OPERATIONAL AMPLIFIER PHE10

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OPERATIONAL AMPLIFIER(PHE-10)

• Shubha Gokhale
• School of Sciences, IGNOU
• MARCH 30, 2007

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Typical Op Amp IC Packages
Metal Can Package
Dual-in-Line package (DIP)
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Op Amp Pin-out connectionsDual-in-Line Plastic
Package
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Op Amp Pin-outs Metal Can
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WHAT IS INSIDE AN OP AMP?
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Phases of output signal wrt input
input
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Common Emitter Amplifier
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Two matched transistors form a differential
amplifier
VC1VCC-I1R2
VC2VCC-I2R4
I1
I2
input
0 V
-VEE0.7 V
Constant current I1I2
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Transisterized Differential Amplifier
Double ended input - double ended output diff.
amp voutA (v1-v2)
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Transistorized Differential Amplifier
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Single Ended Differential Amplifier (commonly
used in op-amps)
Inverting input
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Op Amp stages with pin-outs of IC741
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Op amp Comparator

• Open loop gain of op amp is defined as
• AOL Vo / VD
• where VD VNI VINV
• Open loop gain of op amp is very high
  (ideally infinite).
• Any small difference between VNI and VINV
  results into saturation of output voltage VSAT
• i.e. for VNI lt VINV output is VSAT and
• for VNI gt VINV output is VSAT
• Value of VSAT is limited by the supply voltage of
  op amp

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Op amp Voltage Level Detector
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Important features of Op Amp

• High open loop gain (ideally infinite) which
  implies that even the smallest difference between
  the two inputs results into saturated output
  voltage
• High input impedance (ideally infinite) implies
  that there is no current flowing into the input
  of an op amp

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Virtual Ground

• These two conditions give rise to VIRTUAL GROUND,
  where the voltages at both the inputs are
  maintained at exactly same level.
• To achieve this condition, a feedback circuit
  between the output and the inverting input
  terminal of the op amp is necessary.
• This results into many applications of op amp,
  which qualify it to be OPERATIONAL adder,
  subtractor, multiplier, divider etc.

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Op amp Applications
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Inverting Amplifier

• Multiplier/Divider circuit, whose gain is decided
  by the values of resistors

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Inverting Amplifier Analysis
Due to virtual ground Iin Vin / Rin If
( Vout / Rf ) ?Vout (Rf / Rin ) Vin Rf gt
Rin ?multiplier Rf lt Rin? divider
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Non-inverting Amplifier
Due to virtual ground VINV VNI Vin If
(VoutVin) / Rf Iin Vin / Rin ?Vout 1 (Rf
/ Rin ) Vin
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Unity Gain Amplifier (Non-inverting Buffer)
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Inverting Adder
Vout Rf (V1 / R1) (V2 / R2) (V3 / R3)
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Subtractor
Vout Rf (V2V1)/ Rin
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Subtractor Analysis
Vout V1Rf / Rin 1 (Rf / Rin ) V
Rf (V2 V1) / Rin
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Op amp integrator
Iin Vin / Ri n If Cf ( dVout / dt )
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Op amp Differentiator
Iin CindVin /dt If Vout / Rf
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Op amp Characteristics
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Common Mode Rejection Ratio (CMRR)

• It is the ability of an op amp to reject the
  signal which is present at its both inputs
  simultaneously i.e. the common mode signal
• CMRR AOL / ACM, where ACM is common mode
  voltage gain defined by Vout / VCM
• Ideally CMRR is infinite
• For IC 741 it is 90 dB

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Slew Rate

• Maximum rate of change of output voltage (when
  typically a step voltage is given at the input
  terminal)

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Slew Rate (cont)

• Slew rate is caused by the finite response time
  of the circuit elements of an op amp
• It limits the highest possible frequency of
  operation

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• Open Loop Gain
• AOL is ideally infinite
• In case of typical practical op amp like IC 741
  it is of the order of 105 i.e. about 100 dB
• Input Resistance
• RI is ideally infinite
• In case of IC 741 it is 2 MO
• Output Resistance
• RO is ideally zero
• In case of IC 741 it is 75 O

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• Input Offset Voltage
• Small voltage needed to be applied between INV
  and NI terminals to get zero output voltage
• Ideally it should be 0 V (CM operation)
• Output Offset Voltage
• Under common mode operation, output voltage
  should be zero, but due to mismatch is devices it
  is non-zero (Can be corrected by applying
  voltage between Pins 1 and 5 of IC 741)

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Summary

• Op amp can be used for various (mathematical)
  operations like addition, subtraction,
  multiplication, division, differentiation,
  integration etc.
• Infinite open loop gain and infinite input
  resistance give rise to VIRTUAL GROUND
• Op amp is available in a simple to use IC form
  which require dual supply
• Many other applications can be implemented using
  op amp

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ACKNOWLEDGMENT

• Some circuits in this presentation are based
  on following materials
• HANDBOOK OF OPERATIONAL AMPLIFIER APPLICATIONS
• by Texas Instruments
• www.williamson-labs.com

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Thank you !