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Old 24th January 2004, 01:55 PM   #1
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Question VAS transistor choice

Hi!

I am looking for a good complementary pair of transistors for my VAS stage (rail voltage is +/-42V). I have searched the forum and it seems that the MJE340/350 are mostly used, but considered not to be the best... I have found some other BF/Sanyo/etc. parts, but most of them seem to be discontinued or I don't find a reseller... further, I would be very happy to get the Spice models for the transistors somewhere...

Any suggestions?


Thanks,
stegmaie
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Old 24th January 2004, 06:22 PM   #2
pjacobi is offline pjacobi  Germany
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Hi,

Also depends on the VAS topology, eh?

For a start, BD139/BD140 aren't that bad and cost essentially nothing.

Judging from datasheets and models KSA1381/KSC3503 look fine. But unless odering a larger quantity from a Fairchild distributor, you would most likely only get 2SA1381/2SC3503 and then you must hope, that the originals are as good as the copies.

http://www.fairchildsemi.com/models/...ransistor.html

Panasonic 2SA794/2SC1567 is available through Reichelt and not bad at all.

10mA Iq enough? Panasonic 2SA1124/2SC2632 in 'stretched' TO-92 will do. Several german mail-order stores have them.

Moderately nostalgic feeling: BC141/BC161 in TO-39 metal case!

Prefer more power dissipated: Sanyo 2SA1011/2SC2344 or Sanken 2SA1488/2SC3851 in TO-220AB/TO-220FM

Regards,
Peter Jacobi
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Old 24th January 2004, 09:05 PM   #3
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Stegmaie,

As general rule of thumb, the higher the VCE the transistors can handle and the higher power they can dissipate, the lower their gain and the lower their GBW product. So, from a performance point of view for +/- 42 V, you should take the one that has a Vce limit of say 100 or 120V, but no more, and that can easily handle the power required, but not very much more.

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Old 24th January 2004, 11:17 PM   #4
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Other popular types seem to be 2SB649/2SD669 and the more
modern Toshibas 2SA1837/ and 2SC4793.
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Old 26th January 2004, 12:26 AM   #5
dulel is offline dulel  Yugoslavia
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Default Re: VAS transistor choice

Quote:
Originally posted by stegmaie
Hi!

I am looking for a good complementary pair of transistors for my VAS stage (rail voltage is +/-42V). I have searched the forum and it seems that the MJE340/350 are mostly used, but considered not to be the best... I have found some other BF/Sanyo/etc. parts, but most of them seem to be discontinued or I don't find a reseller... further, I would be very happy to get the Spice models for the transistors somewhere...

Any suggestions?


Thanks,
stegmaie
Hello everybody, this is my first post.

I would recommend using bf871/872 pair. They can withstand very high voltages, colector base capacitance is very low (2 or 3 pf), and ft is 60 Mhz. Packaging case is TO 202, which means that they can dissipate 300 or 400mW without overheating. Don't use TO 92 and TO 39 devices, they would probably overheat (unless your VAS current is few milliamps). BF471/472 offers similar caracteristic in TO 126 package.

Dule
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Old 26th January 2004, 03:48 AM   #6
AKSA is offline AKSA  Australia
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Stegmaie,

Speed and low base/collector capacitance is important.

As Jan says, fix the voltage a little over twice the rail voltage; for a 42V rail 100-120V is fine for Vceo.

The higher the beta, the more the open loop gain of the amp, and the more is the feedback factor. Generally, this is good. So try to find a device no slower than 100MHz and a beta of 60 minimum.

I have used 2SC1819 and 2SC3955 with good results, but both these devices are now obsolete. Sanyo have a very good manufacturing process which gives high speed, low Cbc, and good beta. IMHO the voltage amp is the pivotal stage in a power amplifier. Watch the dissipation; no more than one third the rating for the device, and keep temperatures low for reliability.

Cheers,

Hugh
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Old 26th January 2004, 09:20 AM   #7
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Thanks everybody!

I played a little bit with the KSA1381/KSC3503 (in the simulation) and they look fine... I think I will get them resp. the 2SA/2SC types. I had a look at the datasheet of the 2SA/2SC types and its pretty the same as for the KSA/KSC. So, I hope they will behave somewhat like the models.

With respect to the criteria given, Sanyo has some really nice types. But unfortunately I do not find any models for them. So, I will use what I have in my simulation and think of an update if the amp works as expected... BTW: Any chance of making a more or less good model out of the specs of a datasheet?


stegmaie
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Old 26th January 2004, 04:04 PM   #8
mlloyd1 is offline mlloyd1  United States
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Suppliers I use are still able to source 2SA1210 and 2SC2912 from Toshiba. These work well as VAS stage amps. These parts are in a TO-126-like package

mlloyd1
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Old 26th January 2004, 04:29 PM   #9
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Beware, Reichelt will often sell you second sources even if they list the real part number in the catalogue. As far as I know, the Japanese to not licence out the numbers, so any second sources are self appointed. I know of cases where there is a data sheet available for a Taiwanese second source, and about all that agrees is the wattage. I hate to think of those copies for which there aren't even published data sheets.

Best way is to order 1-2 pieces and if they are genuine, order more telling them what the code is.

Segor will do a very good job of telling you the manufacturer.
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Old 26th January 2004, 04:40 PM   #10
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Quote:
Originally posted by capslock
Beware, Reichelt will often sell you second sources even if they list the real part number in the catalogue. As far as I know, the Japanese
Thanks for the hint... I would have ordered from Reichelt...

Quote:

Segor will do a very good job of telling you the manufacturer.
I don't know why, but I never liked Segor...
But, price is nearly the same (and in stock), so I will probably get them there. I have seen that the transistors have different ratings concerning Beta... I will ask them if its possible to get specific ones, resp. which ones they sell...


Thanks,
stegmaie
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