VAS transistors

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ChocoHolic

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Joined 2003
#1
Hi Everybody !
May be I am just ignorant and did not find the relevant
threads....

But I am not succesful in looking for high end VAS transistors.
For the output stage there are often often mentioned:
2SC2922 & SA1216 (BJT)
MJL4281A & MJL4302A (BJT)
or in MosFet designs 2SJ201 & 2SK1530...

But for the VAS I am missing comparatively high rated
transistors (mainly looking for BJT here).
Isn't there something like 200V/2A/20W/200MHz/gain200 ??
😀

Also for the input stage I am missing 120V/100mA/1W/1GHz/gain500....

Any hints or proposals are welcome, you don't need to meet my dream specifications.

Bye
Markus
 
#3
For the MJL4281 and MJL4302, you can order free samples from ONsemi.com. They are usually really good at fast orders, and you can order up to 25 of each at no charge (but include $11 for shipping).

For small signal BJT transistors, I'm not sure if this is what you're looking for, but I like to use Zetex FZT857 (npn) and FZT957 (pnp). They are surface mount packages rated at 3 watts, 350V breakdown. Digikey has these.
 
#6
For a VAS, my work has shown you don't need more than 200MHz, 50mA and beta 150. A parameter of importance is Cob, the base/collector input capacitance, which should be under 3pF if you can find it. However, going higher in speed could well cause more problems with stability, and making an amp stable with lag compensation appears to have sonic repercussions across the entire audio range.

Toshiba have devices which are tailored to VAS audio duties.

Hope this is helpful,

Hugh
 
#7
ChocoHolic said:
Hi Everybody !

But for the VAS I am missing comparatively high rated
transistors (mainly looking for BJT here).
Isn't there something like 200V/2A/20W/200MHz/gain200 ??
😀
Click to expand...

How about importance of Early voltage? VAF*Hfe is some sort of benchmark in semiconductor processes. Or how about VAF*Hfe*ft multiplied by voltage rating ? 😉

there seem to be general tradeoff between VAF and Hfe in similar transistors like BC550A or BC550C

Heterojuction transistors might be the way to go, but these RF-devices are usually low-voltage. Might be usefull for input stage if you cascode it to reduce voltage on input transistors...
 
#13
VAS Transistor

I like the Fairchild semiconductor:
KSC2690A (NPN) / KSA1220A (PNP) and
KSC2682 (NPN / KSA1142 (PNP)

some of the best medium power transistors at any price, excellent candidates for any HQ VAS design.

160 Vceo
200 beta
low Cbc

and less than $.30 each, you can't beat them. They should be easy to find anywhere.
 
#16
all these specs sound fine and dandy, but what kind of bias currents are we talking about here? isn't 2 amps a bit much in a VAS stage?
Click to expand...

Yes, thats I'm asking myself too. Something like 200V, 0.1A, 1W should be sufficiant for a VAS. 2SC2911 and its complementary part 2SA1209 are a good choice. I wouldn't look for a low beta device also, as they have typically a higher early voltage / output resistance which give a linearer operation and a higher gain. Cob and Crss are more relevant parameters than a high ft as they directly influence the high frequency linearity.
 
#17
AKSA said:
For a VAS, my work has shown you don't need more than 200MHz, 50mA and beta 150.
Click to expand...

Mr Dean,
then why did you recommend the 2SA1540 and 2SC3955 for the Vas ?
I got a bunch of them from Nikko's, thinking these might be an improvement over lower Ft/Beta devices.(i see Pavel Air likes them as well )

Markus did not mention nor asked for driver devices, Vas only. Now he is getting both, nice open door.
 
#18
Hi All !
Great , that's exactly the information I was looking and
somehow was to blind to find the older threads.

I used BF469/470 & BF759/760 about 15 years ago
and still have some of them.
But within 15 years BJTs should have improved
and they have. Thanks a lot.

I will check through tons of data sheets next.



VAF*Hfe*ft:
Isn't VAF always resulting from ft and Hfe ?
Typically 1st order behaviour for normal BJTs ( & f < ft) ?
Up to now I had the approach to look for high ft in order to
have enough gain at high frequencies. Second to look for
high gain- and in case of to much gain and undesired low
VAF ==> coming around with a single stage current feedback...
.. not OK...?

Bye
Markus
 
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