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Old 12th February 2004, 02:17 AM   #1
subwo1 is offline subwo1  United States
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Default VAS pair

Hi all. I am searching for a transistor pair faster than MPSA42/92 which can handle 250 volts and at least 10mA. I think the 2N5401/2N5551 are good to 150 volts, and I don't know how fast they are. Actually, I would like a pair to replace those 2N's as well in order to speed up the differential input stage, hmm.
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Old 12th February 2004, 04:18 AM   #2
sonnya is offline sonnya  Denmark
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2SC2910/SA1208 or 2SC2911/SA1209..

150MHz@10mA, very low Cob. VCEO = ~160V
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Old 12th February 2004, 05:02 AM   #3
sam9 is offline sam9  United States
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With an fT range of 100-300Mhz and Cobo of 6pF for the 2N5401/2N5551, is near the top end of readily avaiable units.

The Sanyo transistors mentioned, above, 2SC2910/SA1208 have slightly greater Vce an lower Cob ratings so might be a little better to the extent those charateristics are important to your project. Most good things come with a trade-off and in this case it is the Ic rating with is quite a bit lower than for 2N5401/2N5551. This may or may not be important depending on how you use them.

I suggest you go "google" for the data sheets and relate those to your objectives.
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Old 12th February 2004, 05:43 AM   #4
subwo1 is offline subwo1  United States
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Thanks for your answers. I am also considering the 2SC3334/2SA1321. Those are two 250 volt types I have since found to exist. But they may be a little slow, I am not sure. If I go with 160 volt transistors, the power supply has to be less than +/-70v. Actually, only the NPN on the lower rail should have to swing the full power supply voltage plus an extra 12v.

Edit: No, the complement comes close to swinging as much voltage as the NPN one. The PNP one needs to handle as much as only several volts less than the NPN one on the lower rail.
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Old 12th February 2004, 08:31 AM   #5
Eva is offline Eva  Spain
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Could you post a simple schematic of your input stage?

I couldn't imagine why an input stage should have such a huge voltage swing

Some time ago I was also looking for fast and linear 150V-250V bipolar transistors and the best I found was from Toshiba, but I don't remember part numbers

As you've said, for 150V 2N5401 and 2N5551 are a good choice

For 100V I like BD139 and BD140, sometimes I've used them for switching and they showed to have vey fast turn-off, but beware of low quality clones

For 200-300V there are some alternatives to Toshiba but I don't like them very much : MPSA42/92 [I don't like these at all], BF420, BF421, BF471, BF472, BF871, BF872 [these appear to be similar dies in different packages] and MJE340/350

Afther not being able to easily get good high voltage small signal bipolars, I decided it was time to explore floating supply topologies because they allow powering everything but output devices and drivers from +-15V regulated supplies and working with quasi-constant Vce
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Old 12th February 2004, 03:40 PM   #6
subwo1 is offline subwo1  United States
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I happen to be using some floating supplies, but more for the purpose of supplying more current more efficiently. I have included the first half of the amp circuit which is not too extraordinary topologically, I think. The PNP transistor on the right is for the CFP output stage. It must swing the full power supply voltage as well as Q6.
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Old 12th February 2004, 04:44 PM   #7
Eva is offline Eva  Spain
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Depending on the output stage topology, Q6 and Q16 may have to swing the full |+Vcc|+|-Vcc| range, but there are classic solutions like using fast 50-60V transistors cascoded with BF871/872 or similar

You could place two couples of series antiparalell 1N4148 or similar across C2 to prevent the capacitor and the input stage to track te output in DC and blow in case of any amplifier failure causing the output to latch to positive or negative rail

You could also place a simple capacitor in paralell with a zener, a resistor or a Vbe multiplier, or a cascode, in series with collectors of Q3 and Q4 to reduce its working Vce allowing to use fast 50-60V transistors

Using a big capacitor requires the supplies to rise slowly to prevent overvoltage across Q3 and Q4 so this has to be handled with care

Beware of cascoding, it's an evil technique since it allways adds some delay to the signal path and reduces phase margin, independently of possible THD reduction or higher bandwith due to less miller effect
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Old 12th February 2004, 06:39 PM   #8
pjacobi is offline pjacobi  Germany
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It seems, the possible solutions are well explained already.

If you want to go without increasing the number of transistors, you essentially can't get high beta and high VCEO.

For high VCEO devices with still fair beta, I'll suggest KSA1381/KSC3503 or (2SA1381/2SC3503).

http://www.fairchildsemi.com/ds/KS/KSA1381.pdf
http://www.fairchildsemi.com/ds/KS/KSC3503.pdf

Regards,
Peter Jacobi
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Old 12th February 2004, 09:21 PM   #9
subwo1 is offline subwo1  United States
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Thanks. I think the 2sa1381/2sc3503 are going to be the best option since that makes a simpler circuit with lower parts count and should ease construction. I may try them as input devices also to see how the operation of the circuit is affected. The problem of over-voltage in case of amplifier failure, never thought of that one either. DC output from shorted power transistors reminds me of the days with the old Tiger amp.
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