AC amplifier with fixed output range

acamplifierfrequency

Lets say I have a sine frequency sweeping circuit (1kHz-10MHz) that goes down in magnitude, from say 3V to 1.5V when it reaches its maximum frequency. I've tried buffering it (and even terminating with 50 ohms before, its impedance), however the voltage still varies, and much so more when the frequency gets higher.

What I desire for example.. A small AC will have the specific gain to reach 5V peak, or a larger AC (unlikely) attenuate to reach 5V peak, so I get a flat frequency response.

How would I amplify it to specifically go from 0V to 5V with its centre point at 2.5V for example? Will I have to employ some sort of buck/boost converter, or can it be done with some form of feedback with an emitter amplifier? An op amp might have something, but those destabilise at 1MHz, however if that is all I can do I can upgrade them.

Additional info. for Jippie: – about this "sweep device". It is simply a function generator (Hantek 3×25) and it tends to vary its voltage lower at higher frequencies, even with terminated output due to the quality of the device or whatnot (even though the sine wave is perfectly suitable that comes out.) It could be anything however, an XR function generator, just some variables sweep of some kind (a VCO in the future?). My scope and its probes are 40x the highest bandwidth my gen can put out, so not to worry for that 😛

Best Answer

An automatic gain control (AGC) appears to be what you want. It will try and keep the output constant by using a variable gain cell (usually a JFET) driven by a voltage that represents the peak magnitude of the output. If the output voltage signal level is too small the gain cell is rapidly compensated to achieve the correct amplitude.

You might also consider using a high frequency boost filter just to raise the amplitude of the input signal so that things are levelled out.