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More Non-Ideal Properties

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

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  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. Switching occurs when: But, Schmitt Trigger Analysis

  13. Microcap Demo 2

  14. (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

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

  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.

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