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More Non-Ideal Properties. Bias Current Offset Voltage Saturation Applications of saturation. Bias Current. 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.

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More non ideal properties l.jpg
More Non-Ideal Properties

  • Bias Current

  • Offset Voltage

  • Saturation

  • Applications of saturation


Bias current l.jpg
Bias Current

  • 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 l.jpg
Offset Voltage

  • 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 op-amp 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 l.jpg
D.C. Equivalent Circuit

  • 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 l.jpg
Saturation

  • VOUT cannot exceed the supply voltages.

  • In fact, typically VOUT can only get to within about 1.5 V of the supplies.


Consequences of saturation l.jpg
Consequences of Saturation

  • Unwanted when:

    • Linear amplification was required

  • Wanted when:

    • A clipping effect is required (e.g. distortion effects popular with guitarists)

  • Essential when:

    • The op-amp is used as a comparator


Non linear op amp applications l.jpg
Non-Linear Op-Amp Applications

  • Applications using saturation

    • Comparators

    • Comparator with hysteresis (Schmitt trigger)

    • Oscillators

  • Applications using active feedback components

    • Log, antilog, squaring etc. amplifiers

    • Precision rectifier


Comparators l.jpg
Comparators

If A0 is large, practical response can be approximated as :

VIN > 0 Þ V+ > V-Þ VOUT = +VSAT

VIN < 0 Þ V+ < V-Þ VOUT = -VSAT



Hysteresis l.jpg
Hysteresis

  • 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 l.jpg
Schmitt Trigger

  • The Schmitt trigger is an op-amp comparator circuit featuring hysteresis.

  • The inverting variety is the most commonly used.


Schmitt trigger analysis l.jpg

Switching occurs when:

But,

Schmitt Trigger Analysis



Input output relationship l.jpg

(i)

(i

i

)

(i

ii

)

V

V

V

OUT

OUT

OUT

+

V

+

V

+

V

SAT

SAT

SAT

-

V

-

V

THRESH

THRESH

0

0

0

+

V

V

V

+

V

V

THRESH

IN

IN

THRESH

IN

-

V

-

V

-

V

SAT

SAT

SAT

VIN increasing

VIN decreasing

(i) & (ii) combined

Input-Output Relationship


Asymmetrical thresholds l.jpg
Asymmetrical Thresholds

  • We don’t always want the threshold levels to be symmetrical around 0 V.

  • More general configuration features an arbitrary reference level.


Analysis l.jpg
Analysis

Using Kirchoff’s current law:


Realising v ref l.jpg

But,

Providing and

Realising VREF

Solving

often gives a value of VREF that isn’t available.


Summary l.jpg
Summary

  • 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.