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Old 9th May 2003, 01:41 PM   #1
rbroer is offline rbroer  Netherlands
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Default Cascoded jfet buffer

I've been looking into some buffers like Borbely and others and tried to add some ideas together.
Output DC offset will be handled upstream of the fet input
The input fet now operates constant power.

Any pitfalls and hints and suggestions ?
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Old 21st December 2005, 04:22 AM   #2
lineup is offline lineup  Sweden
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Default Re: Cascoded jfet buffer

Quote:
Originally posted by rbroer
Any pitfalls and hints and suggestions ?
No I cant see any pitfalls.
A perfect circuit!


You can bet your last dime, if was anything to criticize
you would have had your share - and more ....
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Old 21st December 2005, 04:43 AM   #3
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Default Re: Cascoded jfet buffer

Quote:
Originally posted by rbroer
I've been looking into some buffers like Borbely and others and tried to add some ideas together.
Output DC offset will be handled upstream of the fet input
The input fet now operates constant power.

Any pitfalls and hints and suggestions ?
Is this for driving loudspeakers or something?

se
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Old 21st December 2005, 04:47 AM   #4
lineup is offline lineup  Sweden
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possibly a subwoofer
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Old 21st December 2005, 04:57 AM   #5
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Quote:
Originally posted by lineup
possibly a subwoofer
Hehehe. Well just seems to be an awful lot of um... stuff just to drive a few feet of wire and a high impedance input.

se
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Old 21st December 2005, 06:19 AM   #6
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Hi rbroer,

I trust the input FET is going to be gate biassed from somewhere?

Also I think you're biassing T4 from the +18V rail, wouldn't it be a good idea for C1 to bypass to that?

Seems to me unnecessarily convoluted for a simple need. please define your intent.

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Greg
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Old 21st December 2005, 06:35 AM   #7
lineup is offline lineup  Sweden
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Default Re: Re: Cascoded jfet buffer

Quote:
Originally posted by Steve Eddy
Is this for driving loudspeakers or something?

se
Quote:
Originally posted by amplifierguru
Hi rbroer,

I trust the input FET is going to be gate biassed from somewhere?

Also I think you're biassing T4 from the +18V rail, wouldn't it be a good idea for C1 to bypass to that?

Seems to me unnecessarily convoluted for a simple need. please define your intent.

Cheers,
Greg
Click the image to open in full size.

I found link to this very old thread (with 1900 views and no reply)
at Pedja Rogic gainclone page.
http://users.verat.net/~pedjarogic/a.../gainclone.htm

He refers his solution to this thread
and also to a Nelson Pass paper:
Cascode Amp Design: by Nelson Pass
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Old 21st December 2005, 06:39 AM   #8
lineup is offline lineup  Sweden
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Pedja Rogic
also recommends a more usual and simple alternative to buffer LM3875:

Click the image to open in full size.
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Old 21st December 2005, 07:35 AM   #9
mirlo is offline mirlo  United States
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What is the value/purpose of C2? C1 will form a well defined pole with R1, but the C2 seems to form a pole with the rather variable ( part-to-part and temperature dependent ) transconductance of the input JFET.
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Old 21st December 2005, 05:31 PM   #10
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Quote:
Originally posted by amplifierguru
Hi rbroer,

I trust the input FET is going to be gate biassed from somewhere?

Also I think you're biassing T4 from the +18V rail, wouldn't it be a good idea for C1 to bypass to that?

Seems to me unnecessarily convoluted for a simple need. please define your intent.

Cheers,
Greg
In the circuit in post 1:

The input FET should have some resistance to 0V reference or "ground" of the +/-18V supply, 1M to 10M is fine. No "ground" point is shown in the schematic, a glaring omission.

C1 provides low AC impedance from T4 gate to output, for current to drive T4 Cgd, just what you would want for a bootstrap cascode with highish Cgd. With 10mA sink current I would choose a JFET with high Idss and Vp instead of a power MOSFET, something like PN4391, connect R4 to output and dispense with C1,R6,R7. Yes there should be power supply bypass caps but in this schematic there is no 0V to connect to.

I have to wonder how stable the T1/T2/T3 compound feedback triple is. I've had enough trouble with oscillation in CFPs. Has anyone actually built this?

Post 1 is about 31 months old but rbroer has posted recently so a reply may come.

Quote:
What is the value/purpose of C2? C1 will form a well defined pole with R1, but the C2 seems to form a pole with the rather variable ( part-to-part and temperature dependent ) transconductance of the input JFET.
The link in post 7 doesn't offer any clear explanation. I agree with this, but given 1/gm=33 ohms the -3db of C2 and 33 ohms is about 150kHz, well outside the audio range, so the variability might not matter. I would check for any signs of oscillation on the 2SK170 source.
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