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−