Why does MOSFET distort the input wave?
the first hint that there is a problem is the input signal. If you have a unipolar rc filter, the waveform must be symmetrical. It is obvious that you have not correctly polarized the fet for what I imagine to be the desired result of an analog amplifier.
In addition to learning how to draw a line of charge, you must learn how to present your data. there is no scaling on the image, and no accompanying scheme. you also failed to mention your goal.
The losses of this transistor are huge – hard switched transistor. I would also like to check the comparator with the triangular wave and the signal voltage. Here, I would keep the work cycle below 45%. the duty cycle of your switching transistor is greater than 50%. Remember that the transistor requires a longer cooling period than the period of use.
this double-span trace is not the usual description of what the output current does in response to an input voltage to an irf630. for this device, a source gate voltage greater than 4 volts activates the source to drain the current. an internal diode picks up the drain when its voltage drops below the drain voltage.
The waveforms show no reasonable relationship between input and output, so the oscilloscope leads are incorrectly wired at points other than gate and drain; and that the inlet may be excessive, the internal diode may be burned and the drain load uncertain. provide the values of the input and output components, as well as the deviation sensitivities of the trace and ground reference level can help better describe
– but does this inflection have roughly the location of your cursor?
it is almost certainly due to the miller effect. When you have a capacitance through a gain stage, the capacitor appears as if it were multiplied by the gain. go find it on wikipedia if you need it.
there is a parasitic capacitor between the gate and the drain of a mosfet. this cap is small but not negligible. because of the milling effect, this capacity will appear amplified by the gain of the transistor.
The operation of your circuit drives the transistor in different operating regions (from extinction to saturation through the deep into linear) – that’s why we see the heavily clipped red signal. the gain of the transistor is very low in the off and linear sections, it is much higher in the saturation region. thus, the circuit which controls the gate of the transistor sees a capacitive load varying in time.
If you want the green trace to look more like a simple rc-filtered version of the input, you need to make sure that the green node’s capacity is strongly dominated by an unmapped capacitor (ie, increase the ‘c’ and reduce the ‘r’ in your low pass filter), or you will need to put some kind of buffer (like an opamp follower circuit) between the filter and this nonlinear stage.