amplifier


5/15

Coupling and Biasing Input and output coupling

capacitors may be requiredto remove d.c. bias voltages

If input coupling capacitorsare used, a d.c. bias currentpath to the transistors basesmust be established

Extra base resistors

accomplish this These will appear in parallel

with the input impedance


6/15

Constant Current Source

Current, I, should be constant regardless ofvarying VE

In practice, during small signal operation VEdoesnt vary by more than a fraction of a voltso a resistor is a good approximation (as inthe lab experiment)

For a better approximation, a current mirroris often used


7/15

Current Mirror

VBE

VBE is unknown, but should be around 0.5 V

So, ( )REFREF

BERC

RRVII

REF5.14150 =+=

=

T

BESC

V

VII exp

Exactequilibrium value of VBE is set by negative

feedback and can be found from:


8/15

Current Mirror (cont)

=

T

BESC

V

VII exp

VBE is identical for both transistors and

So, 4343 CCBEBE IIVV ==

But we know,REF

CR

I 5.143 =

REF

C

RI

5.14

4=


9/15

Practical Amplifier with Coupling


10/15

Non-Ideal D.C. Effects If operation down to d.c. is required, the

coupling components are omitted

This leads to some effects that are peculiar tod.c. operation: Offset Voltage Bias Current


11/15

Offset Voltage With zero differential input, the collector

currents and, therefore, the collectorvoltages should be identical

This assumes that: The transistors are identical The loads are also identical

In practice, loads will vary and thequiescent conditions will not be perfectly

symmetrical There will be an offset voltagebetween

the actual output and the idealassumption


12/15

Bias Current In order to bring the transistors into the

active region, a small d.c. base biascurrent is required

This d.c. current must be supplied by thesignal source

This is a separate issue to the current

drawn by the input impedance Note that bias currentand offset voltage

effects are identical to those observedwith op-amps

/CxBx II =


13/15

Applications Differential inputs and outputs

Useful when negative feedback is required in a multi-stageamplifier

Also useful for balancedsignals

Transmitter

Noisy Channel

Noisy receivedsignals

DifferenceAmp

Output


14/15

Comparisons with CE Amp Common Emitter Features

One transistor required Single input, single output Maximum input amplitude for linear operation around 1 mV High gain possible with high input impedance

Differential Features At least two transistors required Differential input, differential output Maximum input amplitude for linear operation around 50 mV

Reduced gain possible with high input impedance


15/15

Multi-Stage Amplifiers With both common-emitter amplifiers and differential

amplifiers, a design compromise must be struckbetween: Voltage gain Input impedance Output impedance

Simultaneously achieving specified requirements may

not be possible using a single amplifier Solution: cascade more than one amplifier in series

More on this next time

Documents

10-DiffAmpDesign