1. B.Sc-II Paper-VI Chapter- Operational Amplifier.
Prof. Dr. V. S. Jamdade

2. OPERATIONAL AMPLIFIER (PHE-10)+ -
Shubha Gokhale
School of Sciences, IGNOU
MARCH 30, 2007

3. Typical Op Amp IC Packages
Metal Can Package
Dual-in-Line package (DIP)

4. Op Amp Pin-out connections Dual-in-Line Plastic Package
OFFSET NULL

5. Op Amp Pin-outs Metal Can

6. WHAT IS INSIDE AN OP AMP?

7. Phases of output signal wrt input

8. Common Emitter AmplifierRb RL
Input signal
Inverted output signal

9. Two matched transistors form a differential amplifier
VC1=VCC-I1R2
VC2=VCC-I2R4 I1 I2
input
0 V
Constant current
= I1+I2
-VEE+0.7 V

10. Transisterized Differential Amplifierv1 v2
vout - +
Double ended input - double ended output diff. amp:
vout=A (v1-v2)

11. Transistorized Differential Amplifier
Differential Mode signal
Common Mode signal

12. Single Ended Differential Amplifier (commonly used in op-amps)●
Single ended output
Non-inverting input
Inverting input

13. Op Amp stages with pin-outs of IC7412 4 7 6 3

14. 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 < VINV output is – VSAT and
for VNI > VINV output is + VSAT
Value of VSAT is limited by the supply voltage of op amp

15. Op amp Voltage Level Detector
VOUT + -
VREF
VIN
= +VSAT for VIN>VREF
= -VSAT for VIN<VREF

16. 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

17. 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.

18. Op amp Applications

19. Inverting Amplifier
Multiplier/Divider circuit, whose gain is decided by the values of resistors Rf
Rin
Vin
Vout +

20. Inverting Amplifier Analysis
Iin Rf
Rin
Vin
Vout If +
Due to virtual ground:
Iin= Vin / Rin= If = – ( Vout / Rf )
Vout= – (Rf / Rin ) Vin
Rf > Rin →multiplier Rf < Rin→ divider

21. Non-inverting Amplifier
Vin
Vout
Rin Rf
Iin If +
Due to virtual ground:
VINV = VNI = Vin
If = (Vout–Vin) / Rf = Iin= Vin / Rin
Vout = [1+ (Rf / Rin )] Vin

22. Unity Gain Amplifier (Non-inverting Buffer)+ –
Vin
Vout=Vin

23. Inverting Adder R1 V1 R2 V2 R3 Rf V3 Vout+
Vout
Vout= Rf [(V1 / R1)+ (V2 / R2)+ (V3 / R3)]

24. Subtractor
Vout V2 V1 Rf
Rin
Vout= Rf (V2–V1)/ Rin

25. Subtractor Analysis V1 V2 Rin V’ V’ = V2Rf / (Rf+Rin) Vout Rf
Vout = – V1Rf / Rin + [1+ (Rf / Rin )] V’
= Rf (V2 – V1) / Rin

26. Op amp integrator Cf Rin If Iin Vin Vout
Vout= – (1 / Rin Cf ) ∫ Vin dt
Iin If
Iin= Vin / Ri n= If = – Cf ( dVout / dt )

27. Op amp Differentiator Cin Rf If Iin Vin Vout
Vout= – Rf Cin (dVin / dt ) If
Iin
Iin= CindVin /dt = If = – Vout / Rf

28. Op amp Characteristics

29. 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

30. Slew Rate
Maximum rate of change of output voltage (when typically a step voltage is given at the input terminal)
Step Input
Vout t
dVout / dt

31. 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
Vin
Vout
Expected Voltage t

32. Open Loop Gain Input Resistance Output Resistance
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 MΩ
Output Resistance
RO is ideally zero
In case of IC 741 it is 75 Ω

33. Input Offset Voltage Output 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)

34. 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

35. ACKNOWLEDGMENT
Some circuits in this presentation are based on following materials:
HANDBOOK OF OPERATIONAL AMPLIFIER APPLICATIONS
by Texas Instruments

36. Thank you !