Op amp output impedance is an important factor in the design of any circuit. It determines the level of output voltage reduction when a load is applied.
The output impedance or resistance is also important because any voltage dropped within the op amp itself will dissipate power and this may cause the op-amp temperature to rise significantly.
Whilst the output impedance of most op amp circuits is low, this does not necessarily mean they can drive low resistance loads with high power levels. Their drivers have limits to the amount of power they can deliver.
Op amp output impedance & resistance basics
The output impedance of an operational amplifier, often designated Zo, arises from the fact that the output driver circuit and the associated connections have a defined impedance.
The output impedance can be split for many applications. The resistance element is of primary importance and is the major component of the overall impedance. However for some cases the reactance may also be an issue and this is caused mainly by the series inductance. To be fair, the reactive elements are normally small and are ignored for most op amp applications. Typically the frequencies at which op amps are used, the reactance levels will be small and not affect the circuit operation unduly. However they should not be forgotten as they may have an effect in some instances.
Accordingly the effective equivalent circuit for an op amp with its output resistance can be seen below.
As can be seen from the diagram, the op amp output resistance is the DC resistance that appears in series with the output from an ideal amplifier located within the chip. In other words the output resistance can be measured by looking at the voltage drop caused when a defined load is added to the output.
In most cases the output resistance is very low and very little drop will be seen. The major issue is normally that if reaching the limit of the current that the op amp will supply.
Also the reactance should not be ignored. High frequency operational amplifiers are available and the reactance can be such that it needs to be cosnidered for any calculations.
When looking at data sheets to discover the output impedance. Dependent upon the manufacturer, data sheets may list the output impedance under one of two different conditions. Some list closed-loop output impedance while others list open-loop output impedance. Confusingly both tend to use the designation Zo.
For many op amps the small signal impedance values fall between from about 50 Ω and 200 Ω.
Op amp out impedance can particularly be a design issue when using rail-to-rail output op amps to drive heavy loads. Under these circumstances the op-amp is required to drive a much higher voltage range, and current levels are higher, as well as requiring the output stage to reach voltages very close to the rails If the load is mainly resistive, the output impedance will limit how close to the rails the output can go - if voltages very close to the rails are required, this can cause problems. If the load is capacitive, the extra phase shift that this introduces can erode phase margin and lead to instability.
Op amp output drive capability
Another aspect that is linked to the output impedance of an op amp is the output drive capability.
Output drive capability is dependent upon a variety of aspects including the internal and external circuit and other conditions.
Internal factors include aspects such as the output-stage bias current, drive level, circuit architecture and capability as well as the process on which the chip was made.
External factors also influence the drive capability. However these can be controlled more easily as they are affected by the external circuit, although some are less controllable. External factors for the op amp drive capability include output voltage headroom, i.e. the voltage difference relative to supply rails; input overdrive; total supply voltage; dc- vs. ac-coupled load; and junction temperature.
It is obviously necessary to be able to specify the drive capability. Generally this is achieved by taking the output short-circuit current parameter. In general the manufacturer will specify the level of current that guaranteed to flow when the output is tied to ground. For situations where In a single-supply situation, the output is tied to one-half the supply voltage, called Vs/2.
Often two figures may be given, one for conditions where the op amp is sourcing current and another for the situation where the op amp is sinking current.
Using these figures it is possible to determine the behaviour of the op amp where the voltage swing across the load is low, and therefore the internal output-stage is able to maintain a large voltage headroom to the respective supply rails.
The overall op amp circuit output impedance is normally low and usually purely resistive. However aspects like the drive capability of the op amp need to be carefully considered as most chips have a very limited capability as they are not expected to drive large loads. Where large loads and high currents are needed, additional components can be added to provide the additional capability, or high power op amp chips can be used.
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