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How to Calculate Bias Current

An operational amplifier, or op-amp, is an electronic device that takes two voltage inputs and then outputs a voltage many times larger than the difference of its input voltages. Ideally, no current or voltage runs through the device when you apply a zero-volt voltage difference across the input terminals, but in practice this is not the case. Imperfections in the components of an op-amp lead to an internal "offset voltage" and small "bias currents" run along the inputs. To calculate bias current, you must first measure offset voltage.

DC voltage supply
Voltmeter
Two 100-kilo ohm resistors for bipolar op-amps (1 mega-ohm for FET op-amps)

Instructions

- 1
  Determine the offset voltage by using a DC voltage regulator to apply an equal voltage to the two input terminals of the op-amp. Measure the output voltage with the voltmeter. The result is the op-amp's offset voltage.
- 2
  Put the 100 kilo-ohm resistors in series with the input terminals of the op-amp.
- 3
  Repeat Step 1, applying an equal voltage to the input terminals (the resistor terminals now) and measuring the output voltage with the voltmeter.
- 4
  Subtract the voltage in Step 1 from the voltage in Step 3. For example, 0.03 volts minus 0.02 volts equals 0.01 volts.
- 5
  Divide Step 4's result by 100 kilo-ohms (1 mega-ohm for FETs). The result is your op-amp's bias current. Using 0.01 volts from above, you would obtain a bias current of 10^-7 amps, or 0.1 micro-amps.

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