Summing circuit operational amplifier problem?

Question

The question goes like this

A) design an inverting amplifier with an input resistance of 20 kohms and a gain of negative ten (-10).

B) Include a resistor to compensate for bias current

C) Design a summing amplifier such that Vout = -(10*V1+2*V2+0.5*V3).

So I think I have the design sort of. Down here is a link for part A and I think I have that right.

http://s274.photobucket.com/albums/jj243...

What I don't get is part B. Where should I add the resistor to compensate for bias current?

Also for the summing circuit, I know it should look something like this

http://s274.photobucket.com/albums/jj243...

But what resistor values should I use? I'm guessing that you do want any gain so the Rf resistor should be the same as your input resistor. But which would be your input resistor. And do you use a value that is 10 times lower than your feedback resistor to get a voltage 10 times the first one or use a value that is 2 times bigger than your feedback resistor to get .5 gain out of voltage 3? Some one please help

Answer 1

Photobucket pictures are blocked by my server so I am 'blind'.  But, I can still answer.

A) Rf = 200K ; Rin= 20K

Rf is connected from the OpAmp output to its inverting (-) input

Rin is connected from the Input to the inverting (-) input

B) the compensating resistor is connected from the OpAmp non-inverting (+) input to ground.  The value will be the parallel combination of 200K and 20K.  200K || 20K = 18.1818...K  (Use 18.0K)

Now, the bias currents from both + and - inputs will 'see' the same resistance to ground and the difference voltage due to those bias currents will be minimized.

EXTRA NOTE: IF you knew something about the source impedance (it probably won't be a pure voltage source) then you can add that value to the 20K and recalculate the compensation resistor value.

C) For the summing amp:

Use the same feedback resistor: 200K

Connect V1 to a 20K resistor to the (-) input (summing junction)

Connect V2 to a 100K resistor to the (-) input

Connect V3 to a 400K resistor to the (-) input

If you want to compensate, connect a 200K || 20K || 100K || 400K = 15.3K resistor from the (+) input to ground.

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