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

2. 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.

3. 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.

4. 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)

5. Saturation VOUT cannot exceed the supply voltages.
In fact, typically VOUT can only get to within about 1.5 V of the supplies.

6. 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

7. 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

8. 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

9. Microcap Demo 1

10. 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

11. Schmitt Trigger
The Schmitt trigger is an op-amp comparator circuit featuring hysteresis.
The inverting variety is the most commonly used.

12. Schmitt Trigger Analysis
Switching occurs when:
But,

13. Microcap Demo 2

14. Input-Output RelationshipV IN
OUT +
THRESH
(i)
SAT -
VIN increasing V IN
OUT -
THRESH (i i ) +
SAT
VIN decreasing V IN
OUT +
THRESH (i ii )
SAT -
(i) & (ii) combined

15. Asymmetrical Thresholds
We don’t always want the threshold levels to be symmetrical around 0 V.
More general configuration features an arbitrary reference level.

16. Analysis
Using Kirchoff’s current law:

17. Realising VREF Solving
often gives a value of VREF that isn’t available.
Providing and
But,

18. 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.