Simulating input bias current

[Freddybaby]

Is it possible to simulate input bias current using the DC Bias analysis ?

I tried but my simulation requires a voltage source. Can I fool the software ? I'm using 5spice. Want to
simulate an LM358 with 1M across the + input, no signal in....

I'm new to Spice but not to op amps...

Fredrick

 

[audioguru2]

Hi fredrick,
It is easy to calculate the effect of input bias current with an input resistor for an opamp.

The typical input bias current for an old LM358 is 45nA and the max is 250nA. The inputs are PNP transistors so a resistor to ground will have the bias current in it causing the input voltage to rise.
45nA across 1M= 45mV.
250nA across 1M= 250mV.

Connect another 1M resistor as the bias feed (or feedback resistor) to the other input so the currents cancel and leave only the input offset current of 9nA to 50nA.

Or use an opamp with FET inputs which has virtually no input bias current.