VAS transistors

[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]

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]

Choices for Vas transistor

VAS transistor choice

[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]

Originally posted by AKSA
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...