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Old 7th February 2003, 09:37 PM   #11
jcarr is offline jcarr  United States
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If the 2SA1540/2SC3955 meet your requirements, similar Sanyo TO-126 devices include the 2SA1478/2SC3788, and 2SA1209/2SC2911. Perhaps also the 2SA1538/2SC3953.

In a molded TO-220 package, the 2SA1697/2SC4474. Definitely.

Then there's also Toshiba's 2SA1360/2SC3423...

hope this helps, jonathan carr
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Old 7th February 2003, 11:40 PM   #12
AKSA is offline AKSA  Australia
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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
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Old 8th February 2003, 12:58 AM   #13
jcarr is offline jcarr  United States
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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
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Old 8th February 2003, 11:44 AM   #14
AKSA is offline AKSA  Australia
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Hi Jonathan,

My thanks again! This is invaluable information, and I cannot find a single point of disagreement. 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
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Old 13th February 2003, 10:41 AM   #15
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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.
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Old 13th February 2003, 10:52 AM   #16
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Quote:
Originally posted by capslock
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.
You can add a cascode transistor to the CCS to speed it up.
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Old 13th February 2003, 11:09 AM   #17
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Limits voltage swing, too, and does not do a thing dynamically (f_T remains the same if it's the same type).
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Old 13th February 2003, 11:17 AM   #18
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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.
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Old 13th February 2003, 11:37 AM   #19
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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.
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Old 13th February 2003, 12:58 PM   #20
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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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