More Non-Ideal Properties
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Bias Current
Offset Voltage
Saturation
Applications of saturation
Bias Current
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All op-amps draw a small constant d.c. bias currents
at their inputs.
Typical value for a 741 is around 100 nA.
This is only notable when very high impedance
sources are used.
In such cases, an alternative op-amp with lower bias
current should be used.
NB. Bias current is separate to input impedance. It is
equivalent to a current source in parallel with the
input impedance.
Offset Voltage
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When both input voltages are equal, the output
should be zero.
Actually it probably won’t be due to an offset voltage
between the inputs.
Typically, this is around 2 mV.
This isn’t much but is magnified so much by the opamp gain that it will probably saturate.
Offset voltage is automatically compensated by a
negative feedback network.
Can be a problem for precision comparator
applications.
D.C. Equivalent Circuit
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Both the offset voltage
and bias current are d.c.
A.C. operation is not
affected by them (they
just add an offset)
Negative feedback reduces
the effect of both
Steps can be taken to
reduce them (further
reading)
Saturation
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VOUT cannot exceed the supply voltages.
In fact, typically VOUT can only get to within
about 1.5 V of the supplies.
VOUT
VOUT
t
Desired Output W aveform
t
Actual Output Wa veform
Consequences of Saturation
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Unwanted when:
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Wanted when:
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Linear amplification was required
A clipping effect is required (e.g. distortion effects
popular with guitarists)
Essential when:
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The op-amp is used as a comparator
Non-Linear Op-Amp Applications
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Applications using saturation
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Comparators
Comparator with hysteresis (Schmitt trigger)
Oscillators
Applications using active feedback
components
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Log, antilog, squaring etc. amplifiers
Precision rectifier
Comparators
VOUT
Ideal response
VOUT = A0VIN
Practical response
(clipped)
VIN
If A0 is large, practical response can be approximated as :
VIN > 0  V+ > V- VOUT = +VSAT
VIN < 0  V+ < V- VOUT = -VSAT
Microcap Demo 1
Hysteresis
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A comparator with hysteresis has a ‘safety margin’.
One of two thresholds is used depending on the
current output state.
V
Upper threshold
time
Lower threshold
Schmitt Trigger
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The Schmitt trigger is an op-amp comparator circuit
featuring hysteresis.
The inverting variety is the most commonly used.
Schmitt Trigger Analysis
Switching occurs when:
R1
VIN  V  V  VOUT
R1  R2
But,
VOUT  VSAT
VTHRESH  VSAT
R1
R1  R2
Microcap Demo 2
Input-Output Relationship
(i)
(ii)
VOUT
(iii)
VOUT
+VSAT
VOUT
+VSAT
+VSAT
-VTHRESH
-VTHRESH
0
+VTHRESH
-VSAT
VIN increasing
0
VIN
-VSAT
VIN decreasing
0
VIN
+VTHRESH
-VSAT
(i) & (ii) combined
VIN
Asymmetrical Thresholds
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We don’t always want the
threshold levels to be
symmetrical around 0 V.
More general
configuration features an
arbitrary reference level.
Analysis
Using Kirchoff’s current law:
VOUT  V VREF  V

0
R2
R1
VOUT VREF V V
R1  R2


   V
R2
R1
R2 R1
R1R2
 V  VOUT
R1
R2
 VREF
R1  R2
R1  R2
Realising VREF
R1
R2
 VREF
Solving VTHRESH  VSAT
R1  R2
R1  R2
often gives a value of VREF that isn’t available.
But,
Providing R1  r1 || r2 and VREF
r2
 VS
r1  r2
Summary
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Saturation of op-amps is exploited by
comparator circuits.
Their function is to decide whether an input
voltage is greater or less than a reference
level.
Hysteresis is often applied to provide some
resilience against noise.