How to amplify only the DC detla voltage ?

[lordvader88]

I was trying to make a little 555 freq. to voltage converter, (not really working I don't think). But anyways at 1 point I was getting from an LM358 output, about DC 9.00-10.60V from for 3 ranges of frequencies, from say 60Hz to 87kHz, about 0.500V for each range, as I sweep the coarse freq control.

How can I take that DC change of about 1.50V and amplify it on a centered 2.5V say ? Ignoring my circuit thats probably not working, I thought take the lm358 out1 and split it down to 2.5V  (+1.50Vdelta) with a voltage divider. What circuit will basically just take the DC 1.5V change and multiply it by 2 or 3 so I'd get 2.5V to 7V for example?

[paulca]

Not sure but are just after a differential (subtractor) amp?

http://www.electronics-tutorials.ws/opamp/opamp_5.html

You can subtract the two voltages and then multiply the difference by the gain.

[lordvader88]

I forgot they could do that, and thats 1 thing I used to model on circuit wizard a year ago.