# Transfer function of a real opamp circuit

I know how to calculate the transfer function of an ideal opamp. But when calculating for a real opamp, should I use the Input Bias Current values specified in the datasheet? If so, use the Min, Typ, or Max value?

Note: the links provided are only an example, not of the circuit I trying to figure.

How should I approach finding the transfer function, and essentially the value at VOUT?

• Assume the worst case, which is maximum input bias current since the input bias current is ideally 0. – Null Oct 21 '14 at 14:44
• So, if I am interested to find the input bias current of the current circuit, I modify the inputs setting, and subtract the two, but use a variable for the input current, right? – KingsInnerSoul Oct 21 '14 at 14:49
• Are you asking how to do this without a simulator? – copper.hat Oct 21 '14 at 15:12
• Ughh, For that circuit I think I would just simulate it. In general I don't think the input bias current will effect the gain that much.. (mostly a DC offset). After ideal, the next thing you want to add in is the GBW of the opamp. (Assuming the typical single pole gain roll-off.) – George Herold Oct 21 '14 at 15:18
• But how to calculate this multi stage opamp circuit? I know how to do a single step, but not sure about this, and just want to make sure I am approaching this right – KingsInnerSoul Oct 21 '14 at 15:32

As indicated already by G. Herold - forget the bias currents. These currents have only a minor effect on the circuits function.

Overall transfer function: It is not easy to read your drawing, but I guess the overall feedback resistors are R5 and R6, OK?

That means, your circuit contains a main amplifier A - consisting of two opamps (A1 and A2 with internal feedback) in series with an RC lowpass in between. Therefore, as a first step you should find the expression for the overall gain A. Then, as a next step, you apply Black`s feedback formula for the closed-loop gain Acl:

Acl=A/(1+A*k)

with k=feedback factor k=R6/(R5+R6) .

This gives you the gain referenced to the non-inv. input of the 1st opamp. As a final step, you can consider the passive circuitry at the non-inv. input.

Comment (edit): The first opamp in your circuit has no internal feedback. Therefore, if you assume ideal opamp properties (open-loop gain infinite) the gain A also will be infinite, and the closed-loop gain reduces to Acl=1/k. Otherwise (for real opamp properties), you must use the frequency dependent gain function for each of the two opamps.

• I want to find the effects of the Caps on the transfer function. How can that be found from those calculations? – KingsInnerSoul Oct 21 '14 at 15:59
• There are only two capacitors (C3 and C4) which are to be considered. I suppose, you know how to find the gain function for one single opamp with capacitive feedback? – LvW Oct 21 '14 at 16:04