# 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:
• 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.
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