Download DIFFERENTIAL PAIR:

DIFFERENTIAL PAIRS
Goal:
To construct a basic differential pair and to find its
a) Common Mode gain,
b) Differential gain,
c) Common Mode Rejection Ratio and
d) Slew Rate.
Set up:
SIMPLE DIFFERENTIAL PAIR
+10 V
10k
10k
x
y
-
+
A
10k
-10 V
Bias details:
1)
Connect the (+) and (–) terminals to the Vdd.
2)
Now adjust the 10 k pot so that x and y are approximately near 0V.
3)
Now disconnect + and – from the Vdd.
Operation:
OUTPUT OFFSET VOLTAGE
1) To find the output offset voltage, connect both (+) and (–) to ground.
2) Measure the output voltage between x and y. This is the o/p offset voltage.
COMMON MODE GAIN
+10 V
10k
10k
y
x
+
0.01F
-
10k
A
10k
-10 V
1) Here the main idea is we are finding the gain of the opamp by giving a
common signal to both the inputs.
2) First fix the DC point of X and Y by adjusting R bias (if necessary).
3) Give the same input signal [sine-50mVpp-100Hz-zero DC offset] both to
(+) and (–) terminals.
4) Using the oscilloscope, trace Vin and Vout.
5) Now Common mode gain = Vout / Vin.
How does the waveform at A look like. Why?
DIFFERENTIAL MODE GAIN
+10 V
10k
10k
x
0.01F
F
y
0.01F
10k
10k
10k
-10 V
PROCEDURE:
1) Here the idea is to find the gain of the opamp by wiggling the inputs in the
opposite directions i.e. we give a highly differential input.
2) First fix the DC point of X and Y by adjusting R
bias’
(if necessary).
3) Give the input signal [sine-50mVpp-100Hz-zero DC offset] to the (+) and (–)
terminals, as shown in the figure.
4) Using the oscilloscope, trace Vin and Vout.
5) Now Differential mode gain = Vout / Vin.
How does the waveform at A look like. Why?
COMMON MODE REJECTION RATIO
The Common mode rejection ratio is
CMMR = Differential mode gain / Common mode gain
SLEW RATE
+10 V
10k
10k
x
0.01F
F
y
0.01F
10k
10k
10k
-10 V
PROCEDURE:
1) Give a square wave [50mVpp – 100Hz-zero DC offset] to the differential input and
find the slope of the output waveform. This gives the maximum slew rate of this
differential pair.
Now replace the tail resistor with a current source. Repeat the whole procedure
and compare the values of common mode gain, differential mode gain, CMMR, slew
rate and justify.
RESULTS:
For the simple differential pair with a Tail Resistor,
a) Common Mode gain is ___.
b) Differential gain is ___.
c) Common Mode Rejection Ratio is ___.
d) Slew Rate is ___.
For the simple differential pair with a Tail Current Source,
d) Common Mode gain is ___.
e) Differential gain is ___.
f)
Common Mode Rejection Ratio is ___.
d) Slew Rate is ___.