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Old 22nd March 2007, 01:09 AM   #1
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Default BJTs for VAS Stages

In the old days, I used 2N3440 & 2N5415 transistors. Obviously, these are not a particularly contemporary choice.

I haven't seen a lot of discussion on transistor choices for anything but the output and driver transistors.

I like the availabilty (at least in the US) of the ON Semi parts and I can be confident that they are "real"

What are some favorite complementary pairs?
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Old 22nd March 2007, 02:17 AM   #2
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A fast and low cob part is most ideal. I personally love to use either the
800mw 2sc2705/a1145 parts or the 5w 2sc3423/a1360 for the vas 1.8pf for the npn/2.5pf for the pnp. These are 200mhz 50ma devices with an average Y-beta of about 200.. I see most designs using 1amp parts but these are emtremely unneccesary not to mention have an average collector capacitance anywhere from 10-30pf..


Colin
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Old 22nd March 2007, 02:55 AM   #3
jcx is offline jcx  United States
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Default Re: BJTs for VAS Stages

Quote:
Originally posted by danville
In the old days, I used 2N3440 & 2N5415 transistors. Obviously, these are not a particularly contemporary choice.

I haven't seen a lot of discussion on transistor choices for anything but the output and driver transistors.

I like the availabilty (at least in the US) of the ON Semi parts and I can be confident that they are "real"

What are some favorite complementary pairs?


but did you do any searching?

like for instance "vas" + "search in tiltle"?

maybe you would have picked up on:

VAS transistors

(admitedly I cheated, rather than look thorough all 82 threads I also know to look for AKSA as poster)

(which also brings up the apparent issue that "search in title" currently seems to pick up on more than just Thread Titles - should it do this?)
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Old 22nd March 2007, 04:03 AM   #4
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Yeh, I searched VAS but I missed the earlier thread. Too many irrelevant hits.....

I've looked at many of the suggestions and they look good but most are thru hole parts. I plan to build my VAS stage with SMT parts. This really restricts the recommendations and most of the parts don't seem to have SMT equivalents.

I haven't seen a selector guide from either Sanyo or Toshiba. This might make it a little easier to search.
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Old 22nd March 2007, 03:26 PM   #5
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Quote:
A fast and low cob part is most ideal. I personally love to use either the
800mw 2sc2705/a1145 parts or the 5w 2sc3423/a1360 for the vas 1.8pf for the npn/2.5pf for the pnp. These are 200mhz 50ma devices with an average Y-beta of about 200.. I see most designs using 1amp parts but these are emtremely unneccesary not to mention have an average collector capacitance anywhere from 10-30pf..

Colin

I have looked at a lot of transistors since last night and the Toshibas that Colin mentioned would be excellent for my purposes. I haven't found anything better on paper than the 2SA1145/2sc2705 pair.

The only catch I see is that there doesn't seem to be SMT equivalents. I have looked at Sanyo, Zetex, Toshiba, ON-Semi & Fairchild so far for similar performance. This seems a bit odd since high volume today is in SMT.

Any new suggestions?
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Old 22nd March 2007, 06:29 PM   #6
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Here's a picture of a doubled 2SC2705 in the Vas stage of an expensive buggar from the 90s.
A single 2SC3955(+2SA1540) only costs 10 cents more.
Attached Images
File Type: jpg doubled 2sc2705 vas.jpg (55.2 KB, 1138 views)
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Old 22nd March 2007, 06:33 PM   #7
sajti is offline sajti  Hungary
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Try 2SC2911/2SA1209! High voltage, TO-126 for cooling, linear hfe, and very low capacitances.

sajti
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Old 22nd March 2007, 07:06 PM   #8
Dejak is offline Dejak  Finland
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I would suggest 2SA1538/2SC3953 as a VAS stage transistors. fT 400Mhz, Cre 1.7pF. They are cheaper than 2N5415/2N3440, at least in Finland
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Old 22nd March 2007, 10:57 PM   #9
CBS240 is offline CBS240  United States
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If you prefer lower power VAS as I do, these are from Fairchild, which can be ordered from Mouser here in the US.

http://www.fairchildsemi.com/ds/KS/KSA1174.pdf
http://www.fairchildsemi.com/ds/KS/KSC2784.pdf


http://www.fairchildsemi.com/ds/KS/KSA992.pdf
http://www.fairchildsemi.com/ds/KS/KSC1845.pdf

Now if you like to cascode your VAS as with a common base, were Vce is of non-issue, the 5089 has Hfe, I measured almost 600!
http://www.fairchildsemi.com/ds/KS/KST5088.pdf

Or the good old BC850 / BC860

I'm sure there are those who find these lacking in power for this purpose, but I like to use a smaller VAS and utilize current gain of the output stage. Lower Ic and higher Hfe in the VAS stage, means less current change in front end, depending on design of course.


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Old 23rd March 2007, 05:12 PM   #10
AndrewT is online now AndrewT  Scotland
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Hi,
if 1360 is not good enough consider
1370,
1380,
1406.

I never put 1209 or 1145 on my list of suitable VAS devices due to superior specs available and/or cheaper prices from the above.

I have not looked for equivalents in SMT, sorry.
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