Sevensoft logo

Opamp input currents

An ideal opamp input would not draw any input current. In a precision DC application, the effect of input currents flowing through a source resistance must be included in the error budget.

The values listed for the non-inverting input bias current are the typical values at 25 degrees C.  This parameter is used in preference to either the maximum bias current at 25 deg C or the maximum bias current over temperature because it is quoted for nearly all devices. Not all applications are subjected to a wide temperature range.

The behaviour of the bias current depends on the opamp input circuitry. If bias current is important in your circuit, you will need to understand the behaviour of the different types of input circuit.

JFET inputs

Opamps with FET inputs will have small input currents which are just leakage currents. The input currents may be positive or negative, and can have different polarities on the two inputs.  FET gate leakage currents also typically double with every 10 degree rise in chip temperature. This graph shows how the bias current varies with temperature for a typical device.
JFET bias against temperature
This graph suggests that very low bias currents can be achieved. An important caveat is that the inputs must be operating within all the manufacturer's limits, which will almost certainly include limits on common mode voltages and differential mode voltages. The next graph shows how protection diodes kick in when the differential voltage becomes too large.
JFET bias current with differential voltage
This limit on differential voltage should not become a problem during normal operation because the differential input voltage is normally negligible when the loop is closed.

Bipolar inputs

Bipolar transistors require a significant bias current of a predictable polarity. NPN transistors require positive current to flow in to the base, and PNP transistors require negative current to flow out of the base. The polarity of the input bias current is important when polarised capacitors are used to  create some form of AC filtering, as is common in audio applications. Although the base currents can be reduced at will, the tradeoff is an inevitable increase in noise and decrease in speed. As the bias currents for the two inputs are approximately equal, their effect can be cancelled out if the circuit uses matched bias resistors. In this case, only the difference in bias currents will contribute to circuit errors. This difference in bias currents is the "Input offset current", and may be specified by the device manufacturer.  

Some device contain additional current sources to provide the requisite input current.