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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. 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 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 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. VOUT VOUT t Desired Output W aveform t Actual Output Wa veform Consequences of Saturation Unwanted when: Wanted when: Linear amplification was required 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 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 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: 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 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 R1 R2 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 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.