Class A2 SET using source follower driver - diyAudio
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Old 30th November 2006, 08:45 AM   #1
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Default Class A2 SET using source follower driver

Hi,
I hope no one will crucify me by asking SS help here...

I was attracted by Tubelab's approach of driving output stage into A2 using his Power Drive circuit. I would very much like to try in on some of my experimental SETs.

As I'm new to electronics, I don't know how to use & design the 2sk2700. Could anyone please show me how to design a Mosfet Source follower? a 2sk2700 to be exact.

Cheers

A_I
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Old 30th November 2006, 10:03 AM   #2
lineup is offline lineup  Sweden
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Default toshiba high voltage mosfet - 2SK2700

hi
I did not know about 2SK2700.

Applications: Chopper Regulator, DC–DC Converter and Motor Drive

It is a Toshiba N-channel MOSFET.
Looks like an isolated ( 3.1 C/W ) TO220 size case.

High voltage device: 900 Vds
Max DC Current: 3 A
Max Power: 40 W
RDS(on): ~4 Ohm
Ciss, input capacitance: 750 pF
Coss, output capacitance: 70 pF
Rise/Fall time: 15/30 nS

---------------------------

Comment:
The input capacitance is maybe a bit high for Audio.
there might be better choices reagarding this data.
Seems like it works linear below 1000mA, and Vgs 4-5 Volt.
---------------------

Questions:
At what working condition would you use this?
.... input signal voltage, power supply, output load impedance
What is A2?
I guess it is a power amplifier.
------------------------------

Here is the datasheet, anyway,
for those who knows MOSFET applications a bit better:
Toshiba 2SK2700 Datasheet PDF

regards
lineup
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Old 30th November 2006, 05:06 PM   #3
Sheldon is offline Sheldon  United States
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If this is used as a source follower, wouldn't crss be the issue? It looks low at 10pf for this part, but it jumps pretty dramatically below about 20Vds.

Sheldon
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Old 30th November 2006, 11:38 PM   #4
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I am interested in his design too. I decide to buy a PCB from him directly.

The ciss is not a problem because it is used as source follower.

A2 means the situation that a tube works with significant grill current. A1 means almost no grill current.
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Old 1st December 2006, 12:52 AM   #5
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Quote:
The ciss is not a problem because it is used as source follower.
The gate capacitance is not of much concern in a source follower. However, the reverse transfer capacitance of the FET is important. A low and reasonably constant reverse transfer capacitance is necessary. A large reverse transfer capacitance shorts HF info. to ground.

The IRFBC20 meets the need WELL. IRFBC20 data sheet here.
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Old 1st December 2006, 02:52 AM   #6
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Hi,
Thank for the respond.

Quote:
---------------------

Questions:
At what working condition would you use this?
.... input signal voltage, power supply, output load impedance
What is A2?
I guess it is a power amplifier.
------------------------------

regards
lineup
It's for the driver of a tube power amplifier.

My understand of A2 is limited, kick me if I'm wrong, here goes, The power tube works at lower power in A1 condition i.e. -ve bias and negligeble grid current, but once tha bias turn +ve, it goes into A2 for higher power. in +ve bias region, the grid will have significant current flowing.... That's why a "high" current cathode follower or source follower will be benificial to min distortion...

Don't really know about the output load impedence. but the input signal voltage is around 260Vp-p I think....

Quote:
HighVividity
I am interested in his design too. I decide to buy a PCB from him directly.
Ditto, but they are not available until now...

[QUOTE]Eli Duttman
[The IRFBC20 meets the need WELL. IRFBC20 data sheet here./QUOTE]

I have problem understanding source follower and how to design one.
Please teach.

Cheers

A_I
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Old 1st December 2006, 03:26 AM   #7
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Quote:
Originally posted by Audio_idiot
My understand of A2 is limited, kick me if I'm wrong, here goes, The power tube works at lower power in A1 condition i.e. -ve bias and negligeble grid current, but once tha bias turn +ve, it goes into A2 for higher power. in +ve bias region, the grid will have significant current flowing.... That's why a "high" current cathode follower or source follower will be benificial to min distortion...
Yes, so far, so good.

Don't really know about the output load impedence. but the input signal voltage is around 260Vp-p I think....

Quote:
I have problem understanding source follower and how to design one.
Please teach.
You can design from the characteristics provided by the manufacturer. Do keep in mind that power MOSFETs are enhancement devices, and not depleation devices like VTs. This means making the gate some volts more positive (or less negative, as the case may be) than the source. To DC couple to the grid of the Class A2 final, adjusting the Vgs bias can allow you to set final Q-Points. It's not all that much different, really.

However, power MOSFETs have g(m)'s that are way higher than any VT. For all intents and purposes, it's a unity gain device with very low Zo's. since r(don) is also quite low, it will source more than enough current to drive any Class A2 final.
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Old 1st December 2006, 03:26 AM   #8
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Quote:
I have problem understanding source follower and how to design one.

Read MOSFET Follies here.
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Old 1st December 2006, 03:52 AM   #9
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Hi,
I'd came accross the mosfet folly article some time ago, but don't know how to implement...

Please forgive my ignorance, but could anyone teach me how to determine the +Vs+, -Vs, the 220ohm and 10k resistor in the circuit attach?? how to set bias voltage for the mosfet or even to determine the bias voltage? I'm a complete idiot when it comes to SS (among others
..... )

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Old 1st December 2006, 04:03 AM   #10
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Oop, this is the circuit I meant
Attached Images
File Type: gif mosfol2.gif (10.0 KB, 362 views)
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