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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.

Thanks!

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    \$\begingroup\$ "... as I increase the frequency, the amplitude of the signal reduces." That's because your op amp doesn't have a high enough GBW for your application. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Jun 13, 2015 at 1:46
  • \$\begingroup\$ Thanks for the response. I just did some research on the "gain bandwidth" 741 op amp and sure enough one of the webpages references exactly my problem, that at higher frequencies, my signal would drop in amplitude because the 741 has a hard drop off and it appears to occur in the audible range. Any suggestions on a good op amp that should be able to handle voltage gains uniformly at in the audible freq range? Don't need rail to rail, just something stable enough for normalizing freqs in the audible range. Thanks again! \$\endgroup\$
    – Dave Guenther
    Jun 13, 2015 at 3:35
  • \$\begingroup\$ If you can find one, the LMP8671 should be all the amp you need. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Jun 13, 2015 at 3:57
  • \$\begingroup\$ I think, in your case it is the limited SLEW RATE (very bad for 741 types) that reduces the amplitude for rising frequencies. Hence, watch in particular this parameter while choosing another opamp. \$\endgroup\$
    – LvW
    Jun 13, 2015 at 7:16
  • \$\begingroup\$ DaveG, question to you: What is the final waveform to be achieved? Squarewave or sinus (because you are mentioning audio applications)? \$\endgroup\$
    – LvW
    Jun 13, 2015 at 7:17

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I'm closing this post because it has been answered. The real Issue I had was that the gain would reduce as I went into a higher frequency range because I used an op amp with too low a gain bandwidth. For what it's worth, I hooked up an LM3900N and ran the higher frequencies that the lm741 couldn't handle and the LM3900N did just fine. Thanks to all for help with this. I'll do additional research on VCOs with a volt/oct range, but that is a separate issue and from what I have read it is easier to get through a micro controller than it is to build that circuit from components.

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