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Homework Statement
A particular inverting op-amp has a gain of -100 V/V, and an input resistor 100 K. The output voltage is 9.32 V when measured with the input open and has a 9.10 V. when measuring with the input grounded. What is the input offset voltage and bias current?
The circuit is attached
Homework Equations
The Attempt at a Solution
From the gain and the input resistor I know that R_2 has to be 10 MΩ. But I just can't seem to set my equations up properly.
I get confused because bias current flows from output to V_1. But then offset voltage is at the inverting terminal. Since current is flowing opposite of V_os, I thought it'd be simply V_os - I_B1 * R_2 for when it is grounded. I don't know how to get the equation for when the input is open though since we're not given V_in? I feel like I'm complicating this way more than needed.
When grounded I got this..
[itex] \large 9.10={v_{os}}- I_{B1}{10MΩ}[/itex]The answer for the equations are actually, for open input
[itex] \large 9.32 = v_{os} + 10000 I_{B1} [/itex]
and for ground..
[itex] \large 9.10 = v_{os}*101 +10000 I_{B1}[/itex]
I don't get how they got those directions, all the terms are positive. And for when the input is grounded how did they get the gain of 101 but for input there is gain of only 1 in front of v_os?
If anyone could explain I would appreciated very much, really want to get this concept down.