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Operational Amplifiers

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Chapter 30 Operational Amplifiers Introduction Characteristics High input impedance Low output impedance High open-loop gain Two inputs One output Usually + and ... – PowerPoint PPT presentation

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Title: Operational Amplifiers


1
Chapter 30
  • Operational Amplifiers

2
Introduction
  • Characteristics
  • High input impedance
  • Low output impedance
  • High open-loop gain
  • Two inputs
  • One output
  • Usually and dc power supplies

3
Introduction
  • Ideal Characteristics
  • zin (inverting) 8
  • zin (non-inverting) 8
  • zout 0
  • Av 8

V
Inverting Input
-
Output

Non-Inverting Input
V-
4
Introduction
  • Uses
  • Comparators
  • Voltage amplifiers
  • Oscillators
  • Active filters
  • Instrumentation amplifiers

V
Inverting Input
-
Output

Non-Inverting Input
V-
5
Introduction
  • Single-ended amplifier
  • One input grounded
  • Signal at other input
  • Double-ended amplifier/Differential amplifier
  • Signals at both inputs

V
Inverting Input
-
Output

Non-Inverting Input
V-
6
Differential Amplifier and Common-Mode Signals
  • Basic differential amplifier
  • Q1 identical to Q2
  • RC1 RC2
  • IC1 IC2 and emitter currents equal
  • Also, IC IE for high ß
  • and VBE 0.7 V
  • Similar calculation of Bias

7
Differential Amplifier and Common-Mode Signals
8
Differential Amplifier and Common-Mode Signals
  • Apply same signal to both Bases
  • Vout Vout1 Vout2 0
  • Eliminates common-mode signals
  • 60 Hz
  • Noise

9
Differential Amplifier and Common-Mode Signals
  • Apply sinusoids to both bases
  • Same amplitude, 180 difference in phase,
  • if Vin1 Vin2
  • Vout 2Vin

10
Differential Amplifier and Common-Mode Signals
  • Common-mode signals
  • Differential voltage gain
  • also called open-loop voltage gain
  • 20,000 Av 200,000

11
Differential Amplifier and Common-Mode Signals
  • Common-mode signals
  • Common-mode voltage gain

12
Differential Amplifier and Common-Mode Signals
  • Common-mode rejection ratio (CMRR)
  • Equations
  • Values

13
Differential Amplifier and Common-Mode Signals
  • Noise
  • Static in audio signal
  • Increases as signal is amplified
  • Common mode signal
  • Significantly reduced by differential amplifier

vnoise
-
vin

__ _ -
__ _ -
14
Negative Feedback
  • Op-amp
  • Large differential, open-loop voltage gain
  • Avol 100,000
  • Small input yields saturated output (VCC or VEE)

15
Negative Feedback
  • Negative feedback
  • Returns a portion of output signal to the input
  • Open-loop voltage gain decreased

16
Negative Feedback
  • Input impedance still high
  • Output impedance low
  • Circuit voltage gain, Av
  • Adjustable
  • Stable

Negative Feedback
-

vout
vin
__ _ -
17
Inverting Amplifier
  • Basic circuit

18
Inverting Amplifier
  • Output 180 out of phase with input
  • Significant decrease in gain
  • Gain now called closed-loop voltage gain
  • Output impedance 0
  • vd 0

19
Inverting Amplifier
  • Inverting input at virtual ground, vin(-) 0
  • iin to op-amp 0
  • Input current only dependent on vin and R1
  • Avcl only dependent on input resistor and
    feedback resistor

20
Inverting Amplifier
  • Model
  • vd 0
  • Rin 8
  • iin if
  • zin R1

RF
R1
i 0
if
-

iin
vd
Rin
vin
-
-
Rout
vout(OC)
-
__ _ -
Avolvd internal
__ _ -
21
Inverting Amplifier
  • Low output impedance

RF
R1
i 0
if
-

iin
vd
Rin
vin
-
i1
Rout
iout(sc)
-
__ _ -
__ _ -
Avolvd internal
__ _ -
22
Non-Inverting Amplifier
  • Circuit

23
The Non-Inverting Amplifier
  • Very high input impedance
  • Voltage gain related to the two resistors
  • Very low output impedance
  • Excellent buffer

24
Non-Inverting Amplifier
  • Differential voltage
  • vd 0
  • Input current to op-amp
  • i 0
  • Closed-loop voltage gain (Avcl) is a resistor
    ratio

25
Non-Inverting Amplifier
26
Non-Inverting Amplifier
  • Model
  • Input impedance

vin

iin
vd
Rin
zin
vout
-
Rout
-
-
Avolvd internal
R1
__ _ -
-
__ _ -
RF
if
27
Non-Inverting Amplifier
  • Model
  • Output impedance

i2

iin
vd
Rin
-
Rout
-
-
Avolvd internal
R1
__ _ -
-
__ _ -
__ _ -
RF
if
iout(sc)
28
Non-Inverting Amplifier
  • Very high zin
  • Very low zout
  • Good buffer circuit
  • Also called voltage follower (gain 1)
  • Or adjustable gain gt 1

29
Non-Inverting Amplifier
  • Voltage Follower Buffer Circuit
  • Gain 1
  • High impedance source drives low impedance load

30
Op-Amp Specifications
  • LM 741 series
  • Inexpensive
  • Widely used
  • Good general specifications
  • Characteristic of all op-amp specifications
  • Provide Minimum, Typical, and Maximum ratings

31
Op-Amp Specifications
  • Input Offset Voltage, Vio
  • LM741C, Vio typical is 2 millivolts
  • Model is voltage source with value, Vio in series
    with input

32
Op-Amp Specifications
  • Input Offset Voltage, Vio
  • Without feedback this would saturate output with
    no input
  • With negative feedback, output due to Vio is
    closed-loop gain times Vio

33
Op-Amp Specifications
  • Input Offset Current, Ios
  • Ios Difference between bias currents at and
    inputs of op-amp
  • 741C typical Ios is 20 nanoamps
  • Multiplying resistor used to measure Ios

34
Op-Amp Specifications
  • Input Resistance
  • 741C minimum .3 M?, typical 2 M?
  • Open-Loop Voltage gain (Avol)
  • 741C Avol Large Signal Voltage Gain
  • minimum 20,000, typical 200,000
  • Closely related to Bandwidth, BW

35
Op-Amp Specifications
  • Gain-bandwidth product
  • 741C 1,000,000 106 MHz

36
Op-Amp Specifications
  • Gain versus frequency curve for op-amp

37
Op-Amp Specifications
  • Slew rate
  • Maximum rate of change of output voltage
  • 741C maximum slew rate 0.5 V/µsec

38
Op-Amp Specifications
  • Fastest time for output to go from 0 to 10 volts
    is 20 µsec
  • Can distort waveforms that have too fast a rise
    time

39
Op-Amp Specifications
  • Slew rate required for Sinusoid with frequency f
    and amplitude A
  • Maximum amplitude of a sine wave with frequency f
    for a given slew rate

40
Op-Amp Specifications
  • Bias Compensation use RC R1RF

41
Troubleshooting an Op-Amp Circuit
  • Problems occur when circuit is first built
  • Most important
  • Correct connection of dual power supply
  • Connecting a supply to a input (or vice
    versa) can burn out an op-amp
  • Single earth ground
  • Short connecting wires
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