hi guys. I am an undergrad student majoring in electrical engineering

I have a project which is to design and make my own electric circuit, and I chose Theremin as my topic.

Problem is, my project doesnt allow the usage of any transistors, diodes or nand gates. The units that i can use are op amps, resistors, inductors and capacitors.

Is it possible to design a theremin with only those elements? If so, please tell me how to implement it or how the overall flow diagram should be like.

Please help😭

# Reaching for help for my project...Theremin masters Please Help

Hi guy.

The answer is yes. Build two ~130kHz Wien bridge oscillators around an op amp each, choosing carefully the feedback/amplification factor to maintain the oscillation on one side and to prevent saturation (clipping) on the other side. Add the signals together with a "wrong" DC bias, using a third op amp which will then act as a mixer, due to the non-linear characteristic induced by the DC bias. A fourth op amp as a 7kHz Sallen-Key LPF, and you are done.

Hi guy.The answer is yes. Build two ~130kHz Wien bridge oscillators around an op amp each, choosing carefully the feedback/amplification factor to maintain the oscillation on one side and to prevent saturation (clipping) on the other side. Add the signals together with a "wrong" DC bias, using a third op amp which will then act as a mixer, due to the non-linear characteristic induced by the DC bias. A fourth op amp as a 7kHz Sallen-Key LPF, and you are done.

Wow....thanks a million😭

Could you be a little more specific about what ‘adding signals together with a“wrong” DC bias’??

I’d reeeeally really appreciate that

Imagine a “normal” summing amplifier which would just add two signals, no demodulation would happen. For demodulation, you need basically a multiplication, which can be achieved by summing in a non-linear (ideally exponential, that’s how slide rules work for multiplications) way.

A non linear characteristic can also be achieved by adding a DC bias which will help driving the op amp into saturation. It has to be dimensioned in a way that the op amp’s output is saturated when the sum of the input signals is 0. The effect is like an amplifying diode, since only the positive half of the sum signal will pass. And it will contain (among other distortion products) the difference of the input frequencies which you’ll get then exclusively behind the final LPF.

*"A non linear characteristic can also be achieved by adding a DC bias which will help driving the op amp into saturation. It has to be dimensioned in a way that the op amp’s output is saturated when the sum of the input signals is 0. The effect is like an amplifying diode, since only the positive half of the sum signal will pass. And it will contain (among other distortion products) the difference of the input frequencies which you’ll get then exclusively behind the final LPF." - Thierry*

Op-amps can take a while to recover from saturation. Using diodes in the feedback path (so that output saturation is avoided) might give better bandwidth?

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