# Homework Help: Operational Amplifier

1. Apr 11, 2012

### yasef

I need a circuit that adds an AC signal with magnitude of 25mV and a variable DC signal between 0 to 5V, and applies it to a diode.

2. Apr 11, 2012

### yasef

How to superimpose AC signal With DC signal with operational amplifier?

3. Apr 11, 2012

### Staff: Mentor

What do you want to do with the "output" of the diode?

An op-amp can have any number of input resistors, each connected to the (-) input. If each Rin is made equal in value to the single feedback resistor, Rf, then the op-amp forms a unity-gain summer (albeit inverting). Is that good enough for your need?

4. Apr 11, 2012

### psparky

What's the input? What's the output?

What do you mean by superimpose?

5. Apr 11, 2012

### DragonPetter

Hi yasef,

This depends on the output impedance of your AC source, but some simple methods are to use bias resistors and an AC coupling coupling capacitor. This can form a high pass filter, so you need to choose appropriate values. You could also use a summing OpAmp.

6. Apr 11, 2012

### DragonPetter

7. Apr 11, 2012

### psparky

If you just mean you want a DC and AC signal on the output....then dragonpetter's idea will work.............

Or just connect your AC source to the "- input" and connect DC source to "+ input". And plan their gains accordingly.

8. Apr 11, 2012

### yasef

For the diode I will measure the C- V characteristic of the diode. Just thank you for your answer and could you tell me if the output signal of the summing amplifier will consist DC and AC coming together so that I can measure Capacitance and voltage simultaneously.

9. Apr 11, 2012

### yasef

This is what İ want to do:İ want to design a circuit to measure C-V curve of a reverse biased diode. Due to the specifications İ should use a circuit that adds an AC signal with magnitude of 25mV and a variable DC signal between 0 to 5V, and applies it to
a diode. Then İ measure AC current through the diode with a lock-in amplifier by using a built in function. Do you have any ideas how İ can do this?

10. Apr 11, 2012

### yasef

Thanks for your answers.This is what İ want to do:İ want to design a circuit to measure C-V curve of a reverse biased diode. Due to the specifications İ should use a circuit that adds an AC signal with magnitude of 25mV and a variable DC signal between 0 to 5V, and applies it to
a diode. Then İ measure AC current through the diode with a lock-in amplifier by using a built in function. Do you have any ideas how İ can do this?

11. Apr 11, 2012

### Staff: Mentor

I doubt that a summer is needed. Just couple the AC through a capacitor and join it with a relatively high resistor from your variable DC supply. You'll be measuring a ratio of voltages to allow you to estimate C, will you? This is a varactor experiment?

12. Apr 11, 2012

### f95toli

I've always used wide-band transformers for this, so-called modem transformers to be specific.
The DC goes through the secondary and the AC is "added" via the primary.
Works quite well as long as you can tolerate the extra inductance in series with the DC.
A capacitor might also work, but you need to watch out for the RC time-constant.

13. Apr 11, 2012

### yasef

This is not a varactor experiment. Actully now I had desined a circuit.
I added a circuit that adds AC signal with Magnitude of 25 mV a variable DC signal between 0 to 5 using with op amp, and applied to a diode. I will measure the AC current through the diode with a lock in amplifier. I will use lock in amplifier built in function generator and auxillary output port. From this experimental setup how can I measure C-V current of a reverse biased diode.

14. Apr 11, 2012

### Staff: Mentor

Okay.
Then what is C, since it's not capacitance?

15. Apr 11, 2012

### yasef

Actually I am not very experienced about these things but I think that varactor is a special case. This is just a diode. C come from diode.

16. Apr 12, 2012

### Staff: Mentor

Okay, so it's an improvised varactor diode. https://www.physicsforums.com/images/icons/icon14.gif [Broken]

Will you be using actual devices, or is this going to be a computer simulation?

Is this to be a semi-automated demonstration, or is your goal simply to obtain an accurate graph
of C vs. Vrev for a particular diode?

Last edited by a moderator: May 5, 2017
17. Apr 12, 2012

### yasef

We will use actual device. I am not sure what do you mean about semi outomated demonstration but our signal source for AC and DC is lock in amplifier. At the and I should abtain a graph of C vs V.