Operational Amplifiers. What is an Op Amp? High voltage gain IC with differential inputs –Designed...
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Transcript of Operational Amplifiers. What is an Op Amp? High voltage gain IC with differential inputs –Designed...
Operational AmplifiersOperational Amplifiers
What is an Op Amp?What is an Op Amp?High voltage gain IC with High voltage gain IC with differential inputsdifferential inputs– Designed to have Designed to have
characteristics near idealcharacteristics near ideal
Inexpensive, widely used ICInexpensive, widely used IC
Available in wide range of Available in wide range of packagespackages
Typically require a positive Typically require a positive and a negative power supplyand a negative power supply
Uses: amplifier, buffer, summer, differentiator, Uses: amplifier, buffer, summer, differentiator, integrator, comparator, instrumentation amp, integrator, comparator, instrumentation amp, Schmitt trigger, negative impedance, super Schmitt trigger, negative impedance, super diode, logarithmic/exponential output, simulated diode, logarithmic/exponential output, simulated inductorinductor
History of Op AmpsHistory of Op AmpsOriginally designed for analog computers using vacuum Originally designed for analog computers using vacuum tubestubes– Output voltage is a function of input voltage (addition, subtraction, Output voltage is a function of input voltage (addition, subtraction,
integration, differentiation, exponential, logarithm…)integration, differentiation, exponential, logarithm…)
K2-W: first mass produced op amp (1952)K2-W: first mass produced op amp (1952)– Invented by George PhilbrickInvented by George Philbrick– $22. Gain ≈ 20,000. Gain-bandwidth ≈ 1MHz. $22. Gain ≈ 20,000. Gain-bandwidth ≈ 1MHz.
Input impedance ≈ 50kInput impedance ≈ 50kΩΩ– See picture (K2-W with and without shell)See picture (K2-W with and without shell)
History of Op AmpsHistory of Op Amps
First transistor op amp (μA702) used 12 BJTsFirst transistor op amp (μA702) used 12 BJTs– Invented in 1964 by Bob WidlarInvented in 1964 by Bob Widlar– Sold for $300Sold for $300– Prone to short circuitsProne to short circuits
μA709 improved on μA702 (used 14 transistors)μA709 improved on μA702 (used 14 transistors)– Invented in 1965 also by Bob WidlarInvented in 1965 also by Bob Widlar– Higher gain, more bandwidth, cheaper, more robustHigher gain, more bandwidth, cheaper, more robust– Sold for $70 to start, then $2 within a few yearsSold for $70 to start, then $2 within a few years
741 Op Amp741 Op AmpIntroduced in 1968, and versions still Introduced in 1968, and versions still available and in use todayavailable and in use today
μA741 Op AmpμA741 Op Amp
Op Amp FlavoursOp Amp Flavours
High output powerHigh output powerHigh speedHigh speedlarge bandwidthlarge bandwidthLow noiseLow noiseLow power Low power Low voltageLow voltagePrecisionPrecisionRail-to-RailRail-to-RailSmall packageSmall package>1 Op Amp per package>1 Op Amp per package
Ideal Op AmpIdeal Op Amp• Input resistance is infinite
• Output resistance is zero
• Bandwidth is infinite
• Input current is zero
• Open loop gain is infinite
• Output voltage is zero when input voltages are zero
AV
V V
V
VOU T OU T
IN
ROU T 0
I 0
R IN
I 0
Ideal Op Amp GainIdeal Op Amp Gain
If open loop gain of ideal op amp → ∞, what’s If open loop gain of ideal op amp → ∞, what’s the use?the use?– Any input will result in + or – saturationAny input will result in + or – saturation
Even non-ideal op amps not useful with open Even non-ideal op amps not useful with open looploop– Only small input voltages before + or - saturationOnly small input voltages before + or - saturation
Op amps almost never use open loop.Op amps almost never use open loop.– Use feedback to control the actual gainUse feedback to control the actual gain
Feedback!Feedback!
