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Old 17th April 2001, 03:13 PM   #1
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I'm displaying my ignorance here, I'm sure, but where better to erase it..

If one of the major problems with mosfets is their input capacitance, isn't a smaller capacitor in series with the input capacitance, in order to reduce the overall capacitance, an option? Is this stupid, commonplace, or impossible?

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Old 17th April 2001, 03:56 PM   #2
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Jakeh,
MOSFETs have a *very* high input impedance. Putting a capacitor in the circuit would produce a filter. If the cap is in series, you'd produce a high pass; if in parallel, you'd have a low pass.

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Old 17th April 2001, 04:40 PM   #3
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hi Jakeh ;-)

Not a stupid Q.
A way to come around the capacitive problem with all transistors are very well described by our guru, Mr. Pass, in this article : http://www.passlabs.com/articles/cascode.htm
and by Mr. Borbely in his article at: http://www.borbelyaudio.com/
JFETs: The New Frontiers, Part 1 and 2.
Happy reading
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Old 17th April 2001, 08:29 PM   #4
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Default Key is gate charge

The capacitance of MOSFET's unfortunately tends to vary with both gate drive an applied voltage.

In general third generation devices (IRF nomenclature) are preferred for audio. You should avoid devices made for switching.

!NOT A STUPID QUESTION!

Petter
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Old 19th April 2010, 08:53 PM   #5
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A series cap would not fix things because a large portion of the signal would be dropped across said cap. Say that the Cgs value is 4.5 nf (it's non-linear as well, but we're keeping it simple), and we add a 0.5 nf series capacitance. The overall equivalent is 0.45 nf, one tenth of Cgs. But nine tenths of the input signal is lost by being dropped across the 0.5 nf cap. Hence the FET receives an input that is 0.1 times the original signal (-20 dB).

Does this help?

Claude
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Old 19th April 2010, 08:59 PM   #6
Eva is offline Eva  Spain
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The problem is not high capacitance but highly non-linear capacitances.

High input capacitance is not really a problem if gate drive is buffered, at least not in linear circuits.
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Old 19th April 2010, 09:21 PM   #7
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The gate input capacitance can play havoc with a VAS stage as it loads the VAS output.
What people generally do is put a resistor in series (22R to 470R) with the gate to slightly decouple it from the VAS.

IRFP240 is quite a good MOSFET because its input capacitance is relatively low.
Watch out for high power MOSFETs like the IRFP250 which is two MOSFETs on the same die which gives twice as much capacitance as IRFP240.
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Old 20th April 2010, 12:36 AM   #8
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Quote:
Originally Posted by nigelwright7557 View Post
The gate input capacitance can play havoc with a VAS stage as it loads the VAS output.
What people generally do is put a resistor in series (22R to 470R) with the gate to slightly decouple it from the VAS.

IRFP240 is quite a good MOSFET because its input capacitance is relatively low.
Watch out for high power MOSFETs like the IRFP250 which is two MOSFETs on the same die which gives twice as much capacitance as IRFP240.

Nigel, I think what people should do is to always use a driver before the output mosfets.
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Old 20th April 2010, 12:44 AM   #9
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Quote:
Originally Posted by homemodder View Post
Nigel, I think what people should do is to always use a driver before the output mosfets.
Even with a driver you are still driving into a pure capacitance which isnt good.

Even with my quasi which has extra drivers I still put in gate resistors.
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Old 20th April 2010, 04:34 AM   #10
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Mosfets inherently form a Colpitts oscillator and/or essentially a Hartley oscillator when you take into account the lead inductance. Gate stopper resistors simply form a low pass filter in order to snub the resonance of the oscillator that exists in every mosfet. In addition a gate Zobel filter placed on the gate pin as close to the package as possible can help dampen the oscillation even better in turn allowing a smaller gate stopper, increasing the usable bandwidth of the mosfet. The real problem, as Eva stated, is the non-linear overall capacitance, which to a significant degree is dependent on Vds. You aren't just driving Cgs, but also Cds and Cgd. A quick look at a datasheet will show this to you.


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