Op Amp Offset Null

- overview of op-amp offset null and how it can be used to reduce small DC offsets for DC amplifiers..

The op amp offset null capability is one that is available on many op-amp chips.

The offset null capability is used to reduce small DC offsets that can be amplified. These can be important in DC amplifiers where these small voltages can then become significant where large gains are required.

Op amp input offsets

An op amp is a differential amplifier. This means that when there is no difference between the two inputs, e.g. when the inputs are shorted together, there should be no voltage on the output.

Unfortunately under these circumstances there is always a small offset because no op amp is never perfect and completely balanced. There is always a small input offset voltage.

This input offset voltage is small and arises from mismatches in the differential input stage of the op amp chip. These small offsets are caused by a variety of unavoidable issues within the manufacture of the op amp. They include aspects including mismatched transistor pairs, collector currents, current-gain betas (β), collector or emitter resistors, etc..

Typical input circuit of an op amp showing the long tailed pair and the way in which mismatches can cause an offset.
Typical op amp differential input circuit

The op amp offset arises particularly as a result of mismatches in the input. If the transistors Q1 and Q2 are perfectly matched, the current, I, from the current source is divided equally between them. If there is a mismatch, then the current in either leg is different.

A mismatch in transistors usually occurs because of mismatches in emitter areas for bipolar junction transistors and W/L ratios for MOSFETs.

A mismatch in the collector resistors, Rc1 and Rc2 also causes different voltages to be developed across them resulting in different voltages being passed on to the following stages of the op amp.

The output can be made completely zero if a small offset voltage is applied to the inputs. This is known as the input offset voltage.

Implementing offset null within the op amp

It is possible to add some additional connections to enable the results mismatches in the chip to be significantly reduced. Using an offset null capability it is possible to reduce the level of the offset voltage significantly.

To incorporate an offset null capability within the op amp, a number of different actual circuits can be adopted. The op amp offset null circuitry adds a balance capability to the input circuitry as shown in the example of the 741 op amp input stage.

The input circuit of a 741 op amp showing the offset null circuitry.
Op amp offset null circuit of a 741

The op amp offset null connections enable the input circuit balance to be obtained by applying external circuitry.

Op amp offset null

For circuits where it is necessary to remove or null the offset, many op-amp chips provide two pins that enable this to be done. Using the offset null adjustment requires a potentiometer with its wiper connected to the negative supply with some op amps or to 0 V with others so it is necessary to check the data sheet. The value for the potentiometer may typically be around 10 KΩ to 100 KΩ but again check the data sheet for the most suitable value.

The simple addition to the circuit outside the chip to enable the op amp offset null to be implemented includes a single adjustable potentiometer.
Op amp offset null adjustment

On op amps with an offset null capability two pins are provided as shown in the diagram.

Diagram showing the pin connections of the 741 op amp and in particular the offset null connections.
741 op amp pin connections including those for offset null

The offset null capability for op amps is often used in instrumentation applications. For example where the small DC voltages produced by thermocouples or other sensors need to be amplified. The offset null may be used in other applications where DC amplification is required and the offsets need to be removed.

Although the most common method is to use a preset potentiometer, many instruments may use self-calibration where the DC offset is measured under zero input conditions and then a digital value used to counter the offset that is measured. This could be achieved by applying the required voltage to the offset null, applying an equal and opposite DC offset, or applying the measured offset in any digital processing that may be undertaken.

By Ian Poole

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