B.Sc-II Paper-VI Chapter- Operational Amplifier.

1. B.Sc-IIPaper-VIChapter- 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. OFFSET NULL OFFSET NULL Op Amp Pin-out connectionsDual-in-Line Plastic Package

5. Op Amp Pin-outs Metal Can

6. WHAT IS INSIDE AN OP AMP?

7. Phases ofoutput signal wrt input input

8. RL Rb Inverted output signal Input signal Common Emitter Amplifier

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

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

11. Common Mode signal Differential Mode signal Transistorized Differential Amplifier

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

13. 7 2 6 3 4 Op Amp stages with pin-outs of IC741

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 VINVresultsinto saturation of output voltage ±VSAT i.e. for VNI < VINV output is –VSAT and for VNI > VINV output is +VSAT • Value of VSATis limited by the supply voltage of op amp

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

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

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

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

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

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

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

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

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

27. Cin Rf If Iin Vin Vout Vout= – Rf Cin(dVin / dt ) Op amp Differentiator 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. Step Input dVout / dt Vout t Slew Rate • Maximum rate of change of output voltage (when typically a step voltage is given at the input terminal)

31. Vin Expected Voltage Vout t 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

32. 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 MΩ Output Resistance • ROis ideally zero • In case of IC 741 it is 75 Ω

33. 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)

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 • www.williamson-labs.com

36. Thank you !