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8/3/2019 04 Non Linear Apps or

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More Non-Ideal Properties

Bias Current

Offset Voltage

Saturation

Applications of saturation


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Bias Current

All op-amps draw a small constant d.c. bias currentsat 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 theinput impedance.


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Offset Voltage

When both input voltages are equal, the outputshould be zero.

Actually it probably wont be due to an offset voltagebetween the inputs.

Typically, this is around 2 mV.

This isnt much but is magnified so much by the op-amp gain that it will probably saturate.

Offset voltage is automatically compensated by anegative feedback network.

Can be a problem for precision comparatorapplications.


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D.C. Equivalent Circuit

Both the offset voltageand bias current are d.c.

A.C. operation is notaffected by them (theyjust add an offset)

Negative feedback reducesthe effect of both

Steps can be taken toreduce them (furtherreading)


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Saturation

VOUT cannot exceed the supply voltages.

In fact, typically VOUT can only get to within

about 1.5 V of the supplies.

Desired Output W aveform Actual Output Wa veform

VOUT VOUT

t t


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Consequences of Saturation

Unwanted when:

Linear amplification was required

Wanted when:

A clipping effect is required (e.g. distortion effectspopular with guitarists)

Essential when:

The op-amp is used as a comparator


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Non-Linear Op-Amp Applications

Applications using saturation

Comparators

Comparator with hysteresis (Schmitt trigger)

Oscillators

Applications using active feedbackcomponents

Log, antilog, squaring etc. amplifiers

Precision rectifier


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ComparatorsVOUT

VIN

Ideal responseVOUT = A0VIN

Practical response(clipped)

If A0 is large, practical response can be approximated as :VIN > 0 V+ > V- VOUT = +VSATVIN < 0 V+ < V- VOUT = -VSAT


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Microcap Demo 1


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Hysteresis

A comparator with hysteresis has a safety margin.

One of two thresholds is used depending on the

current output state.V

time

Upper threshold

Lower threshold


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Schmitt Trigger

The Schmitt trigger is an op-amp comparator circuitfeaturing hysteresis.

The inverting variety is the most commonly used.


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Schmitt Trigger Analysis

But,

21

1

RR

RVV

VV

SATTHRESH

SATOUT

s!@

s!

21

1

RR

R

VVVV OUTIN

!!!

Switching occurs when:


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Microcap Demo 2


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Input-Output Relationship

VIN

VOUT

+VTHRESH0

(i)

+VSAT

-VSAT

VIN increasing

VIN

VOUT

-VTHRESH

0

(ii)

+VSAT

-VSAT

VIN decreasing

VIN

VOUT

+VTHRESH0

(iii)

+VSAT

-VSAT

-VTHRESH

(i) & (ii) combined


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Asymmetrical Thresholds

We dont always wantthe threshold levels to be

symmetrical around 0 V. More general

configuration features anarbitrary reference level.


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Analysis

Using Kirchoffs current law:

012

!

R

VV

R

VVREFOUT

21

21

1212 RR

RRV

R

V

R

V

R

V

R

VREFOUT

!!

21

2

21

1

RR

RV

RR

RVV

REFOUT

!


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Realising VREF

21

2

21

1

RR

RV

RR

RVV

REFSATTHRESH

s!Solving

often gives a value of VREF that isnt available.

211 || rrR !21

2

rr

r

VVSREF

!Providing and

But,


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Summary

Saturation of op-amps is exploited bycomparator circuits.

Their function is to decide whether an inputvoltage is greater or less than a referencelevel.

Hysteresis is often applied to provide someresilience against noise.

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