Topic 12 Reference n Conc

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Topic 12: Differential Amplifier

Objectives of this Chapter

Having completed this chapter we will be able to:

• Determine by measurement the offset null voltage in a differential

amplifier circuit.

• Calculate voltage gain from voltage measurements.• Determine quiescent voltages for a differential amplifier circuit.

• Recognize differential circuit action for a BJT pair.

• Diagnose a fault in a differential amplifier circuit.

E!ipment "e!ired for this Chapter

• Circuit # of D!""" $.$ %emiconductors$ &odule.

• %horting lin's and connecting leads.

• %ignal (enerator.

• )scilloscope.

• *nalog &ultimeter +ith a ,"- DC range.

• Digital &ultimeter +ith $""m- DC and $"- *C ranges.


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E#ercise $%1: DC and &!iescent Conditions

Both transistors share a common emitter circuit. The current in this branch is fied. *ny increase in

current in one of the transistors must be matched by a reduction in the other.

*n increase of current in one transistor +ill cause its collector voltage to fall/ due to the voltage

dropped across its collector load resistor. The current in the other transistor must fall/ causing its

collector voltage to rise by a similar amount.

The output can be ta'en either differentially bet+een the t+o collectors or from only one of the

collectors to ground. 0f the output is ta'en differentially then the voltage difference +ill be t+ice that

measured bet+een one of the collectors and ground.

• %horting lin's bet+een soc'ets .1 2 .3/ .,, 2 .,$/ ., 2 .,4 and .$" 2 .$, +as

connected

• The digital multimeter/ s+itched to the $""m- DC range to soc'ets . 5positive6 and .7

5common6 +as connected• The analog multimeter/ s+itched to the ,"- DC range to soc'ets .8 5positive6 and .$$

5common6 connected

• The module po+er supplies +as s+itch )9.

• The setting of -R! +as varied and the analog multimeter +as +atched as -R! is turned

cloc'+ise the meter pointer moves countercloc'+ise.

• The positive connection of the analog multimeter +as transferred to soc'et .,7. Repeat the

variation of -R! and note that this time the pointer moves cloc'+ise in the same direction as

-R!.

• -R! +as read:usted to give 1- indicated on the analog multimeter/ then the common

terminal of this meter +as transfered from soc'et .$$ to the other collector at soc'et .8 tomeasure the differential output voltage. The reading of about ;$-.

• -R! +as ad:usted again to reduce this to zero/ changing to a more sensitive range as the

circuit approaches null balance.

• The analog multimeter +as removeed from the circuit.


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• The digital multimeter +as used to read and record the voltages +ith respect to ground at the

points indicated in Table .,/ changing the range as required in order to obtain accurate

readings<

TR7 TR1 Junction of

=mitter

ResistorsCollector Base =mitter Collector Base =mitter

.8 . .," .,7 .$" .,1 .,

- m- - - - - -

Table $%1

'ote< The offset null voltage is the difference bet+een TR7 and TR1 base voltages. >ith the base

of TR1 grounded/ the offset null voltage is equal to the base voltage of TR7.

$%1a Enter (o!r val!e of offset n!ll voltage in m)%

????????..

9ote that the offset null voltage may be appreciable/ because of differences bet+een the

characteristics of the t+o transistors.

E#ercise $%2: Alternating *ignal Drive

*n alternating signal needs to be applied to the base of only one of the transistors. %ignal currents

+ill be coupled to the other transistor via the common emitter circuit.

%tage gain +ill be lo+ because of the lac' of decoupling of the emitter resistors. The differentialnature of the t+o outputs allo+s signals of either polarity to be available/ or t+ice the output voltage

if ta'en differentially bet+een the t+o collectors.


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• shorting lin's bet+een soc'ets . 2 .4/ .1 2 .3/ .,, 2 .,$/ ., 2 .,4 and .$" 2

.$, +ere connected.

• The signal generator +as set to ,'@z sine+ave/ minimum amplitude setting.

• The oscilloscope +as set up as follo+s<

Timebase to ".$msAdiv/ trigger selector to *C/ dual trace operation.

The C@., located trace t+o divisions do+n from the top of the display.

The C@.$ located trace t+o divisions up from the bottom of the display.

C@., amplifier gain to $-Adiv/ *C input.C@.$ amplifier gain to $-Adiv/ *C input.

• $mm connecting leads +as used to connect C@., of the oscilloscope to soc'et .$ to monitor

the input to TR7/ and C@.$ to soc'et .8 to monitor its output.

• The module po+er supplies +as s+itch )9.

• The input 5signal generator amplitude control6 +as increased to give 3-pp output 5C@.$6.

• The +aveforms of the input and output voltages +as s'etched on the graticule provided

belo+/ labeling each +aveform 5for eample/ TR7 base/ etc6.

• The C@.$ $mm connecting leads +as transferred from soc'et .8 to soc'et .," to monitor

the emitter signal +aveform and add this to your s'etch in >aveform %'etch .,/ labeling itas before.

• The C@.$ $mm connecting leads +as transferred to soc'et .,7 to monitor TR1 collector

signal +aveform and add this to your s'etch in >aveform %'etch .,/ labeling it as before.

'ote: ou cannot inspect the differential output +aveform bet+een the t+o collectors because of

the ground connections of the oscilloscope.

• The oscilloscope $mm connecting leads +as removed from the circuit.

• the digital multimeter +as s+itched to read $"- AC and connected to read the base input

voltage at soc'ets .$ 2 .7. This time it is reading signal voltage and not the DC 5quiescent6conditions. 9ote the value in Table .$.


