• 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’ A_OL
• 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)
  • V_out = output voltage (V)
  • V_S+ = positive voltage of the power supply (V)
  • V_S− = negative voltage of the power supply (V)

• V_out is defined by the equation:

V_out = A(V₊ − V₋)

• An ideal operational amplifier has the following characteristics:
  • Infinite input resistance (no current from the supply)
  • Infinite open-loop gain (A_OL) (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)

• 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 V_out as possible
• Properties of an op-amp are:
  • High gain (A = 10⁵ – 10⁶)
  • Used for feedback circuits
  • Has a differential input (output voltage V_out 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, V_out 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 V_out will have a magnitude equal to V₊
  • If V₊ is slightly smaller in magnitude than V₋, then V_out will have a magnitude equal to V₋