# Op-Amp help

Discussion in 'Homework Help' started by turtal, Dec 13, 2008.

1. ### turtal Thread Starter New Member

Dec 13, 2008
2
0
I need to design an op-amp circuit that has the following input/output relationship: Vout = ln(Vin) and I am told to use a logathrimic op amp with values NVT = 47.6mV, Is = 3.035 x 10^-9 A, and R = 1k ohm
The logathrimic op amps Vout is equal to -NVT*ln(Vi) + NVT*ln(R*Is)

So, in a nut shell, I need to get -NVT*ln(Vi) + NVT*ln(R*Is) down to ln(Vi) with only 3 op amps.

I'm not asking for a step by step on this, but I've been at this thing for hours and I just wanna know where I am going wrong.

So far my design consits of 2 logathrimic op amps and one inverter, but I'm not sure how to get my values to knock out the NVT.

Any help would be great.
Thanks

2. ### turtal Thread Starter New Member

Dec 13, 2008
2
0
Alright, I'm closer to what I need to do.
My main problem is how do I remove NVT from the equation
Vout = -NVT*ln(Vi) + NVT*ln(R*Is) using a max of 3 op amps?

Once I can remove NVT, or in thise case 47.6mV, all I need to do is subtract the second part of the equation.

I'm starting to get confused with this, bare with me please, I'm new with these op amps

Thanks

3. ### steveb Senior Member

Jul 3, 2008
2,431
469
I would be interested to see if someone knows a solution to this. As far as I've seen, this is not trivial. The solution I've seen before is to introduce a temperature compensated resistor that offsets the temperature dependence of Vt.

You correctly figured out that the temperature dependence of Is can be eliminated. At least I think so, since you are not worried about the temperature dependence of Is, but only that of nVt. I'm assuming that you are using the usual approach of using two transistors in differential mode.