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MALVINO & BATES

MALVINO & BATES. Electronic PRINCIPLES. SEVENTH EDITION. Chapter. 18. Operational Amplifiers. Topics Covered in Chapter 18. Introduction to op amps The 741 op amp The inverting amplifier The noninverting amplifier Two op-amp applications Linear ICs Op amps as surface-mount devices.

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MALVINO & BATES

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  1. MALVINO & BATES Electronic PRINCIPLES SEVENTH EDITION

  2. Chapter 18 Operational Amplifiers

  3. Topics Covered in Chapter 18 • Introduction to op amps • The 741 op amp • The inverting amplifier • The noninverting amplifier • Two op-amp applications • Linear ICs • Op amps as surface-mount devices

  4. Op amp • Noninverting and inverting input • Single-ended output • A perfect amplifier – a voltage-controlled voltage source • An ideal op amp has: • Infinite open-loop voltage gain • Infinite input resistance • Zero output impedance

  5. The typical op amp has a differential input and a single-ended output. Class B push-pull emitter follower More stages of gain Diff amp Vin Vout

  6. Rout vout v1 Rin AVOL(v1-v2) v2 Op amp symbol and equivalent circuit Symbol Noninverting input +VCC Output Inverting input -VEE Equivalent circuit

  7. The 741C op amp is an industry standard. Rout vout v1 Rin AVOL(v1-v2) v2 Rout = 75 W Rin = 2 MW AVOL = 100,000 Iin(bias) = 80 nA Iin(off) = 20 nA Vin(off) = 2 mV funity = 1 MHz CMRR = 90 dB

  8. funity Bode plot of the 741C op amp AVOL 100 dB 80 dB 20 dB/decade rolloff 60 dB 40 dB 20 dB 0 dB 10 Hz 100 Hz 1 kHz 10 kHz 100 kHz 1 MHz

  9. 741C pinout and offset nulling +VCC 7 RB 3 6 Adjust for null (i.e. 0V at Pin 6) 5 2 1 RB 4 10 kW -VEE

  10. The internal frequency compensation capacitor found in most op amps also limits the rate at which the output can change. SR = 0.5 V/ms (for the 741) Slew rate distortion Slope > SR When a signal exceeds the slew-rate of an op amp, the output becomes distorted and amplitude limited.

  11. dv dv dt dt dv dv dt dt SR fmax = 2pfVp The rate of voltage change (slope) is directly related to both amplitude and frequency: > v t SS = 2pfVp The power bandwidth of an op amp is given by: > v t

  12. Inverting op amp • The most basic op amp circuit • Uses negative feedback to stabilize the closed-loop voltage gain • Closed-loop voltage gain equals feedback resistance divided by input resistance

  13. The negative feedback produces a virtual ground at the inverting terminal. The inverting amplifier Rf R1 A virtual ground is a short for voltage but an open for current.

  14. iin iin vout Rf AO(CL) = = vin R1 Analyzing the inverting amplifier Rf R1 vin vout vin = iinR1and vout = iinRf zin(CL) = R1

  15. f2(CL)@ funity AV(CL) Negative feedback increases the closed-loop bandwidth. AVOL 100 dB 80 dB 60 dB 40 dB 20 dB 0 dB 10 Hz 100 Hz 1 kHz 10 kHz 100 kHz 1 MHz

  16. Negative feedback reduces error • V1err = (RB1 - RB2)Iin(bias) • V2err = (RB1 + RB2)Iin(off)/2 • V3err = Vin(off) • Verror = ± AV(CL)(± V1err ± V2err ± V3err) • V1err eliminated with resistor compensation • Use offset nulling in demanding applications

  17. RB2 = R1 Rf Resistor compensation for V1err Rf R1 vin vout RB2 has no effect on the virtual-ground approximation since no signal current flows through it.

  18. Noninverting op amp • A basic op amp circuit • Uses negative feedback to stabilize the closed-loop gain • Closed-loop voltage gain equals the feedback resistance divided by the input resistance plus 1

  19. The negative feedback produces a virtual short. The noninverting amplifier Rf R1 A virtual short is a short for voltage but an open for current.

  20. i1 i1 Rf+R1 Rf vout = = AV(CL) = + 1 R1 R1 vin Analyzing the noninverting amplifier vout vin Rf vin = i1R1and vout = i1(Rf+R1) R1 zin(CL) 

  21. Op amp application: summing amp • A summing amp has two or more inputs and one output • Each input is amplified by its channel gain • If all channel gains equal unity, the output equals the sum of the inputs

  22. Rf v1 + R1 Rf vout = v2 R2 The summing amplifier R1 Rf v1 R2 v2 vout In a mixer, a summing amp can amplify and combine audio signals

  23. Op amp application:voltage follower • Has a closed-loop gain of unity • Has a bandwidth of funity • Useful as an interface between a high-impedance source and a low-impedance load

  24. The voltage follower Rhigh vout vin The virtual short tells us vout = vin Rlow AV(CL) = 1 zin(CL)  zout(CL) 0 f2(CL) = funity

  25. Other than the 741 • BIFET op amps offer extremely low input currents. • High-power op amps supply amperes of output current. • High-speed op amps slew at tens or hundreds of volts/ms and some have hundreds of MHz of bandwidth. • Precision op amps boast small offset errors and low temperature drift.

  26. Other linear ICs • Audio amps in the mW range optimized for low noise (preamplifiers) • Audio amps in the watt range for driving loudspeakers • Video amps with wide bandwidths • RF and IF amps for receiver applications • Voltage regulators

  27. Op amps as surface-mount devices • Pin out is simple for most op amps • Small outline package (SOP) is the preferred SM style

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