FeedbackFeedback
Positive FeedbackPositive Feedback– Output feeds back to input, and increase Output feeds back to input, and increase
output leading to out of control systemoutput leading to out of control system– E.g., microphone picks up speaker output, E.g., microphone picks up speaker output,
driving speaker output higher which is picked driving speaker output higher which is picked up by microphone…up by microphone…
Negative FeedbackNegative Feedback– Output feeds back to input to allow system to Output feeds back to input to allow system to
adjust and keep output within a certain rangeadjust and keep output within a certain range– E.g., body controlling blood glucose levelsE.g., body controlling blood glucose levels
Feedback in Op AmpsFeedback in Op AmpsIf output is fed back to non-inverting input – If output is fed back to non-inverting input – positive feedback occurspositive feedback occurs– Not usefulNot useful
If output is fed back to inverting input – negative If output is fed back to inverting input – negative feedback occursfeedback occurs– Widely used configurationWidely used configuration
Golden Rules for Op-AmpsGolden Rules for Op-Amps
1. With negative feedback:1. With negative feedback:The op amp drives the output so that The op amp drives the output so that the two inputs are at equal voltagethe two inputs are at equal voltage
2. 2. Assume that the input current is zero.Assume that the input current is zero.
Buffer or Voltage FollowerBuffer or Voltage Follower
No voltage difference between the output and No voltage difference between the output and the inputthe input
Buffer draws no current, so it puts no load on the Buffer draws no current, so it puts no load on the source. Output current supplied by VCC.source. Output current supplied by VCC.
Used to isolate sources from loadsUsed to isolate sources from loads
Non-Inverting AmplifierNon-Inverting Amplifier
Non-Inverting AmplifierNon-Inverting Amplifier
AAVV = V = Voo/V/VII
VVII = V = V22 (i.e., V (i.e., Vin+in+ = V = Vin-in- ) … from “Golden Rules” ) … from “Golden Rules” No current flows into VNo current flows into Vin+in+ (Golden Rule) therefore: (Golden Rule) therefore: VVII = V = V2 2 = V= Vo o * R1/(R* R1/(R11 + R + R22) … voltage divider) … voltage divider
AAVV = V = Voo/V/VII = (R = (R11 + R + R22)/R)/R11 = 1 + (R = 1 + (R22 / R / R11))
V2
Non-Inverting AmplifierNon-Inverting AmplifierOther ConsiderationsOther Considerations
R2 can’t be too low since the Op Amp has R2 can’t be too low since the Op Amp has limited current capabilitylimited current capability
R1 should be much smaller than the input R1 should be much smaller than the input impedanceimpedance
Circuit voltage gain should be much less Circuit voltage gain should be much less than the open loop gain of the Op Ampthan the open loop gain of the Op Amp
The output voltage swing has to be less The output voltage swing has to be less than the supply voltagesthan the supply voltages
Non-Inverting Amplifier Non-Inverting Amplifier ExamplesExamples
(from course package)(from course package)
Inverting AmplifierInverting Amplifier
Inverting AmplifierInverting Amplifier
Current through RCurrent through R11 equals the current equals the current
through Rthrough Rff
No current into the inputsNo current into the inputs
The voltage at both op amp inputs is zeroThe voltage at both op amp inputs is zero
Inverting AmplifierInverting Amplifier
• Current through R1
• Current through Rf
IV
R
V
R11
1
1
1
0
IV
R
V
RO
f
O
f2
0
Inverting AmplifierInverting Amplifier
Why the minus sign for the current through Why the minus sign for the current through RRff??
– The convention for Ohm’s Law is that the The convention for Ohm’s Law is that the current flows from the high voltage to the low current flows from the high voltage to the low voltage for a resistorvoltage for a resistor
– Here the current flows from the low voltage Here the current flows from the low voltage (ground) to the high voltage (V(ground) to the high voltage (VOO))
Inverting AmplifierInverting Amplifier
• The current through R1 must equal the current through R2 since there is no current in the inputs.
• Combining the two equations for the currentsV V
R
ROUT INf 1
Inverting Amplifier ExamplesInverting Amplifier Examples
(from course package)(from course package)
Op Amp ApplicationsOp Amp Applications
ArithmeticArithmetic– Summing, differencingSumming, differencing
CalculusCalculus– Integration, differentiationIntegration, differentiation
Level detectorsLevel detectors– Comparators, Schmitt TriggersComparators, Schmitt Triggers