Choices for Vas transistor

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#5
Sanyo source?

Dear Hugh,

I also like this Sanyo series of video transistors. The problem is it seems to be discontinued, I even got that info in Japan.

While there is still a limited supply of the C3955, its counterpart 2SA1540 seems to be gone for good.

I can still get a pair from the same series that has a lower current rating, but it's no longer possible to order them from the same gain group.

Do you have a special source or know about adequate replacements?

Regards,

Eric
 
#7
Hi Eric,

Well! I did not realize it was discontinued!! Bummer!

But it is available at www.pacificsemi.com (US46c), and www.bdent.com (US92c) in one off and 100+ quantities.

I made a list of transistors suitable for this work some time back but knowing me I've lost the damn thing.......

Sanyo has a proprietary manufacturing process which reduces collector capacitance, and frankly this makes them very desirable as VA devices.

I will seek out more; I buy from Peacock Industrial in Singapore and yesterday sent out an order. I'll let you know.

Cheers,

Hugh
 
#8
Dear Hugh,

thanks for your link. Pacific Semi has good prices, but you'll notice their stock is 0 for all of these parts. I have no idea whether that means the do special order only or if it's simply no longer available.

BD do not say whether they have stock.

Do you have any way of ordering parts form the same h_FE group?

Greetings,

Eric
 
#10
Just to share some info....

I just got some 2SA1540 and 2SC3955 from MCM (in USA).

And no, I haven't checked to see if they are fake or not. 🙁

Both NPN and PNP were "D" grade. That means HFE @10mA range of 60-120.

FYI

mlloyd1

oops, I forgot: price was $1.15 and $1.07 USD (>15 pieces each) respectively
 
#12
Jonathan,

My sincere thanks. I will look into all of them, although I note that 2SC3423 is rated at 200MHz, though it does have collector capacitance of only 1.8pF. (The others are 400MHz I believe).

What's your feeling about the Toshiba 2SC4439 (400MHz, 150V, 8W, 4pF)

Cheers,

Hugh
 
#13
Hugh: The Sanyos that I listed are for the most part 150~200MHz devices, although I think that the 2SA1697/2SC4474 are 300MHz.

OTOH, for linear audio applications, once you clear the 100MHz point, IMO factors like Cob, Cre, Vaf, and linearity matter much more than finding the ft champion. In the Sanyo range, some devices sport both higher ft and higher Cob than other, similar devices, but personally I would prioritize low capacitance over high ft. Besides, as you know, the maximum ft rating is only realized at a specific Ic, usually with a clear drop-off on either side of that IC, the value of which may or may not coincide with the current that the rest of your circuit wants to run at.

>What's your feeling about the Toshiba 2SC4439 (400MHz, 150V, 8W, 4pF)<

I haven't used it yet. No idea of how it performs and sounds in the real world. But the data sheet looks reasonable, at least. Let's run a very rough comparison between it and the Sanyo 2SC3788.

Both the Sanyo and Toshiba clear 100MHz ft by around 3mA Ic, at 10mA, the Toshiba is kissing 200MHz while the Sanyo is still at about 150Mhz. However, the ft of the Sanyo appears pretty uniform at any Ic between 15~40mA, while the Toshiba's ft curve stabilizes between 30~100mA.

The Cob of the Sanyo starts out at perhaps 4pF at 1V Vcb, and drops down to 1.5pF or so at 100V. For the same voltage swing, the Toshiba goes from 10pF~3pF, so I would expect distortion to be higher on the Toshiba.

The hfe-Ic curve is clearly more linear on the Sanyo. The Early voltages are probably somewhat lower with the Toshiba, but both devices look to be pretty mediocre in this parameter. OTOH, the Vbe-Ic stability appears to be superior with the Toshiba.

If I was only allowed to judge from the data sheets, I would probably choose the Sanyo (unless I had specific requirements in mind, which _is_ a common occurance), but the Toshiba does look attractive enough to consider ordering samples and breadboarding some circuits.

My two pence... jonathan carr
 
#14
Hi Jonathan,

My thanks again! This is invaluable information, and I cannot find a single point of disagreement. :nod: I too give preference to collector capacitance and uniformity of beta; these to me are the big ones. I feel that since the application calls for a stage current around 8mA, any more Ic than 100mA is not required, enabling TO-126 packages and generally more nimble performance.

I have continued searching and settled on the Sanyo complementary pair 2SC3600/2SA1406. I do not know if they are obsolete; I hope not. The 3600 (my app is npn) looks very attractive, with 200V, Cob at 1.5pF/70V and dead linear, saturation voltages at around 150mV (very good for negative swing linearity), and hfe as straight as a die from 5 to 45mA. I would suggest this would be a killer Voltage Amplifier.

I have enjoyed your website. True high end; I doffs me cap. And clearly you are to be admired for mastering the only language jokingly described as an '...impediment to communication'!!

Cheers,

Hugh
 
#15
In designs with a single-ended VAS (rather than complementary), common wisdom has it that the constant current source (CCS) transistor may be much slower than the VAS transistor.

Sanyo has some superfast video transistors which don't come in complentary pairs, so one would be forced to use a slower part for the CCS.

Now, it's true that Miller capacitance is not that important in the CCS because the base can be buffered with a large cap. On the other hand, I am not sure that f_T is really unimportant. I think it is still needed for the transistor to react to HF changes in the load conditions. True?

If so, a slow CCS would introduce signicant phase lag. One could put a .5 to 5k resistor in series with the CCS collector, which would probably eliminate HF lag issues, but this would limit voltage swing.
 
#16
capslock said:
Sanyo has some superfast video transistors which don't come in complentary pairs, so one would be forced to use a slower part for the CCS.

Now, it's true that Miller capacitance is not that important in the CCS because the base can be buffered with a large cap. On the other hand, I am not sure that f_T is really unimportant. I think it is still needed for the transistor to react to HF changes in the load conditions. True?

If so, a slow CCS would introduce signicant phase lag. One could put a .5 to 5k resistor in series with the CCS collector, which would probably eliminate HF lag issues, but this would limit voltage swing.
Click to expand...

You can add a cascode transistor to the CCS to speed it up.
 
#18
It will lower the voltage swing a bit, yes, but it does speed up
the CCS since it keeps Vce almost constant for the CCS
transistors, thus avoiding the Miller effect in it.

This was discussed a while ago, by the way, in another thread,
but I can't remember which one right now.
 
#19
The Miller effect should be irrelevant in a CCS because the AC impedance of the control voltage at the base is essentially 0. The cascode transistor will increase (LF) output impedance further by getting rid of the I_C vs. V_CE variation. There is nothing it can do about the limited f_T, though.
 
#20
I will have to think about this. I got the cascoding idea myself
once (it is a pretty obvioous idea) and if my suggestion was
only based on that I would probably yield and assume I was
wrong. However, this idea was later suggested on this forum by
other people. I think it was an EE who suggested it, although I
may be wrong there. I haven't succeded to find the thread yet.

Does anybody else have an opinion on cascoding CCSes?
 
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