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need help to understand MOS in switch mode

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houly

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Hello
I would have to understand how the MOSFET turn ON and turn off when I apply a command on its gate, my circuit seems to works well (I just want to apply a constant current on a load (simulated here by the diode) and the transistors are here to switch the current value.
The first capture represent the signal on the diode which is conform with my needs : a current step to a particular value, and another one.

here is the schematic :




The waveform2 is the signal on the Gate of the transistor M1 and the voltage of Vtest. On Vtest we can see a peak, why and how does it come from ?



I have peaks on the current in the diode and undershoot as you can see on the waveform3.



I hope that you could help me.
 

saikat36

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1st 4 Transistor form 2 current mirror ckt. to ensure good & stable current source for FETs. According to your schematic 2 MOS are n-channel enhanced type. To switch on this type of MOS Gate-Source voltage should be greater than the threshold voltage. To get what is the threshold voltage for your MOS plz check out corresponding datasheet.
 

Milad-D

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The reason that you get this shape for the ID1 is that: somewhere in the circuit you have a pair of pole-zero which are near to each other and are within the bandwidth of the circuit. They call it Doublet.


One suggestion: Put a parallel capacitor with R4 and R6 in a way to satisfy the equation ==> Cp*R6=Cgs*R7. Just sweep the value of Cp and see the effect of it.
 

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