# SparkFun Forums

### Rf resistor?

General project discussion / help
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### Rf resistor?#163098

By 8Volts
#163098
Hello,

I would like to discuss the following circuit described in the attachment below. Please do not post anything in relation to any other op amp typologies such as the differential, inverting ect... configurations... I want to only discuss the configuration shown in the attachment.

My question is, by applying 1Vdc at the inverting input, what rf resistor would one need in order to get -5.96 Vdc at the output. We don't know the current nor the rf resistor.

I know rf's value, since I built the circuit and measured vout. But what I don't understand is how to calculate rf?

It would be very appreciated if one can show the calculations...
Thanks
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### Re: Rf resistor?#163100

By jremington
#163100
The posted circuit won't do anything useful, so there is no point in discussing it.

### Re: Rf resistor?#163106

By 8Volts
#163106
The posted circuit won't do anything useful.
Yes indeed.

But I am curious as to why this circuit behaves the way it does. You may try this circuit yourself and replace rf with values from 10 to 1K and the op amp will output a useful voltage range between aprox +6Vdc to -14 Vdc respectively

But I haven't a clue on whats actually happening ??

Bof, I can simply forget about it and stay ignorant, or I keep on trying until its clear!!!

thanks

### Re: Rf resistor?#163107

By jremington
#163107
The op amp is current limiting, trying to bring the (-) input to 0Vdc. Since the current limit is a function of the particular op amp you are using, the design does not produce predictable results.

### Re: Rf resistor?#163110

By 8Volts
#163110
Okay thanks jremington!

### Re: Rf resistor?#163124

By jremington
#163124
That circuit actually does have a use, if the input to the op amp (-) terminal is a current instead of a fixed voltage. Then it becomes a current-to-voltage converter. This circuit is typically used to convert the output of a photodiode or phototube, which generates a current proportional to the light intensity.

### Re: Rf resistor?#163238

By 8Volts
#163238
That circuit actually does have a use, if the input to the op amp (-) terminal is a current instead of a fixed voltage. Then it becomes a current-to-voltage converter
Yes, that's exactly what I learned yesterday

These op amps .... is there anything they can't do!!!

Thanks for the up keep!
8v

### Re: Rf resistor?#163259

By dschlic1
#163259
The output depends not only on Rf but also on the impedance or series resistance on the 1 VDC voltage source. So the equivalent series resistance of the voltage source then becomes Rin which along with Rf determines the gain.

### Re: Rf resistor?#163309

By 8Volts
#163309
yes thanks dschlic1!
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