# Electronic – MOSFET Oscillations when bringing gate to same potential as drain

mosfetoscillation

I was messing around in the lab today looking for a clever way to make a single FET oscillator. I made this circuit, which is basically a high side switch using an n channel mosfet. I have it oscillating but I am not sure if my explanation is correct. If someone could help explain why it is oscillating or prove my theory, I would greatly appreciate that.

As you can see in the second picture, the gate is connected up to the drain which is connected to Vcc. Let's say it is at 10 volts. The source then connects through a load to ground. My explanation is as follows:

1. The fet turns on because the source is pulled down by the resistor to ground and the potential difference between gate and source is > than Vth (2volts).
2. Once the transistor turns on, the drain is now pulled up to Vcc.
3. The gate is no longer 2 volts above the source, because they are both roughly at the same potential.
4. The transistor then turns off because Vgs < Vth
5. Source is then pulled back down to ground.
6. Goes back to step 1.

Does this make sense? Thanks! In the pic below I used 10 volts, you can see that the source comes up to 8 volts (input minus Vth) which makes sense. The oscillation frequency is 360khz and p-p voltage is 0.5v