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

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Go to Top