UNIT 2 CHARACTERISTICS OF OP-AMP

UNIT-II

CHARACTERISTICS OF OP-AMP

OPERATION AMPLIFIER
An operational amplifier is a direct coupled high gain amplifier consisting of one or more
differential amplifiers, followed by a level translator and an output stage.

It is a versatile device that can be used to amplify ac as well as dc input signals & designed
for computing mathematical functions such as addition, subtraction ,multiplication,
integration & differentiation.

Op-amp symbol

Ideal characteristics of OPAMP

1. Open loop gain infinite
2. Input impedance infinite
3. Output impedance low
4. Bandwidth infinite
5. Zero offset, ie, Vo=0 when V1=V2=0

Inverting Op-Amp

Vout = -Vin(Rf/Ri)

Voltage follower

Vout = Vin

DC characteristics

Input offset current

The difference between the bias currents at the input terminals of the op- amp is called as input
offset current. The input terminals conduct a small value of dc current to bias the input transistors.
Since the input transistors cannot be made identical, there exists a difference in bias currents

Input offset voltage

A small voltage applied to the input terminals to make the output voltage as zero when the
two input terminals are grounded is called input offset voltage

Input bias current

Input bias current IB as the average value of the base currents entering into terminal of
an op-amp
IB=IB(+) + IB(-)


AC characteristics

Frequency Response

HIGH FREQUENCY MODEL OF OP-AMP

Need for frequency compensation in practical op-amps
• Frequency compensation is needed when large bandwidth and lower closed loop gain is
desired.
• Compensating networks are used to control the phase shift and hence to improve the stability

Frequency compensation methods
• Dominant- pole compensation
• Pole- zero compensation


Slew Rate
• The slew rate is defined as the maximum rate of change of output voltage caused by a step
input voltage.
• An ideal slew rate is infinite which means that op-amp’s output voltage should change
instantaneously in response to input step voltage.