06-08-2013, 12:34 PM
DIFFERENTIAL AMPLIFIER
Aim
To construct the Differential Amplifier in
a)Common mode and
b)Differential mode, and to find the common mode rejection ratio (CMRR).
Theory
The Differential amplifier amplifies the difference between two input voltage signals.
Hence it is called differential amplifier.V1 and V2 are input voltages, Vo is proportional to
difference between two input signals.
If we apply two input voltages equal in all respects then in ideal case output should be
zero. But output voltage depends on the average common level of the inputs. Such an average
level of two input signals is called common mode signal
Higher the value of C.M.R.R, better the performance of the differential amplifier. To
improve C.M.R.R we have to increase differential mode gain and decrease common mode
gain
Procedure
1. Connections are given as per the circuit diagram
2. Set Vi=5mV and note down Vo in both differential mode & common mode
3. Calculate the gain for both the modes
4. Calculate C.M.R.R
Formulae
For common mode signal: Gain Ac = Vo / Vi
For differential mode signal: Gain Ad = Vo / Vi
Common Mode Rejection Ratio: CMRR = 20 log (Ad / Ac)
Result
Thus a differential amplifier is constructed in both common mode and differential mode
and the corresponding gains are obtained and the CMRR is calculated.