CIE A Level Physics (9702) 2019-2021

Revision Notes

24.1.1 Ideal Op-amp

Properties of an Ideal Op-amp

  • An operational amplifier (op-amp) is:

A device which is integrated into circuits to produce an amplification or gain to the voltage

  • In other words, the purpose of an amplifier is to produce an output signal larger than that of the input signal
  • This gain in voltage is for all frequencies and has the symbol A
    • This is sometimes referred to as the ‘open-loop gain’ AOL
  • Examples of amplifiers are:
    • Radio and television transmitters and receivers
    • Hi-fi stereo systems
    • Instrument amplifiers
  • An operation amplifier (op-amp) circuit is used in the automation and control of electronic circuits
  • The circuit symbol for an op-amp is a triangle with 5 pins:
    • V+ = plus input (sometimes referred to as ‘non-inverting’ input) (V)
    • V = minus input (sometimes referred to as ‘inverting’ input) (V)
    • Vout = output voltage (V)
    • VS+ = positive voltage of the power supply (V)
    • VS− = negative voltage of the power supply (V)
  • Vout is defined by the equation:

Vout = A(V+ − V)

  • An ideal operational amplifier has the following characteristics:
    • Infinite input resistance (no current from the supply)
    • Infinite open-loop gain (AOL) (gain obtained when no overall feedback is used)
    • Zero output resistance (no ‘lost volts’ when current is supplied)
    • Infinite bandwidth (an ideal op-amp will amplify signals of all frequencies)
    • Infinite slew rate (changes in output are instantaneous to changes in input – no time delay between the two)

Uses of Op-amp

  • A comparator compares two voltage and the output depends on which voltage is greater
  • The function of an op-amp is to use the potential differences between two inputs V+ and V to produce a large an output Vout as possible
  • Properties of an op-amp are:
    • High gain (A = 105 – 106)
    • Used for feedback circuits
    • Has a differential input (output voltage Vout is proportional to the difference in V+ and V input signals)
  • An ideal Op-amp can be used to compare two voltages:
    • When the two voltages are equal, Vout is 0
    • It can compare V+ and V and tell which one is larger
    • This is because an ideal op-amp has infinite resistance
  • A comparator circuit can be used to compare, for example, two temperatures of two light levels
    • If V+ is slightly higher than V then Vout will have a magnitude equal to V+
    • If V+ is slightly smaller in magnitude than V, then Vout will have a magnitude equal to V

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