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Differential Amplifier
M.S.P.V.L Polytechnic college,
Pavoorchatram.
• A differentiator circuit produces an output that is proportional to the
derivative or rate of change of the input voltage over time.
• Differentiator circuit can be constructed as shown using an operational
amplifier, a resistor, and a capacitor.
• Unlike an ideal integrator circuit where the slightest DC offset in the input
eventually drives the output into saturation, for the differentiator we need
not be concerned about a DC offset in the input since the derivative of a
constant is always zero. For this circuit, it can be shown that:
Differentiator Circuit
• Since the output voltage of a differentiated is proportional to
the input frequency, high frequency signals (such as electrical
noise) may saturate or cutoff the amplifier.
•
For this reason: a resistor is placed in series with the capacitor
in the input as shown in Figure .
•
This establishes high frequency limit beyond which
differentiation no longer occurs:
• To achieve greater attenuation
at
higher
frequencies
(or
prevent oscillation), a feedback
capacitor is added in parallel
with the feedback resistor.
• This establishes another break
frequency
that
can
be
calculated as in the integrator.
Stable Differentiator Circuit
Figure 1. Circuit Diagram for a Dual-Supply Op Amp Differentiator
Figure 1. Circuit Diagram for a Single-Supply Op Amp Differentiator
• The circuits shown in Figures 1 and 2 are differentiator circuits, which are
also sometimes referred to as 'differentiation amplifiers'. The main
component of these circuits is the operational amplifier, configured in such
a way that its output voltage is proportional to the derivative of its input
voltage.
• The circuit in Fig. 1 operates on two supplies, while that in Fig. 2 is a
single-supply
differentiator.
However,
what
makes
them
both
differentiators is the combination of the feedback resistor (R2 in both
examples) and the capacitor at the inverting input of the op amp (C1 in both
examples).
Cont..,
•
To illustrate how these circuits perform differentiation, consider the circuit in
Figure 1. Since the current going into the inverting input is ideally zero, then the
current through capacitor C1 is practically equal to the current through R2. The
current through C1 is just C1 times the rate of change of the voltage across it,
dVc/dt. If R1 << R2, then this current is approximately C1(dVin/dt).
•
The output voltage Vout of this circuit is equal to the negative of this current times
the resistance of R2. Thus, Vout = -R2C1(dVin/dt), which clearly shows that the
circuit is indeed a differentiator. As a graphical example, the input voltage in both
circuit examples is a triangle wave. This emerges as a square wave at the output of
the circuits (the derivative of a triangle wave is a square wave).
•
Differentiator circuits like this are commonly seen in wave-shaping and functiongenerating circuits.
Improved Differentiator Amplifier
•
The basic single resistor and single capacitor differentiator circuit is not
widely
used
to
reform
the
mathematical
function
of Differentiation because of the two inherent faults mentioned above,
Instability and Noise.
• So in order to reduce the overall closed-loop gain of the circuit at high
frequencies, an extra Resistor, R2 is added to the input as shown below.
Improved Differentiator Amplifier Circuit
•
The circuit which we have now acts like a Differentiator amplifier at low
frequencies and an amplifier with resistive feedback at high frequencies
giving much better noise rejection. This then forms the basis of a Active
High Pass Filter as seen before in the filters section.
Applications of Differential Amplifier
•
•
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•
•
•
Integrator
Differentiators
Difference amplifier
Instrumentation amplifier
AC amplifier
V to I converters
I to V converters
Cont..,
•
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Buffers
Comparators
Multi vibrators
Triangle wave generator
Square wave generator
Log and anti log amplifiers
Precision rectifiers.