UNITII Characteristics of OpAmp UNIT II CHARACTERISTICS OF

UNIT-II Characteristics of Op-Amp UNIT II CHARACTERISTICS OF OPAMP Ideal OP-AMP characteristics, DC characteristics, AC characteristics, , differential amplifier; frequency response of OP-AMP; Basic applications of op -amp – Inverting and Non-inverting Amplifiers-V/I & I/V converters , summer, differentiator and integrator. 1 KIOT-Department of EEE ; LIC/UNIT -2

OPERATION AMPLIFIER 2 KIOT-Department of EEE ; LIC/UNIT -2

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Op-amp • Terminals (2 i/p ; 2 supply ; 1 o/p) • DC offset null terminal 2 • Power Range ± 5 V to ± 22 v KIOT-Department of EEE ; LIC/UNIT -2 KIOT-Department of EEE ; LIC/UNIT -2 4

Internal circuitry of 741 5 KIOT-Department of EEE ; LIC/UNIT -2

Ideal characteristics of OPAMP 1. 2. 3. 4. 5. Open loop gain infinite Input impedance infinite Output impedance low Bandwidth infinite Zero offset, ie, Vo=0 when V 1=V 2=0 6 KIOT-Department of EEE ; LIC/UNIT -2

-ve Feedback As the open loop DC gain of an operational amplifier is extremely high Feedback is the process of “feeding back” a fraction of the output signal back to the input. but to make the feedback negative, we must feed it back to the negative or “inverting input” terminal of the op-amp using an external Feedback Resistor called Rƒ. This feedback connection between the output and the inverting input terminal forces the differential input voltage towards zero. 7 KIOT-Department of EEE ; LIC/UNIT -2

Inverting Op-Amp Circuit that amplifies i/p and provides o/p with 180º phase shift. KIOT-Department of EEE ; LIC/UNIT -2 KIOT-Department of EEE ; LIC/UNIT -2 8

Inverting Op-Amp 9 KIOT-Department of EEE ; LIC/UNIT -2

Non-Inverting Amplifier • Circuit that amplifies i/p and provides o/p without any phase shift. • Value of R 1 is very large as op-amp draws –ve current from signal source. • Rf and R 1 forms potential divider. Rf 10 KIOT-Department of EEE ; LIC/UNIT -2

Non-Inverting Amplifier 11 KIOT-Department of EEE ; LIC/UNIT -2

Voltage follower • o/p is equal to i/p in both magnitude and phase. • o/p follows i/p. • i/p impedance is high (MΩ) ; • o/p impedance is Zero. • Used as buffer for impedance matching. 12 KIOT-Department of EEE ; LIC/UNIT -2

Voltage follower 13 KIOT-Department of EEE ; LIC/UNIT -2

DC characteristics Practical OP-AMP characteristics does not match ideal OP-AMP • Due to temperature • Internal transistor mismatch. DC characteristics : 1. 2. 3. 4. Input bias current Input offset voltage Thermal drift 14 KIOT-Department of EEE ; LIC/UNIT -2

DC characteristics Input bias current IB as the average value of the base currents entering into terminal of an op-amp Ø Ideal opamp – no current drawn through i/p terminal. Ø But practically, input do conduct small current. Ideally, if vi=0; vo should be 0 ; but vo=(IB+)Rf (IB+) - The Small input current cause o/p voltage across vo 15 KIOT-Department of EEE ; LIC/UNIT -2

DC characteristics Input bias current This effect can be compensated by adding a compensation resistor Rcomp. 16 KIOT-Department of EEE ; LIC/UNIT -2

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 17 KIOT-Department of EEE ; LIC/UNIT -2

DC characteristics 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. 18 KIOT-Department of EEE ; LIC/UNIT -2

DC characteristics Thermal Drift Bias current, offset current and offset voltage change with temperature. A circuit carefully nulled at 25 oc may not remain so when the temperature rises to 35 oc. This is called drift. 19 KIOT-Department of EEE ; LIC/UNIT -2

AC characteristics Frequency Response HIGH FREQUENCY MODEL OF OPAMP 20 KIOT-Department of EEE ; LIC/UNIT -2

AC characteristics Frequency Response OPEN LOOP GAIN VS FREQUENCY 21 KIOT-Department of EEE ; LIC/UNIT -2

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 22 KIOT-Department of EEE ; LIC/UNIT -2

Frequency compensation methods • Dominant- pole compensation • Pole- zero compensation 23 KIOT-Department of EEE ; LIC/UNIT -2

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 24 KIOT-Department of EEE ; LIC/UNIT -2

Differentiator • Performs mathematical operation of differentiation. • O/P wave is the derivative of I/P wave. 25 KIOT-Department of EEE ; LIC/UNIT -2

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Integrator • Performs mathematical operation of integration. • O/P proportional to time integral of I/P & R 1 Cf. 27 KIOT-Department of EEE ; LIC/UNIT -2

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Differential amplifier 29 KIOT-Department of EEE ; LIC/UNIT -2

Differential amplifier • Circuit that amplifies the difference between the two i/p signal and provides o/p. • Used in instrumentation amplifier. 30 KIOT-Department of EEE ; LIC/UNIT -2

Differential amplifier (R 2 / R 1) is Gain; ie if R 2=100 R 1 then Vd is Amplified 100 times. 31 KIOT-Department of EEE ; LIC/UNIT -2

Summer • Output is inverted sum of the inputs. 32 KIOT-Department of EEE ; LIC/UNIT -2

Summer Output is Sum of input If R 1= R 2= R 3= Rf Output is Average of input If R 1= R 2= R 3= 3 Rf 33 KIOT-Department of EEE ; LIC/UNIT -2

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