Op Amp Offset Null

The op amp offset null capability is used to null any small DC offsets at the output for DC amplifiers.

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The offset null connections present on many operational amplifiers chips can be used to null any small DC offsets that might appear if the capability were not used.

Although the op amp offset null may not be important where AC coupling is used, there are some electronic circuits where DC amplifiers are used. Here it can be of particular importance, enabling high DC gains to be employed without large offsets that can be present.

Using the offset null capability is easy, and often just requires the addition of a single electronic component: a potentiometer to the circuit, although in some instances digital techniques may be also employed. There are few changes to the electronic circuit design that are needed: typically the addition of the single potentiometer to null the offset, or the digital nulling capability.

Op amp input offsets & input current

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 operational amplifier is ever 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 and the drive currents required fort he input biassing and drive, etc. 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.

An op amp input uses a long tailed pair to provide the differential input
An op amp input uses a long tailed pair

The operational amplifier 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.

Typical offset voltages (with both inputs connected together) can be around 1 mV, although some precision operational amplifier chips may have offset voltages of around 10µvolts.

Offset voltage drift

One issue for precision operational amplifiers is the drift of the offset voltage. This can occur with both temperature and somewhat less so with time.

Figures of around 10µV/°C may be appropriate for an ordinary operational amplifier, and less for a precision one.

However the drift may be of concern, because even with an offset null capability, it will mean the offset could grow again with temperature changes.

Offset null voltage definition

Input Offset Voltage:   Input offset voltage is defined as the voltage that must be applied between the two input terminals of an op amp to null or bring the output voltage to zero.

The offset null effectively applies this voltage to ensure that the offset is removed from the output.

Where offset drift is an issue the electronic components used to provide the offset null capability could include a temperature compensation to null the effect of the operational amplifier offset drift.

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 operational amplifier, 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.

Typical input circuit of an op amp - that shown is for a 741
Typical input circuit for an op amp showing the offset null

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 when undertaking the electronic circuit design, check the data sheet for the most suitable value.

Simple potentiomenter based offset null adjustment circuit for op amps like the 741
Simple op amp offset null adjustment

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

741 op amp pin connections
741 op amp pin connections including those for offset null

Operational amplifier offset null capability is used for applications where DC amplification is required. The small offsets caused by the input circuitry are amplified and is left unchecked they can become an issue.

One of the major areas where the offset null capability is used is within instrumentation and sensor applications. Here, small DC changes need to be monitored and op-amps provide the idea way of amplifying the DC voltages before they are measured or converted to digital formats. As such the offset null capability is essential for the operation of these circuits.

The most straightforward method for applying the offset null is to use a simple potentiometer, but increasingly digital feedback is used and the value is measured at set automatically by the system. Nevertheless the same principle is used within the op-amp.

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