I stumbled upon this nice video and thought I'd try to reproduce the experiment. Basically the video shows how to build an AM transmitter circuit using an opamp, some resistors and a capacitor. This is the circuit schematics for the oscillator that should generate the carrier wave:


simulate this circuit – Schematic created using CircuitLab

The carrier wave should be at the OUT node right? My problem is that I do not understand how can this circuit oscillate since it is getting constant voltages as input and the video does not say anything about any function generator. My guess is that instead of 9 Volts in the voltage divider I should use a function generator with the carrier wave (possibly a 9V sine wave). Am I right or did I miss some component/key concept? Also by simulating the circuit with the potentiometer set to 0 on LTspice I get no oscillation at the OUT node but just a constant voltage (as expected) of 3.6V.

EDIT: the circuit should work in the 300kHz - 1.2MHz range


closed as unclear what you're asking by Scott Seidman, Sparky256, Daniel Grillo, laptop2d, Transistor Aug 8 '16 at 19:09

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    \$\begingroup\$ Please include the potentiometer, it shouldn't be difficult to add in circuitlab. It may be important to understand the "spirit" of the circuit. No one's going to sit through a video to answer a question. \$\endgroup\$ – pipe Aug 7 '16 at 20:05
  • \$\begingroup\$ @pipe potentiometer added. \$\endgroup\$ – mickkk Aug 7 '16 at 20:10
  • \$\begingroup\$ Hm, but what's the potentiometer wiper connected to? \$\endgroup\$ – pipe Aug 7 '16 at 20:11
  • \$\begingroup\$ @pipe It's a manual potentiometer such as this upload.wikimedia.org/wikipedia/commons/b/b5/Potentiometer.jpg \$\endgroup\$ – mickkk Aug 7 '16 at 20:16
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    \$\begingroup\$ When designing with potentiometer it is sometimes better to consider the high case (10kΩ) and the low case (1Ω) when analyzing your circuit \$\endgroup\$ – laptop2d Aug 8 '16 at 18:44

the video shows how to build an AM transmitter circuit using an opamp, some resistors and a capacitor.

It's a relaxation oscillator and here's a simplified version using a schmitt trigger: -

enter image description here

However, as a carrier wave it's poor because it's basically a square wave and not a sinewave. If you are going to transmit (illegally or not) don't use a square wave as an oscillator because you'll broadcast your transmission at the fundamental frequency of the carrier (say) 1 MHz and every odd harmonic up into hundreds of MHz i.e. you become a polluter.

Anyway, back to your question, When the inverter output changes state from zero to one, it charges the capacitor up (via the resistor) until the cap voltage triggers the upper limit in the schmitt trigger. This forces the output to zero and the capacitor discharges via the resistor. Back to square one and the process repeats.

If you can follow the above circuit operation then you should be able to figure out that your circuit is identical in nature.

Hint: inside my circuit hides R1, R2 and R4 and, because they are hidden internally, there is no need to show the non-inverting input.

  • \$\begingroup\$ thanks for the reply, I'll work on it and try to figure it out. Thanks for the hint on the square vs sine wave. As for the legal side, the idea is simply to replicate the experiment in the video and recreate that very limited range (1, 2 m) transmitter so that should be within the legal boundaries I think. \$\endgroup\$ – mickkk Aug 7 '16 at 21:25

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