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Old 24th February 2009, 02:19 PM   #1
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Default cascode output stage what device?

Hi folks,

i was wondering what would the best device to use for cascoding the IRF9610/610 on the output stage possibily be.
Also, if you guys have some suggestion on possible better aternative for the 610/9610.


thanks for the attention.


Best.
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Old 24th February 2009, 03:00 PM   #2
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I don't know what your circuit parameters are, but the first and most obvious choice would be to use another IRF610/9610.
If you want to use another, similar device, look up the Fairchild equivalents. If you can get away with a somewhat less robust device, I used the Toshiba 2SK2013/2SJ313 MOSFETs in the GR-25 and was pleased with their performance. Watch the ratings, they're not quite the same as normal TO-220 devices. If you want to go bipolar, start with the MJE15030/31 and fan out from there.

Grey
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Old 24th February 2009, 04:06 PM   #3
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I've used 2SC2238 for the same purposes with good results.
In my circuit bypolar had some advantages respect the "all hexfet" configuration (i.e. another IRF).
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Marcello
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Old 24th February 2009, 06:20 PM   #4
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Quote:
Originally posted by GRollins
I don't know what your circuit parameters are, but the first and most obvious choice would be to use another IRF610/9610.
If you want to use another, similar device, look up the Fairchild equivalents. If you can get away with a somewhat less robust device, I used the Toshiba 2SK2013/2SJ313 MOSFETs in the GR-25 and was pleased with their performance. Watch the ratings, they're not quite the same as normal TO-220 devices. If you want to go bipolar, start with the MJE15030/31 and fan out from there.

Grey
Hi Grey,

i actually cascoded the 2 devices using another 610/9610 and didn't see any benefit in terms of THD by doing that.
I thought it might be due to the fact that the VDS is forced to be at around 4V which is too low to guarantee a more linear output stage.
I was thinking if there is a good device with higher VGS something like k246 and complementary in to220.
Otherwise i was thinking on use BJTs devices (like Tortello) as toshiba A1358/C3221.

Just quick description of my project: I am building a RIAA phono equalizer.

The first stage will simply drive the 75uS,passive config, time constant.
Than i have arranged another stage with the other 2 time constants in active configuration.

I don't know if it would be optimal to drive an high pass with a pair of J74/K170 biased at 5-6mA.
That's why i am using more robust devices like the IRFs, but, What do you think?
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Old 24th February 2009, 10:06 PM   #5
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Vgs = 4V?
Um...might I suggest a voltage divider from rail to ground to set the Gate reference voltage to a somewhat higher voltage? 15-20V, minimum. Better still, 30-40V. There's no point in trying to cascode if you're not going to let the parts breathe.
If your gain device (the one on the bottom) has only 4V to work with, you're modulating the everlivin' pee-widdle dookey out of the Drain, thus invoking the Early-riser demons. Higher Vds reduces the percentage.

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Old 25th February 2009, 12:15 AM   #6
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If you learn to pit those demons against the transconductance
characteristic, you can get very low distortion.

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Old 25th February 2009, 12:28 AM   #7
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Quote:
If you learn to pit those demons against the transconductance characteristic, you can get very low distortion.
how would one go about doing such a thing?

I think there was a simular statement in the ZV3(?) article, where a loveltech j-fet was cascoded. But I could never figure out exactly how you go about doing that. I had to settle for setting the voltage on the gate of the cascode device at a value that allowed the circuit to best reproduce a sine wave -- no flat spots and not overly peaky looking either.

JJ
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Old 25th February 2009, 12:50 AM   #8
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Quote:
Originally posted by jupiterjune


how would one go about doing such a thing?

I think there was a simular statement in the ZV3(?) article, where a loveltech j-fet was cascoded. But I could never figure out exactly how you go about doing that. I had to settle for setting the voltage on the gate of the cascode device at a value that allowed the circuit to best reproduce a sine wave -- no flat spots and not overly peaky looking either.
Take a close look at the decisions made around the Lovoltech JFET
and read all the text, but go to the ZV9 article (ZV3 will assist you
with a power supply).

JFETs are really good for this, but you can perform the same trick with
a Mosfet.

Unless you like tubes...

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Old 25th February 2009, 02:23 PM   #9
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Hi Nelson,

thanks for your reply.
I have read the ZV9 article.
I couldn't understand how the bootstrap C5 against the CCS with one leg to ground would improve THD vs the same C5 from out to CCS.
In such a configuration C5 simply looks like a bypass cap for P1 awhile in the other the rule was clear, this is a lack of my knowlege.

I don't know if i have understood wrong, but by modultaing the VDS of the active gain devices and by probing it with a spectrum analyzer it is possible to find a "sweet spot" (for the lowtech device was about 2V) where there is distortion cancellation, correct?
I can't understand how an active JFet device would work better at 2V Vds, how come this low value gives best performance?
For a 2sk170/j74 a Vgs of 8-9V is reccomended for best performance...uhhmm..i am sure it depends upone specific device used.

Last thing: the LUV is a power device and i actually just need a medium power device.
Do you have any suggestion over the classic pair 610/9610?
My output stage is a diamond buffer config and the high capacitance of these two guys here makes things a little more complicated

Best.
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Old 25th February 2009, 03:30 PM   #10
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Quote:
Originally posted by Stefanoo
I don't know if i have understood wrong, but by modultaing the VDS of the active gain devices and by probing it with a spectrum analyzer it is possible to find a "sweet spot" where there is distortion cancellation, correct?

I can't understand how an active JFet device would work better at 2V Vds, how come this low value gives best performance?
For a 2sk170/j74 a Vgs of 8-9V is reccomended for best performance...uhhmm..i am sure it depends upone specific device used.

Last thing: the LUV is a power device and i actually just need a medium power device.
1) Yes you can find a sweet spot if you go looking for it.

2) In ZV9 with those parts at that current, the sweet spot occurs
around 3V or so.

3) The Lovoltech part is rated at 69 watts, if you dare...

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