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Best MOSFET for output stages
Best MOSFET for output stages
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Old 16th May 2019, 02:45 PM   #161
adiaz is offline adiaz  Spain
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Quote:
Originally Posted by Bob Cordell View Post
I always use a driver between the VAS and the MOSFET output transistor gates. Input capacitances of the MOSFETs mean they require current. Those capacitances are nonlinear. I believe in not making the VAS work any harder than it needs to. This is true even if the 15 mA is enough to sufficiently charge the MOSFET input capacitances at maximum voltage slew rate and current slew rate.

Cheers,
Bob
Perfect! thanks for your answer.

Then you think that the gate capacities consume some current and that is why you need to add one more stage in the emitter follower, to relieve the load by that capacitance.

Ok, I made an approximate calculation of the load submitted to the VAS directly, under a frequency of 20Khz. The mosfet I use usually have 500pFx2 of gate capacity, this means that at 20khz the impedance would be about 8Kohms, that would be the load that the VAS would see at that frequency, correct?

Below that frequency there would be a greater impedance ...

With an intermediate step I would gain much more impedance and therefore I would work with less load for the VAS.

Thanks again....

Last edited by adiaz; 16th May 2019 at 02:50 PM.
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Old 16th May 2019, 03:04 PM   #162
adiaz is offline adiaz  Spain
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Another question, Mr. Cordell ...

Does the use of an added cascode transistor in the VAS improve current handling capacity for a lower load in the gates of power mosfet?

Or does only the polarization dc current chosen in the VAS matter regardless of whether or not there is a transistor in the cascode?

Last edited by adiaz; 16th May 2019 at 03:07 PM.
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Old 16th May 2019, 08:08 PM   #163
rsavas is offline rsavas  Canada
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A cascode is a common base configuration, current is sourced/sinked by the CCS load or the push-pull VAS bias.
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Old 16th May 2019, 10:30 PM   #164
adiaz is offline adiaz  Spain
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Quote:
Originally Posted by rsavas View Post
A cascode is a common base configuration, current is sourced/sinked by the CCS load or the push-pull VAS bias.
Yes. I already knew that.

The question is: does the cascode improve the load capacity or is it just the chosen CCS current?
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Old 17th May 2019, 03:26 AM   #165
N101N is offline N101N
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Quote:
does the cascode improve the load capacity
No, I would not implement it. I too would use the 2SC3503 at 10mA, not higher as the very important bandwidth would suffer. You can't circumvent this reciprocity.
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Old 21st May 2019, 02:45 PM   #166
UltimateX86 is offline UltimateX86  France
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Best MOSFET for output stages
hello, to come back to the original subject, what makes a good output mosfet?

I recently simulated IRF and it's the IRFP150n that produce the least distortion in the schematic VS 140, 240, 250.

it is only transconductance that gives this result ?
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