Differential amplifiers amplify the difference between two input signals while rejecting input signals that are common to both inputs. They have advantages like excellent stability, versatility, and immunity to noise and interference. The differential gain (Ad) is the gain with which the difference between the two input signals (V1-V2) is amplified to produce the output (Vo). The common mode gain (Ac) is the gain resulting from any common signals applied to both inputs. Differential amplifiers have high differential gain, low common mode gain, and high common mode rejection ratio (CMRR), which is the ratio of Ad/Ac expressed in decibels and indicates the ability to reject common mode signals.

1. Differential amplifier
Mrs.V.SrirengaNachiyar, AP/ECE
Ramco Institute of Technology
Academic Year:2017-2018 (Odd)

2. Differential amplifier
Amplifier:
An amplifier is a device which accept an
input signal and produces an output signal
proportional to the input.
Differential amplifier:
An amplifier which amplifies the
difference between the two input signals is called
differential amplifier.

3. Advantages of differential amplifier
• Excellent stability
• High versatility (adapt to many different
situations)
• Immune to noise
• Immune to interference signals
• Used in most of analog circuits ranging from
d.c to high frequency applications.

4. Basics of Differential amplifier
• The differential amplifier amplifies the
difference between 2 input voltage signals.
Hence it is also called as difference amplifier.

5. Cont…
• V1 & V2 are 2 input voltages.
• Vo is single ended output.
• Each signal is measured with respect to the
ground.
• In an ideal differential amplifier, output
voltage Vo is proportional to the difference
between the two input signals.
Vo α V1 - V2

6. Differential gain (Ad)
• Vo = Ad (V1 - V2 )
• Where Ad is constant of proportionality.
• Ad is the gain with which differential amplifier
amplifies the difference between two input
signals. Hence it is called differential gain of
differential amplifier.
• The difference between 2 input voltages is
generally called difference voltage denoted as
Vd.
• Vo = AdVd.
• Differential gain Ad = Vo/Vd

7. Common mode gain (Ac)
• If we apply 2 input voltages which are equal in
all the respects to the differential amplifier.
i.e., V1 = V2 . Then ideally the output voltage
Vo = Ad (V1 - V2 ) must be zero.
• But the output voltage of the practical
differential amplifier not only depends on the
difference voltage but also depends on the
average common level of 2 inputs.
• Such an average level of 2 input signals is
called common mode signals denoted as Vc

8. Cont…
• Vc =
𝑽 𝟏+𝑽 𝟐
𝟐
.
• The gain with which it amplifies the common
mode signal to produce the output is called as
common mode gain of the differential
amplifier denoted as AC.
• Vo = Ac Vc .
• The total output of any differential amplifier
can be expressed as,
• Vo = Ad Vd + Ac Vc

9. Common mode rejection ratio (CMRR)
• When the same voltage is applied to both the
inputs, the differential amplifier is said to be
operated in a common mode configuration.
• Many disturbance signals, noise signals appear
as a common input signal to both the input
terminals of the differential amplifier.
• The ability of a differential amplifier to reject a
common mode signal is expressed by a ratio
called CMRR.

10. Cont…
• CMRR is defined as the ratio of the differential
voltage gain Ad to common mode gain Ac
• CMRR = ρ =
Ad
Ac
.
• CMRR is also expressed in dB,

11. Feature of differential amplifier
• High differential voltage gain.
• Low common mode gain.
• High CMRR
• Two input terminals.
• High input impedance.
• Large bandwidth.
• Low output impedance.
• Low offset voltage and current.

12. References
• Donald .A. Neamen, Electronic Circuit
Analysis and Design –2nd Edition,Tata Mc
Graw Hill, 2009.
• Adel .S. Sedra, Kenneth C. Smith, “Micro
Electronic Circuits”, 6th Edition, Oxford
University Press, 2010.
• A.P.Godse & U.A. Bakshi,”Electronic
Circuits-I