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Operational Amplifiers The operational amplifier (op-amp). Note the op-amp has two inputs and one output. Thomas L. Floyd Digital Fundamentals, 8e Copyright ©2003 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electrical Characteristics Note these ratings are for specific circuit conditions, and they often include minimum, maximum and typical values. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. CMRR • One rating worth mentioning that is unique to op-amps is CMRR or Common-Mode Rejection Ratio. • Because the op-amp has two inputs that are opposite in phase (inverting input and the non-inverting input) any signal that is common to both inputs will be cancelled. • A measure of the ability to cancel out common signals is called CMRR. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Differential and common-mode operation: (a) differential-mode; (b) common-mode. VC = ½ ( Vi1 – Vi2 ) EXAMPLE Calculate the CMRR for the circuit measurements shown in Fig. + Vd Vi1 =0.5mV + Vo =8V Vo =8V Vd =1mV - - Vi2 =0.5mV (a) + + Vo =12V Vi1 =1mV - - V2 =1mV Vc =1mV (b) Vo =12V Solution From the measurement shown in Figure, using the procedure in step 1 above, we obtain Vo 8V Ad 8000 Vd 1mV The measurement shown in Fig. using the procedure in step 2 above, gives us Ac Vo 12mV 12 Vc 1mV The value of CMRR is Ad 8000 CMRR 666.7 Ac 12 Which can be expressed as Ad CMRR 20 log 1 0 20 log 1 0 666.7 56.48dB Ac Ac equivalent of op-amp circuit: (a) practical; (b) ideal. Robert L. Boylestad and Louis Nashelsky Electronic Devices and Circuit Theory, 8e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Operation of op-amp as constant-gain multiplier: op-amp ac equivalent circuit. Robert L. Boylestad and Louis Nashelsky Electronic Devices and Circuit Theory, 8e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Operation of op-amp as constant-gain multiplier: ideal op-amp equivalent circuit. Robert L. Boylestad and Louis Nashelsky Electronic Devices and Circuit Theory, 8e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Operation of op-amp as constant-gain multiplier: redrawn equivalent circuit. Robert L. Boylestad and Louis Nashelsky Electronic Devices and Circuit Theory, 8e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Virtual ground in an op-amp. Robert L. Boylestad and Louis Nashelsky Electronic Devices and Circuit Theory, 8e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Inverting constant-gain multiplier. Robert L. Boylestad and Louis Nashelsky Electronic Devices and Circuit Theory, 8e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Noninverting constant-gain multiplier. Robert L. Boylestad and Louis Nashelsky Electronic Devices and Circuit Theory, 8e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. A Constant–Gain Amplifier Rf A R1 Rf A 1 R1 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Voltage Buffer Any amplifier with no gain (or loss) is called a unity gain amplifier. The advantages of using a unity gain amplifier: • very high input impedance • very low output impedance Realistically these circuits will be designed using resistors that are equal (R1 = Rf) to void out problems with offset voltages. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Summing amplifier Rf Rf Rf Vo V1 V2 V3 R2 R3 R1 Robert L. Boylestad and Louis Nashelsky Electronic Devices and Circuit Theory, 8e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. virtual-ground equivalent circuit. Robert L. Boylestad and Louis Nashelsky Electronic Devices and Circuit Theory, 8e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. A 4-bit DAC with binary-weighted inputs. Thomas L. Floyd Digital Fundamentals, 8e Copyright ©2003 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Example : A 4-bit DAC with binary-weighted inputs. Thomas L. Floyd Digital Fundamentals, 8e Copyright ©2003 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Active Filters Adding capacitors to op-amp circuits provides an external control for the cutoff frequencies. The op-amp active filter provides controllable cutoff frequencies and controllable gain. • Low-Pass Filter • High-Pass Filter • Band pass Filter Low-Pass Filter First-order low-pass filter. Low-Pass Filter (Cont’d) Second-order low-pass filter: By adding more RC networks the roll-off can be made steeper. High-Pass Filter The lower cutoff frequency: 1 fOL 2πR1C1 Bandpass Filter There are two cutoff frequencies: upper and lower. They can be calculated using the same low-pass cutoff and high-pass cutoff frequency formulas in the appropriate sections. Questions 1. Calculate CMRR (dB) for the following Op Amp values: Vd = 1 mV, Vo = 200 mV dan Vc = 1 mV, Vo = 30uV. Calculate the output voltage if the input voltage is 150mV pmkd 750 k + 10 V 50 k Vo + - 10 V V1 Calculate the output voltage, Vo 10 k 500 k 750 k 850 k + 22 V + 22 V + 22 V 25 k 15 k + + + - 22 V V1 Vo - 22 V - 22 V Calculate the output voltage, Vo 150 k 780 k 15 k 78 k V1 15 mV + Vo 78 k + V2 20 mV Calculate the output voltage, Vo 15 V1 = 10 V V2 = 20 V 650 k V3 = 30 V Vo + Determine the cut-off frequency k k Vo 0.02uF + 0.02uF V1 k k Determine the Low and High cut-off frequency 20 k 5 k 20 k 5 k Vo 0.05uF + + Vi 20 k 0.02uF Calculate the Value of V1 & V2 Robert L. Boylestad and Louis Nashelsky Electronic Devices and Circuit Theory, 8e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Vo = ? Robert L. Boylestad and Louis Nashelsky Electronic Devices and Circuit Theory, 8e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Vo = ?
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