VAS transistors

[ChocoHolic]

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

 

[mastertech]

200Mhz at 2A at 200hfe wow sounds that you need to look in
some rf power transistor manuals, good luck

 

[rtarbell]

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.

 

[jacco vermeulen]

Markus,

if you have a source for devices that do 200V/2A/20W/100MHz/100 gain,that are also obtainable, you can make me a happy bloke.

How about something affordable, and still on this planet:
Sanyo 2SA1606/2SC4159

 

[aparatusonitus]

For VAS BF469/470/471/472, or BF871/872 are probably one of best you can find. Now obsolete, but anyway. Have some sugestion from Toshiba, Hitachi, Panasonic and Sanyo, but I am at work right now, can't remember the numbers

 

[AKSA]

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

 

[mzzj]

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 ??

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...

 

[Ultima Thule]

Hi Markus,

of an occasion I found these earlier today when looking around which happens to be quite close to your match...

Panasonic 2SA1535/2SC3944

180V/1A/15W/200MHz/hfe90-220

Cheers Michael

 

[amplifierguru]

Hugh is right here - the Vas is typically the dominant pole in the conventional (ugh) topology and, widebanding it only means you need to apply extra compensation (with it's associated degradation of PSRR and loading) or stability will suffer.

Best to have no Vas thus eliminating the dilemma of which exotic device to use.



Cheers,
Greg

 

[mcp]

Hello Markus

Toshiba 2SA1930 / 2SC5171 may be worth looking into.

180V / 2A / 20W / 200MHz / hFE 100-320

Isolated TO-220 packing too.

Regards

 

[chicco_36]

Hi Markus,
2SA1930/2SC5171 from Toshiba are witin your spec: 180V/2A/20W/200Mhz. Cob is very important, but usually capacitance goes up with power, so compromise is the key word here…
Best Regards,
Chicco

 

[RetroAudio]

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?

 

[bmcevers]

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.

 

[jcx]

yes we've had these threads before...

http://www.diyaudio.com/forums/showthread.php?s=&threadid=10566&highlight=

http://www.diyaudio.com/forums/showthread.php?s=&threadid=26738&highlight=

 

[mikeks]

ZTX1056A-ZTX796A-ZTX696B

 

[bocka]

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?

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.

 

[jacco vermeulen]

For a VAS, my work has shown you don't need more than 200MHz, 50mA and beta 150.

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.

 

[ChocoHolic]

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

 

[sss]

if u r gonna use MJL4281A & MJL4302A
good drivers (and VAS ) are MJE15035 / MJE15034

 

[Upupa Epops]

Probably best solution are Sanyo's 2SA1540 and 2SC3955...They have Ft 300 MHz, full isolated package and price is very fine. They are my favourites for this stage over last ten years...