In an article I read, I stumbled across a circuit which seemed quite unfamiliar:


simulate this circuit – Schematic created using CircuitLab

According to the article I found this circuit in, a sinusoidal Vin should produce a DC voltage at Vout, which is then used to drive a voltage controlled amplifier. This seems to indicate that this circuit acts as a rectifier, however it doesn't resemble any of the normal rectifier circuits I've encountered so far.

I've tried solving and also simulating this circuit and I am not able to get a DC voltage at Vout as claimed in the article.

Any help as to how to approach this circuit would be greatly appreciated.

Edit: I forgot to mention that C1 is a variable capacitor which can vary by up to 1-2pF. The article also states that changing this capacitance will change the DC output level so that C1 can be used to change the gain of the Voltage controlled amplifier driven by Vout.

Edit 2: The article in question; The circuit is depicted on page 3 (The part labeled "Volume Antenna" with the short description of its functionality on page 2

  • \$\begingroup\$ How about a link to the article? \$\endgroup\$ – Andy aka Feb 27 '18 at 13:20

The bigger picture: -

enter image description here

It's from a theremin. The oscillator is a modified Colpitts (sometimes referred to as a Goral oscillator and this is a variant). The circuit relies on the difference in resonant frequencies between the tuned circuit (formed from L7 to L10 and the volume antenna's self capacitance) AND the colpitts oscillation frequency.

As you bring your hand towards the volume antenna the tuning lowers towards the frequency produced by the Colpitts oscillator and you get resonance. D1 acts like a diode (or envelope) detector and you get an increase in the DC voltage across C27. This DC voltage sets the "volume" of the theremin.

You probably need R35 and R37 present to make your simulation work.


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