I think there are 2 different situations:
VAS with rather low current, say 2-5 mA
Here we might be able to use TO92,
and the there are plenty of fair choices.
VAS with a bit higher current and we need a package
that can take more power.
Here we go for medium power TO126 or similar.
In this case is not so many good transistors to find.
Especiallt not complementary NPN/PNP
In the first case we may not have to Cascode.
In the second case Cascoding is one solution.
To make transistors cope with power.
VAS with rather low current, say 2-5 mA
Here we might be able to use TO92,
and the there are plenty of fair choices.
VAS with a bit higher current and we need a package
that can take more power.
Here we go for medium power TO126 or similar.
In this case is not so many good transistors to find.
Especiallt not complementary NPN/PNP
In the first case we may not have to Cascode.
In the second case Cascoding is one solution.
To make transistors cope with power.
2SA1145/2SC2705 and their TO126 siblings 2SA1360/2SC3423
are very good with a Cob of 2.5 and 1.8pF respectively...
are very good with a Cob of 2.5 and 1.8pF respectively...
BTW, there was another thread some time back discussing this very same subject. I took a quick look but cannot find it. We should take a look at that one again as well as alot of expereinced people commented on this subject at the time.
To-92 is large , for a input section. With SMD there are lots of dual devices to choose from , **** the matching game. VAS wise , Just as some like toobz , nice to-126 pairs on a small heatsink give that feeling of permanence. For a VAS , this is good , it is the one thing in the amp you don't want to ever fail.
OS
Another thread, same subject
http://www.diyaudio.com/forums/solid-state/112227-about-vas-drivers-transistors.html
http://www.diyaudio.com/forums/solid-state/112227-about-vas-drivers-transistors.html
Yes, my favourite as well - but there's no PNP complement or it's too hard to find, but this is not an issue for bootstrapped VAS topologies.
You may try 2SA1539 or 1538 as a complement. I use 2SA1538 (for lack of availability of any other) and their performance is really good.
Yup, VAS failure is a bad thing. Fortunately my VAS is current limited and cannot exceed the current setting or the voltage will degenerate, but this is highly unlikely due to the active bias. I'm a big fan of unconventionality.😀
Just because two devices are in the same package does not mean they are matched even if they are monolithic. The mosfets I use have no particular need to be matched because they are common gate amplifiers. The current amplifying transistors is a different story.😛 If you must use matched devices, and it does have significant advantages, then you should invest in 'matched' arrays. There are several out there, but they are significantly more expensive.😱...
Option to High Voltage Medium (Video) and High Power Devices for Output and VAS Stage
in this case this thread could be of interest:
http://www.diyaudio.com/forums/soli...wer-devices-output-vas-stage.html#post2239333
in this case this thread could be of interest:
http://www.diyaudio.com/forums/soli...wer-devices-output-vas-stage.html#post2239333
I didn't see these listed fairchild KSA1381 KSC3503 , also made by sanyo. 300v, ft 150mhz, hfe 100 current 100ma. Just use a diode string from b to other end of e resistor for current limit. The # diodes depends on the value of e resistor. See old crown schemos for this current limit method.
Edit
Ah, now I see it was covered on page 2, anyway the part about how to current limit 100 ma vas trans may be useful.
Ah, now I see it was covered on page 2, anyway the part about how to current limit 100 ma vas trans may be useful.
These like OK: Sanyo 2SC3601 / 2SA1407
Low Cob, low Cre, high fT, high voltage, high Hfe. Lowish power but who needs a bunch of current through the Vas?
Low Cob, low Cre, high fT, high voltage, high Hfe. Lowish power but who needs a bunch of current through the Vas?