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

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

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

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

Op Amp Pin-outs Metal Can

WHAT IS INSIDE AN OP AMP?

Phases of output signal wrt input

Common Emitter Amplifier Rb RL Input signal Inverted output signal

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

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

Transistorized Differential Amplifier Differential Mode signal Common Mode signal

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

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

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

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

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

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.

Op amp Applications

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

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

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

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

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

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

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

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 )

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

Op amp Characteristics

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

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

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

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

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)

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

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

Thank you !