Negative Feedback Equation

cambria83

Joined Nov 14, 2007
3
Hi,

I am really stuck with a negative feedback equation! I am learning the basics of op-amps on my music tech. course!

If I was to calculate the output voltage of the closed loop gain circuit when the input voltage is 14mV and the negative feedback voltage is 4mV where would I start?

This is probably really simple stuff but I am a complete newbie to all this and finding it a bit hard to get my head around at the moment!

-Neil-

Management

Joined Sep 18, 2007
306
Depending on the configuration of your feedback and if it is inverting or non-inverting you will get different equations. Try reading this: http://www.allaboutcircuits.com/vol_3/chpt_8/4.html

Is that 4mV feedback voltage coming from the output of your amplifier (I am assuming)? Do you have a schematic?

cambria83

Joined Nov 14, 2007
3
Hi,

Thanks for the reply! I did read that article earlier but it didn't really help me to understand how to work out the output voltage from the information I have!

As far as I know it is inverting! The exact question I have here is:

Using the negative feedback equation, calculate the output voltage (Vout) when input voltage (Vin) is 14mV and negative feedback voltage is 4mV.

I have just written a 1500 report on negative feedback but I still don't get how to work this out! I've been trying to find equations to work it out for hours!

Is this any help?

thingmaker3

Joined May 16, 2005
5,083
Vout = Vin * Rf / Riwhere Rf = feedback resistor value and Ri = input resistor value.

cambria83

Joined Nov 14, 2007
3
Vout = Vin * Rf / Riwhere Rf = feedback resistor value and Ri = input resistor value.
I saw an equation similar to this before but I got confused because I don't have any resistor values given to me do I? I think the reason I am so stuck at working this out is because I have only been given those two values! or am I just being really stupid here?

Distort10n

Joined Dec 25, 2006
429
The information you have is incorrect. If it is an inverting attenuator, then the output voltage must be negative because the input voltage is positive. So far, you have give us both positive voltages.
A non-inverting amplifier can never have a gain less than unity (Av > 1), so the 'negative feedback voltage' a.k.a. output voltage, will never be less than the input voltage.

chesart1

Joined Jan 23, 2006
269
I think the teacher means that the there is a voltage divider between the op amp output and the op amp negative input terminal that sets 4 millivolts at the op amp negative input terminal. The statement is negative feedback, not inverted output.

I can't be sure because the wording of the problem is vague.