# Design Circuit to Convert Current to Voltage | LTSpice Also

• Engineering
Hello,

We just finished up midterms and I actually did well--got an A. However, somehow between the midterm (class before last) and last class we jumped to creating a circuit with a diode. I am pretty lost.

1. Homework Statement

## Homework Equations

Voltage / Current = Resistance

## The Attempt at a Solution

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Here is how I designed it in LTSpice:

Ultimately, not only do I not know what the question is asking, but I don't know how to design it on paper nor in LTSpice. I am guessing I use a DC Sweep from the 5-volt source for part of it:

I really hate feeling this lost.

Hopefully you guys can help me through this.

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berkeman
Mentor
It sounds like they want you to model the bias voltage source and photodiode as a variable current source. So just change out the voltage source and vanilla diode in your LTSPICE circuit for a current source, and sweep it across the range they specify. At the max current that they list (still small), you need to drop 1V across the feedback resistor, right?

I don't think there is a simple photodiode model in SPICE (other than a current source).

Do you generally use real opamps in your simulations now, or super-ideal ones? I ask because of issues like input bias current and input offset current/voltage issues in a circuit like this that is dealing with very small currents. If you are using real models, which type of opamp would be better for this (Bipolar or FET input) and why?

EDIT -- I see now they say to use an ideal opamp. I'd still be interested in your answer about which type of real opamp to use for such a circuit.

Last edited:
gneill
berkeman
Mentor
Oh, and congrats on the midterms!

This is just HW for lecture, so I am unsure of which type of real opamp should normally be used.

Here is the circuit I've got so far:

Do you think this is the answer the professor might be looking for?

Here is the DC Sweep of the current source:

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Oh, and congrats on the midterms!
Thank you. :)

berkeman
Mentor
Looks good, but I would only sweep the current source over the range of interest of the problem (0A to 1uA), not all the way to 1A (boom!).

Oh, and you want to plot Vout as a function of Iin -- that would be a good plot to supply as your answer.

I'll let you get to my quiz question when you have some spare time. Just Google tradeoffs between bipolar and FET opamps and enjoy some interesting reading.

Ahh, okay. Something more like this?

Your question: I had to figure out what input bias current was first. If my understanding is correct, it is the small amounts of current in the inverting and non-inverting inputs of an opamp. Counterly, an ideal opamp doesn't represent this input bias current--which is why we call it "ideal".
It looks like one would use a bipolar for less noise and offset, and a FET for more area and power.

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berkeman
Mentor
Great plot, IMO. Good work!
Your question: I had to figure out what input bias current was first. If my understanding is correct, it is the small amounts of current in the inverting and non-inverting inputs of an opamp. Counterly, an ideal opamp doesn't represent this input bias current--which is why we call it "ideal".
Correct. The problem is, when you get to the real world (either in your hobby projects, or your labs, or your first job), you're required to use "real" opamps and other circuitry. That's why it's good to start asking questions about the differences between "ideal" and "real" circuits early.
It looks like one would use a bipolar for less noise and offset, and a FET for more area and power.
Those may be true, but I think of FET input amps when I don't want to have to take input bias currents into my first-order calcs. There are "low-noise" versions of both types of opamps, and I'd have to check out some of the more recent datasheets to see which version is winning the low-noise war currently.

Good job!

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