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0nput %ignal TR7 )utput TR1 )utputDifferential

)utput

- - - -

Table $%2

• The meter positive terminal +as transferred to soc'et .8 to read TR7 output and enter this in

Table .$.• TR1 output +as repeated at soc'et .,7.

• The common connection of the meter +as transferred from soc'et .7 to .8/ to read the

differential output signal bet+een the t+o collectors/ and +as entered in Table .$ as before.

'ote: This can be done since/ unli'e the oscilloscope/ the meter is battery operated and has no

ground connection.

• The stage gain )utput 0nput for TR7/ TR1 and differentially from both +as calculated and

entered in Table .!<

Table $%+

$%2a

Enter

(o!r val!e of T", *tage -ain%

$%2b Enter (o!r val!e of T". *tage -ain%

$%2c Enter (o!r val!e of Differential *tage -ain%

TR7 %tage (ain TR1 %tage (ain Differential %tage (ain


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/or0sheet /$

The anagement Comp!ter has inserted a fa!lt into Circ!it $%

• the circuit of ig .,+as connected and the module po+er supplies +as s+itch )9.

• -R! +as ad:ustedas in =ercise .,/ then the circuit voltages as before witho!t signal

applied 5no shorting lin' bet+een soc'ets . 2 .46 +as measured and recorded.

TR7 TR1 Junction of

=mitter

ResistorsCollector Base =mitter Collector Base =mitter

.8 . .," .,7 .$" .,1 .,

- m- - - - - -

Table $%$

• The shorting lin' bet+een soc'ets . 2 .4 +as connected to in:ect the signal as previously

instructed 5,'@z 3- pp at the output6.

• The output +aveforms at both collectors 5.8 and .,76 +as s'etched on the graticule

provided.

• rom comparison of the voltages against those previously recorded in Table .,/ and from

observation of the circuit +aveforms/ the fault +as deduced.

• * further test or tests 5resistance measurement or characteristic test +ith the oscilloscope6

+as carried out to confirm your diagnosis.

• Diagnosis +as recorded and entered belo+


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"eport

Circuitaulty

Component

9ature of

aultReason for Diagnosis

3a!lt "esponse: Enter the fa!lt which (o! have diagnosed%

"eason Record your reason for your o+n later benefit.

• 'ow that (o! have completed the fa!lt e#ercise4 the fa!lt has been removed b( the

anagement Comp!ter%


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*t!dent Assessment $

,. 0n a differential amplifier the quiescent collector voltages of the t+o transistors should be<

a6 of opposite polarity. b6 the same.

c6 one!

,and the other

!

$of the supply. d6 at ground potential.

$. 0n a differential amplifier +ith t+o transistors TR, 2 TR$/ if the collector current of TR,increases<

a6 collector current of TR$ also increases.

b6 TR, collector voltage increases.

c6 TR$ collector voltage falls.

d6 collector current of TR$ is reduced.

!. 0f both inputs of a differential amplifier are grounded then theoretically there should be no

difference bet+een the t+o collector voltages. >hich of the follo+ing is not a li'ely cause of an

offset voltage being presentE

a6 incorrect value of supply voltage. b6 spreads of parameters of transistors.

c6 tolerance of resistor values. d6 a faulty component.

. 0f a positive signal is applied to TR, input only 5TR$ input grounded6 then<

a6 TR, collector voltage only +ill go positive.

b6 TR, collector voltage +ill go positive and TR$ negative.

c6 TR$ collector voltage only +ill go negative.

d6 TR, collector voltage +ill go negative and TR$ positive.

4. 0n full differential operation the signals +ill be<

a6 applied to both bases in opposite polarity/ and ta'en out bet+een both collectors.

b6 applied to one base only/ and ta'en out bet+een both collectors.

c6 applied to both bases in opposite polarity/ and ta'en out from only one collector.d6 applied to one base only/ and ta'en out from only one collector.

7. Refering to the circuit of ig .!/ the quiescent voltages are given in rectangular boes mar'ed

-,/ -$/ -!/ -. 0f -,F;4.4- and -!F"-/ then<a6 -$ F 4.4-/ - F "- b6 -$ F ;4.4-/ - F "-

c6 -$ F ;4.4-/ - F ".7- d6 -$ F 4.4-/ - F ".7-


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1. Refering to the circuit of ig .!/ the ,' Gvariable resistor -R, is ad:usted for circuit balance so

that<

a6 both emitter voltages are identical. b6 both base voltages are identical.

c6 both collector voltages are identical. d6 both input signals are identical.

3. Refering to the circuit of ig .!/ the signal voltages are given in circles mar'ed %,/ %$/ %!.

>hich of the follo+ing statements is trueE

a6 Differential gain F %,

%!%$ +

and %$ is the same as %! and of the same polarity.

b6 Differential gain F%,

%!%$ −and %$ is the same as %! and of the same polarity.

c6 Differential gain F%,

%!%$ +and %$ is the same as %! but of opposite polarity.

d6 Differential gain F%,

%!%$ −and %$ is the same as %! but of opposite polarity.


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Department of =lectronic =ngineering

aculty of =ngineering

Hniversiti &alaysia %ara+a'

5'6 12,2

*nalog and Digital =lectronic *pplications

%emiconductor 00

Iaboratory &anual

Topic 12: Differential Amplifier

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&atric 9o <

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Topic 12 Reference n Conc