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Lab 5Differential Amplifiers
Objective: This project will focus on single stage differential
amplifiers. Both BT and !"T amplifiers will
be e#amined as will the use of resistor and current source
biasing.
$omponents: %&%%%% BT' %&())) !"T
ntroduction:
The operational amplifier has had a dramatic impact on
electronic circuit design' both analog and digital'over the last %5
+ears. ,hile the comple#it+' speed and capabilit+ of the Op-Amp
have changed
dramaticall+ over this time' the basic operation still depends
heavil+ on the input differential amplifier stage.
t is this differential amplifier stage that will be e#amined in
this project. The differential amplifier isdesigned to effectivel+
shift a constant current between two branches as a function of the
difference between
he two input signals. *deall+' as a result of the changing
current' the amplifier output reflects onl+ the
difference between the inputs. The ualit+ for the amplifier
design is determined' in part' b+ e#amining the
output of the differential amplifier under two specific input
conditions. The ratio of the differential modevoltage gain /AD01
2inputs are eual in magnitude and opposite in sign3 to the common
mode voltage gain
A$01 2both inputs are eual3 is used to determine the $ommon 0ode
4ejection 4atio 2$0443. The
higher the ratio' the better the differential amplifier stage is
able to discriminate between the actualdifference in the signals
present at the two input terminals. The input impedance is another
important
measure of the ualit+ of the differential amplifier stage. These
two items' $044 and in' will be the
primar+ focus of this project.
!igure 6(-6 illustrates a BT based differential amplifier and
!igure 6(-% shows an !"T based stage. Thedifferential output
versions 2!igures 6(-6 2A3 and 6(-% 2B33 have a resistor in each
branch and the output is
measured between the two collectors 2drains3. 0an+ differential
amplifiers are designed as single ended
outputs since the information contained in either of the
collector 2drain3 terminals is sufficient to determinehe
differential input. !igures 6(-6 2B3 and 6(-% 2B3 illustrate the
single ended designs. *n the single ended
ase one of the branch resistors 246 for e#ample3 is removed and
replaced b+ a short circuit. The
determination of the non-inverting 2783 and inverting 27&3
input terminal is made b+ loo9ing at theelationship between a
change on the input terminal and the corresponding change in the
output voltage. The
non-inverting terminal causes an increase in the output voltage
for an increase in the input signal. There is a
); phase shift between the inverting input signal change and the
output signal change. The current sourcellustrated between each
diagram is generall+ implemented using an appropriate BT or !"T
current mirror.
A discussion of current mirrors can be found in 8rojects and
6
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!igure 6( - 6: BT Differential Amplifier
!igure 6( - %: !"T Differential Amplifier
Design:
. Design a single ended BT differential amplifier capable of
providing a = 6) 7 output swing across a 6 9
esistor using = 65 7 D$ power supplies. The switching current
should be supplied b+ a BT current mirror.ndicate the value of 7out
when both inputs are grounded. 7erif+ +our design 8>8*$"?.
%. 4epeat step 6 using a single resistor to provide the
switching current.
. Design a single ended !"T differential amplifier capable of
providing a = 5 7 output swing across a 6 9
esistor using = 65 7 D$ power supplies. The switching current
should be supplied b+ an !"T currentmirror. *ndicate the value of
7out when both inputs are grounded. 7erif+ +our design
8>8*$"?.
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@. 4epeat step < using a single resistor to provide the
switching current .
Lab 8rocedure:
. $onstruct the differential amplifier designed in step 6 of the
design procedures. 7erif+ the circuitoperation with both inputs
grounded. Be careful in ma9ing voltage measurements so as not to
effectivel+
b+-pass +our current source.
%. Appl+ a differential voltage signal 273 to each input. The
individual input voltages should be eual inmagnitude but opposite
in polarit+. 0easure the output voltage and determine the
differential mode voltagegain 2AD03. 0easure the input current for
terminal and determine the effective input impedance as seen b+
he total differential input voltage 2%73. Be careful not to over
drive the amplifier.
. Appl+ a common mode signal 2eual magnitude and same polarit+3
to the two inputs. Adjust +our
ommon mode voltage to the total differential voltage used in
step % 2% 73. 0easure the output voltageand determine the common
mode voltage gain 2A$03. Again' be careful not to overdrive the
amplifier.
Determine the input impedance for this input condition. $ompare
this input impedance with the impedance
determined in step %. Discuss possible causes for an+
differences between the two values.
@. Determine the $044 2AD0A$03 for the amplifier.
5. 4everse the polarities on the inputs for steps % and < and
determine the differential mode gain' common
mode gain' and $044 for the revised inputs. $omment on an+
similarities andor differences.
C. 4epeat steps % - 5 for each of the other three differential
amplifier designs.
(. 8repare a summar+ of the various measurements and results for
all the tests. Anal+e the summar+ dataand provide a brief
discussion of the differencessimilarities between the various
designs.
Euestions:
. $ould an+' or all' of these circuits be designed using a
single D$ power suppl+F "#plain +our answer.
%. ,hat' if an+' limitations are there on the value of the
common mode signalF Are the+ different for the
amplifiers designed using a current source when compared to
those using a single resistor for the currentuppl+F
. Are the two voltage gains' AD0 and A$0 affected b+ the value
of the input voltages usedF "#plain +our
answer.
@. *s the input impedance affected b+ the decision to use a
current mirror versus the single resistorF *f so'
how can the difference be e#plainedF
5. $omment on the benefitdisadvantage of using an !"T current
mirror for the BT based differential
amplifier. 4epeat for the reversed situation.
C. Discuss the benefitsdisadvantages of the single ended output
versus the differential output designs.
