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 function-

generating 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

• Integrator

• Differentiators

• Difference amplifier

• Instrumentation amplifier

• AC amplifier

• V to I converters

• I to V converters

Cont..,

• Buffers

• Comparators

• Multi vibrators

• Triangle wave generator

• Square wave generator

• Log and anti log amplifiers

• Precision rectifiers.