MALVINO Electronic PRINCIPLES SIXTH EDITION

Operational Amplifiers Chapter 18

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

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

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

Bode plot of the 741 op amp funity 100 dB 80 dB 20 dB/decade rolloff 10 Hz 100 Hz 1 kHz 10 kHz 100 kHz 1 MHz funity

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

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.

change (slope) is directly related to both amplitude dv dt dv dt 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: dv dt dv dt v > t fmax = 2pfVp SR

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

Analyzing the inverting amplifier iin iin vin vout vin = iinR1 and vout = iinR2 ACL = = R1 R2 vout vin zin(CL) = R1

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

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

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

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

Analyzing the noninverting amplifier vout vin i1 R2 vin = i1R1 and vout = i1(R2+R1) i1 R1 ACL = = R1 R2+R1 vout vin R2 + 1 zin(CL)  

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

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

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.

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