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# Differential mode

The op-amp is commonly used in situations where very small differences between two voltages have to be amplified.

This is the op-amp being used in differential mode.

## The op-amp in differential mode

A differential amplifier amplifies the potential difference between its two inputs. The circuit for a difference amplifier is shown in the diagram above.

When used in differential mode the gain expression is

This gain expression only holds if the values of the external resistors are chosen so that

Question

An op-amp is used in the differential mode with resistor values

R1 = R2 = 10 kΩ and Rf = R3 = 20 kΩ.

Calculate the output voltage Vo when V1 is +2 V and V2 = -4 V.

Answer

As with calculations on the op-amp in inverting mode, you should check to see if the output waveform is clipped because of the supply voltage used.

Here are some key points to remember for when an op-amp is used as a differential amplifier:

• the gain of the circuit is constant and determined only by the values of the external resistors R1, R2, R3 and Rf
• if the values of all four resistors are equal, then the differential mode gain expression becomes Vo = V2 - V1. This circuit performs the mathematical operation of subtraction
• since a differential amplifier only amplifies the difference between two signals any unwanted signal present on both inputs, called noise, will not be amplified

## Uses of op-amps in differential mode

A differential amplifier can be used to amplify the output produced by a range of different resistive sensors connected in a Wheatstone bridge arrangement to the inputs. If you are unsure about the Wheatstone bridge go to the Resistors revision bite.

The types of resistive sensors that can be used include:

• a thermistor, to respond to changes in temperature
• an LDR (light dependent resistor), to respond to changes in light intensity
• a strain gauge, to respond to movement when load is applied

## Differential amplifier with Wheatstone bridge

The circuit shown uses a thermistor in a Wheatstone bridge arrangement connected to a differential amplifier. Such circuits are used in monitoring and control applications. In this instance it is being used to monitor temperature.

In this circuit, the voltage at the inverting input of the op-amp changes as the temperature of the thermistor changes. The small voltage difference between the two inputs of the op-amp is amplified and can be used for control. The variable resistor is used to balance the bridge initially.

The output of the op-amp is normally used to control the physical change that is detected at the input by the Wheatstone bridge arrangement.

For example a drop in temperature detected by a thermistor at the input of a circuit can be counteracted by the output of the circuit increasing the heat supplied by a heater.

## Amplifier circuit used to monitor and control temperature

External devices such as heaters and motors normally take a larger current than the output of an op-amp can supply. When used to control external devices, a transistor is connected to the output of the op-amp as a power amplifier.

The diagram shows how a complete circuit controls a heater. The thermistor monitors changes in temperature.

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