i/v stage problem

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#21
Ken Newton said:
Lampie519 has the right sort of idea. Try inserting a 1K resistor in series with the gate of Q4. This will create an RC filter pole in conjunction with the gate terminal's parasitic capacitance, and is known as a 'gate stopper'. Solder the resistor close to the gate terminal of Q4.
Click to expand...

Hi,
thank you for for your advise. I resolved my problem with a 1K gate stopper, but i don't understand why i have the problem with a cap in C7/C8 and no problem without cap in C7/C8.

thank you!Maxpou :RÉ
 
#22
Lampie519 said:
What did you find adjusting the resistor value? No change at all?

Another question: Is the output loaded (maybe a small resistor value in series will help (100 Ohm). This will increase the output Z a little but is good practice as it helps protecting the transistors agains short circuit and HF oscillation.
Click to expand...

Hi Lampie
thank you for your advise too, i have modified my schematic and pcb to add a resistor in serie with my output.

Maxpou
 
#24
maxpou said:
Hi,
thank you for for your advise. I resolved my problem with a 1K gate stopper, but i don't understand why i have the problem with a cap in C7/C8 and no problem without cap in C7/C8.

thank you!Maxpou :RÉ
Click to expand...

One guess is that without the bypass cap., 560R resistor R8 causes enough Miller capacitance on the drain to prevent the oscillation. However, having bypass cap. C7 in place prevents the Miller effect from increasing the parasitic capacitance seen by the gate, which works to slow down the device, just as adding the gate stopper resistor slows it down. Again, just a guess.
 
Last edited:
#26
Ken Newton said:
One guess is that without the bypass cap., 560R resistor R8 causes enough Miller capacitance on the drain to prevent the oscillation. However, having bypass cap. C7 in place prevents the Miller effect from increasing the parasitic capacitance seen by the gate, which works to slow down the device, just as adding the gate stopper resistor slows it down. Again, just a guess.
Click to expand...


Hi Ken,
thank you for explanation, very appreciated. Maxpou 😀
 
#28

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