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MALVINO

SIXTH EDITION. MALVINO. Electronic. PRINCIPLES. Operational Amplifiers. Chapter 18. The typical op amp has a differential input and a single-ended output. +V CC. Class B push-pull emitter follower. More stages of gain. Diff amp. -V EE. R out. v out. v 1. R in.

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MALVINO

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  1. SIXTH EDITION MALVINO Electronic PRINCIPLES

  2. Operational Amplifiers Chapter 18

  3. The typical op amp has a differential input and a single-ended output. +VCC Class B push-pull emitter follower More stages of gain Diff amp -VEE

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

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

  6. funity Bode plot of the 741 op amp 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

  7. 741C pinout and offset nulling +VCC 7 RB 2 6 Adjust for null 5 3 1 RB 4 10 kW -VEE

  8. 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.

  9. 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

  10. The negative feedback produces a virtual ground at the inverting terminal. The inverting amplifier R2 R1 A virtual ground is a short for voltage but an open for current.

  11. iin iin vout R2 ACL = = vin R1 Analyzing the inverting amplifier R2 R1 vin vout vin = iinR1and vout = iinR2 zin(CL) = R1

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

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

  14. RB2 = R1 R2 Resistor compensation for V1err R2 R1 vin vout RB2 has no effect on the virtual-ground approximation since no signal current flows through it.

  15. The negative feedback produces a virtual short. The noninverting amplifier R2 R1 A virtual short is a short for voltage but an open for current.

  16. i1 i1 R2+R1 R2 vout = = ACL = + 1 R1 R1 vin Analyzing the noninverting amplifier vout vin R2 vin = i1R1and vout = i1(R2+R1) R1 zin(CL) 

  17. RF v1 + R1 RF vout = v2 R2 The summing amplifier R1 RF v1 R2 v2 vout

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

  19. 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.

  20. 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

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