Low power classAB BJT amp CFP driver transistors

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316a

316a

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Joined 2002
#1
New to BJT and currently experimenting with CFP output stages . Currently using BD139-16/BD140-16 with BD911/BD912 but wondering what else is out there to go in place of the BDs , which appear to be overkill in my application . It's a low power design class AB , single supply Vcc 22V , which may eventually be pushed towards 30V . The amp only needs a handful of watts for my application . I have plenty of BC327.40 and BC337.40 and was considering these . Will I be able to get away with a TO92 device or need to go for something more substantial ?

cheers
 
#2
BD911/BD912 are as big as it gets in a to220 package. Big transistors are a good thing as many people use transistors that are too small. But below 36V (+/-18V), you can probably get away with TIP41/TIP42, and TO92 drivers as long as they are rated for 700mA or more. But again, big transistors can avoid a lot of grief. Silicon today is cheap. Your time is not. One thing is that these old transistors are slow compared to newer parts. That can be a mixed blessing, but generally faster parts are better. About CFP, be sure the driver is much faster than the output, for a stable circuit. Two devices with similar speed is probably unstable.
 
#4
I recommend you use TO-126 packaged devices as drivers. With a 30V single ended power supply you can easily wind up pushing 2-3 ampere peaks into a 4 ohm loudspeaker. So you want beefy output transistors and not-scrawny drivers.

First, best choice: KSC2690AY + KSA1220AY . But you'll have to buy them N.O.S. The semiconductor supply chain hates you. YOU specifically.

Second choice: TTC004B + TTA004B

Third choice: KSC3503 + KSA1381

Fourth choice: BD139 + BD140
 
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#5
BC327/337 are fine. It’s only +/-15 volts here. 3 or 4 amps peak into a loudspeaker does not need an amp and a half driver, even if the outputs only have a beta of 20. I’ve run TIP41/2 with 2N3904/6 on +/-17V and not only survives but sounds good. It was an experiment that used “all parts including transistors from Radio Shack”. Don’t have to special order anything. 15 watts at 4 ohms, 10 at 8. A bit more if the transformer came from somewhere besides Radio Shack, but at least they had the 25.2/2A.

Yes, it had VI limiting - 2 amps into the zero crossing and 6 amps at Vce<5. The TIPs and the drivers held up fine. Without it, it might take out 200 mA drivers in the event of a short.
 
#8
Depending on region.
Easily accessible and good performers.

Either Toshiba or Fairchild legacy transistors
from ON Semi
22 volt rails, yes stick to TO-126 and heatsink
mount. CFP also thermal tracks drivers, not outputs.

TTC/ TTA 004B

KSC/KSA 3503,1381

22 volt rail sounds easy enough.
Could be wrapped up in opamp feedback loop
for low THD.
 
#9
steveu said:
But below 36V (+/-18V), you can probably get away with TIP41/TIP42, and TO92 drivers as long as they are rated for 700mA or more. . . . One thing is that these old transistors are slow compared to newer parts. That can be a mixed blessing, but generally faster parts are better.
Click to expand...
TIP41c/42c may be fine as output transistors. They are noticeably inferior as drivers driving TO3 outputs. Even with my hearing limit of 14 khz, TIP41/42 as drivers sounded bad when the other channel had surviving 2n5320/22. Dull lifeless highs. Datasheet difference, Ft on TIP41/42 is 6 mhz, Ft on 5320/22 is 50 mhz. I have found 20 mhz transistors like 1970's GE D44R4 perfectly adequate as drivers for TO3 outputs. (Which is not the fairchild D44H8. D44R4 Still available from a surplus house with the gold pins last time I looked).
If you are looking for tiny, TO18 transistors with Ft 20 mhz or higher are fine. But they are not cheap, $3.50 each these days. Use heatsinks with TO18 output transistors.
 
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