Can I change Bias? - Page 3

myhrrhleine · 15th February 2006, 02:55 AM

Yes, you can do it.

That small of a bias change won't really get you much.
You'll still have an AB amp, with crossover distortion happening at a different signal level.
Might even put the cross-over at a bad signal level.

Tube_Dude · 15th February 2006, 02:12 PM

Quote:

Originally posted by Eva
Could you explain how a non-saturated MOSFET may have 0.5 ohms of output impedance

According to the data sheet , the Toshiba 2SK1529 transconductance (Gm) at 100 mA drain current is 0,5 S.
As the output impedance of a mosfet follower is 1/Gm, resolving the output impedance become 2 Ohms.

If the output transistor current gain, in even only 50, the open loop output impedance become 2 /50 = 0,04 Ohms certainly a remarkable result and this value is attained , before any overall feedback.

Quote:

MOSFET is going to have more than 1nF effective capacitance between gate and source, thus effectively shunting the input to the output.

In a source follower ( case in discussion), this 1 nF capacity is effectively bootstrapped and we end up with a much lower input capacitance.

darkfenriz · 15th February 2006, 03:44 PM

Quote:

Originally posted by Eva
Did you know that the gate may sink or source current without any Vgs change?

Is this related to gate-drain capacitance?
Miller effect or superconduction.

regards

Eva · 15th February 2006, 04:12 PM

No, this is due to gate capacitace being a dynamic parameter in MOSFETs. Gate capacitance increases and decreases depending on things like Vds and Id. But the amount of charge stored does not change unless some current flows into or out of the gate.

Thats why the gate will sink or source current if you try to keep Vgs constant while changing Vds and/or Id. Also, Vgs will change on its own if you leave the gate open and change Vds and/or Id.

Funny, isn't it?.

In switching circuits you may happen to be sourcing 500mA to a gate during 100nS while not observing any Vgs change at all.

darkfenriz · 15th February 2006, 04:43 PM

Thanks, I understand now, like a capacitive microphone it creates signal, be it voltage or current. Or like a varactor.
Do you think it is very important factor in linear applications?

regards

P.S. sorry for off topic questions here.

Eva · 15th February 2006, 04:49 PM

I think it is very important for linear applications, but I want to see the input current waveform of a MOSFET driving a resistor in class A at 10Khz with considerable Vds and Id swing in order to be sure

I'll try it as soon as I can and I'll post the waveforms.

Don't try simulation, my version of PSpice and/or its models (the stuff that comes with Orcad 9.1) is so lousy that it simulates constant capacitances...

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15th February 2006, 02:55 AM	#21
myhrrhleine diyAudio Member Join Date: Jan 2006 Location: Avalon Island	Yes, you can do it. That small of a bias change won't really get you much. You'll still have an AB amp, with crossover distortion happening at a different signal level. Might even put the cross-over at a bad signal level.

15th February 2006, 04:12 PM	#24
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	No, this is due to gate capacitace being a dynamic parameter in MOSFETs. Gate capacitance increases and decreases depending on things like Vds and Id. But the amount of charge stored does not change unless some current flows into or out of the gate. Thats why the gate will sink or source current if you try to keep Vgs constant while changing Vds and/or Id. Also, Vgs will change on its own if you leave the gate open and change Vds and/or Id. Funny, isn't it?. In switching circuits you may happen to be sourcing 500mA to a gate during 100nS while not observing any Vgs change at all.

15th February 2006, 04:43 PM	#25
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	Thanks, I understand now, like a capacitive microphone it creates signal, be it voltage or current. Or like a varactor. Do you think it is very important factor in linear applications? regards P.S. sorry for off topic questions here.

15th February 2006, 04:49 PM	#26
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	I think it is very important for linear applications, but I want to see the input current waveform of a MOSFET driving a resistor in class A at 10Khz with considerable Vds and Id swing in order to be sure I'll try it as soon as I can and I'll post the waveforms. Don't try simulation, my version of PSpice and/or its models (the stuff that comes with Orcad 9.1) is so lousy that it simulates constant capacitances...