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. 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 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 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 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 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 Applications using saturation Comparators Comparator with hysteresis (Schmitt trigger) Oscillators Applications using active feedback components Log, antilog, squaring etc. amplifiers Precision rectifier

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

Microcap Demo 1

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 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: But,

Microcap Demo 2

Input-Output Relationship V 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

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 Using Kirchoff’s current law:

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

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.