I would like to build building a Voltage Controlled Oscillator (VCO) for audio applications beginning with a relaxation oscillator build from op amps. Adding a potentiometer to the feedback loop allows for frequency control, but as I increase the frequency, the amplitude of the signal reduces. I'l like to build a circuit that takes that signal and adds a dynamic gain to make it so that the final output signal of the oscillator rises to the same peak voltages regardless of frequency. I am having problems constructing such a circuit using op amps and would appreciate any ideas you have to get me on the right track. Beore writing this post, I searched for: Voltage Normalizer Circuit and Voltage Stabilizer Circuirt. Neither searches yielded anything useful.
I have an idea of how I need to accomplish this using plain math, but can't figure out how to translate that into op amps:
Step 1: Create a peak detector on the input signal using a Voltage follower, diode and capacitor to hold the voltage in place. (+-7V swinging about ground for this example). I think I can handle this part.
Step 2: Provide a reference signal to a desired peak input voltage (+-11V for this example)
Step 3: Establish a gain ratio from the desired peak and the input peak (for this example if I were to focus on just the positive side, it would be 11V/7V = ~1.57).
Step 4: Create a non-inverting amplifier off the input signal using the gain ratio determined in Step 3 as the resistance in the negative feedback loop. (For this example, it would essentially take the input signal with a peak amplitude of 7V and amplify it by 1.57 to output a signal with gain that peaks at close to +-11V).
I'm stuck on Step 3 and 4. I don't know how to create a "voltage controlled resistor" that I can use as part of the negative feedback gain loop of an op amp circuit. This looks like it would have a pretty simple solution, but I'm at a loss as to discovering/understanding it. I'd appreciate any help you all can provide on this design issue.