Digital and Interfacing Systems
Lab 9
The Differential Amplifier
CENG 215
Name …………………….….
Objective:
To investigate the properties of a Differential Amplifier using an Op-Amp.
Equipment:
1. DVM, Scope, Function Generator, Parts kit
Introduction:
The differential amplifier (Diff Amp) is useful for increasing signal to noise
ratios. This circuit amplifies a difference signal connected to the 2 inputs. The
Diff Amp, for example, is used to amplify the output from a Wheatstone Bridge
circuit. It will not amplify two signals that are the same (common mode
signals) even if those signals are very large. The result is that desired signals
are amplified and noise voltages are actually attenuated, giving a very
desirable signal to noise ratio. This characteristic of a Diff Amp is expressed as
the Common Mode Rejection Ratio (CMRR) and is usually expressed in dB.
Procedure: Part 1- Differential Mode Gain
1. Set up the circuit shown at right. Use 1% precision resistors available from
your instructor.
2. Calculate the theoretical differential mode
voltage gain AV,DM.
AV, DM = ........................
3. Connect the left (open) end of R1 (VIN,1) to
ground. Set the function generator for about
0.5VP-P sine wave at 1 kHz with no DC offset,
and connect it to R2 at VIN,2
4. Using AC coupling on the scope, connect CH 1
to VIN,2 and CH 2 to VOUT. Measure the VP-P values
of both waveforms and using these values record
the measured differential mode gain AV,DM
VIN,P-P = …………… VOUT,P-P = ……….…..
AV,DM = ........................
Does this value agree with step 2 above ….…...
5. Draw sketches of VIN,2 and VOUT showing exact
voltage levels and phase relationships.
Are VIN,2 and VOUT in or out of phase?…………………….
Should they be in or out of phase and why? ……………………….
Procedure: Part 2 - Common Mode Voltage Gain of the Amplifier
6. To measure the common mode voltage gain AV,CM, of the amplifier
disconnect the left end of R1 from ground and connect VIN,1 and VIN,2
together. Connect the function generator to this common mode input now
referred to as VIN. The function generator signal represents a common mode
noise voltage.
7. Increase the amplitude of VIN to 6 to 8 volts.
Measure the VPK PK
values of both input and output signals. Turn the CH 2 scale to a
very small setting in order to view and measure VOUT. Determine the
measured Common Mode Gain AV,CM
VIN,P-P = …………… VOUT,P-P = ……….….. AV,CM = ………………
8. Draw sketches of VIN and VOUT for Step 7 showing
exact voltage levels and phase relationships.
9. The two gain readings of AV,DM and AV,CM can now be
used to calculate the Common Mode Rejection Ratio
(CMRR) of the amplifier. This ratio can be expressed
as a linear gain ratio or, more commonly, as a dB
ratio. First, the linear ratio is:
CMRR (Linear) = AV,DM / AV,CM
= …………..…………..
This means the desired signal will be amplified by that
factor compared to the noise signals.
The ratio expressed in dB is:
CMRRdB = 20 log CMRR (Linear)
= ……………………….. dB
This dB reading means the desired signal voltage level is that many dB
above the noise voltage level. Remembering that each 3dB drop reduces
the voltage to approximately 0.707 of what it was (or 1/2 power) it can
be seen that this gives a very good signal to noise ratio.
Procedure: Part 3 - Unbalanced Circuit
10. To achieve the best CMRR the differential op-amp relies on the value of RF
being identical to R3, and R1 being identical to R2. Any deviation from this
rule will reduce the CMRR and increase the undesired voltages at the
output. To illustrate this point we will deliberately increase the value of R1
by 10% making it 1kΩ more than R2. This means the non-inverting I/P
will have a larger (10%) signal than the inverting I/P.
11. Add an additional 1 kΩ resistor in series with R1 to increase its value 11
kΩ. Reconfigure the amplifier as you did in step 3 above so that you can
measure the differential mode gain once more.
12. Repeat steps 3 to 9 above to measure AV,DM and AV,CM again. Record the
results with this 10% R1/R2 ratio imbalance:
What is the new calculated value of AV,DM? ……………………….
Measurements
VIN,P-P = ….… VOUT,P- P = …..….. AV,DM = ....................
VIN.P -P = ….… VOUT,P- P = …..….. AV,CM = …………..
Calculate the new linear CMRR.
CMRR (Linear) = AV,DM / AV,CM
= …………..………..
Calculate the new CMRR in dB.
CMRRdB = 20 log CMRR (Linear)
=
dB
13. Calculate the difference in CMRR caused by the 10% variance in R1
=
dB
Difference CMRRdB =
dB
Original CMRRdB
New CMRRdB
=
dB
Question
The VIN voltage in Common Mode operation can be thought of as
representing a noise voltage. What happens to the magnitude of the noise
voltage at the output of the Differential amplifier when the CMRRdB
decreases?
Signed …………………………………………
Date……………………….