Differential amplifier

Basics of Differential Amplifier
Contents
• Introduction
• Block diagram
• Differential gain
• Common mode gain
• CMRR
• Configuration of differential amplifier
• Conclusion and References
Prepared By S ARUN M.Tech Differential Amplifiers

Introduction
• The function of differential amplifier is to amplify the
difference between two signals.
• The need for differential amplifier in many physical
measurements arises where response from d.c to many
megahertz is required. It is also the basic input stage of an
integrated amplifier.

Block diagram of differential amplifier

• The output signal in a differential amplifier is proportional to the
difference between the two input signals.
Vo α (V1 – V2)
Where,
V1 & V2 – Two input signals
Vo – Single ended output

Differential Gain (Ad):
Where,
Ad is the constant of proportionality.
Ad is the gain with which differential amplifier amplifies the difference
between two input signals.
Hence it is known as ‘differential gain of the differential amplifier’.
V1-V2= Difference of two voltage

Common Mode Gain (Ad):
An average of the two input signals is called common mode
signal denoted as Vc.
Hence, the differential amplifier also produces the output
voltage proportional to common mode signals.
Vo = Ac Vc
Where Ac is the common mode gain.
Therefore, there exists some finite output for V1 = V2 due to common
mode gain Ac.
Hence the total output of any differential amplifier can be given as,
Vo = Ad Vd + Ac VcPrepared By S ARUN M.Tech Differential Amplifiers

Common Mode Rejection Ratio (CMRR):
• The ability of a differential amplifier to reject a common mode signal is
defined by a ratio called ‘Common Mode Rejection Ratio’ denoted as
CMRR.
• CMRR is defined as the ratio of the differential voltage gain Ad to
common mode gain Ac and is expresses in dB.

Configurations of Differential Amplifier:
• The differential amplifier in the difference amplifier stage in the
op-amp, can be used in four configurations.
(i) Dual input, balanced output differential amplifier
(ii) Dual input, unbalanced output differential amplifier
(iii) Single input, balanced output differential amplifier
(iv) Single input, unbalanced output differential amplifier
 Out of these four configurations, the dual input, balanced output is the
basic differential amplifier configuration.

9
Dual input balanced output
differential amplifier

Dual input unbalanced output

Single input balanced output

Single input unbalanced output

• Conclusion
From this presentation came to know about
differential amplifier and its types.
• References
Thomas L.Floyd, “Electronic devices” Conventional
current version, Pearson prentice hall.
David A. Bell, “Electronic devices and circuits”, Oxford
University higher education.
Sedra and smith, “Microelectronic circuits”,7th Ed.,
Oxford University Press.

Differential amplifier

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Differential amplifier