Op-amps

Analogue electronics uses electrical signals that are continually changing to represent some physical quantity such as the loudness of a sound.

The main circuit element used in analogue electronics is the operational amplifier (op-amp).

Op-amps were originally developed for early computers to carry out the mathematical operations of addition, subtraction, multiplication and division.

An op-amp can be used to increase the voltage of a signal. The voltage gain (A) of an op-amp is given by

An op-amp has two inputs:

• an inverting input
• a non-inverting input

But it only has one output. It also needs a dual rail supply.

The op-amp

An op-amp amplifies the difference between the voltage applied to the inverting input (V^-) and the voltage applied to the non-inverting input (V⁺). From this we get the equation

V_o = A_o(V⁺ - V^-)

If the amplifier in the example above was operated from a 9 V supply the output voltage would be -6 V. If it was operated from a 5 V supply the output would be clipped (due to saturation of the op-amp) or limited to the value of the negative supply voltage (-5 V).

For an ideal op-amp:

• The input current is zero. This means that it has infinite input resistance.
• There is no potential difference between the inverting and the non-inverting inputs. This means that both input terminals are at the same potential.

An op-amp can be used as a comparator where two voltages are compared.

The op-amp used as a comparator in an alarm circuit

In the alarm circuit shown above, the thermistor and the variable resistor form a potential divider. The bell rings when its top terminal is more positive than its lower terminal.

The voltage at the inverting input (V^-) is set to be just greater than the voltage at the non-inverting input (V⁺ = 0) at room temperature.

If the temperature of the thermistor increases, the voltage at the inverting input falls and the output voltage changes to +15 V and so the bell rings.