High Voltage BjT's for Pre-driver stages of High Power Amplifier - Overview

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soekris

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Basically you really need to look very carefully at the hfe/Ic curves and the SOA curves, and do it with a eye for not just 8 ohm load curves, but also 4 and 2 ohm.

I believe in principles of shortest signal path, and with the newest parts I think that you can do just fine with a two stage EF output.

Soren
 
tiefbassuebertr

tiefbassuebertr

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Cauldron Amplifiers said:
Just a quick reply to this thread on good pre-drivers for audio: Strangely enough, the Mje340/Mje350 drivers are still what I would call a preferred driver for audio. The Mjd340/Mjd350 also look very good in Spice modeling.
In my experience, the KSA1220A/KSC2690A do not respond well as complimentary drivers, as the gain values are way out of balance for any reasonably biased class AB output stage. If you are working with below 50 volt complimentary rails, the best pre-drivers I've ever encountered, were the Motorola MPSU57/MPSU07. I still find them available from time to time, but they've been out of production for a number of years.
Don't know what to say about the 2SA1248/2SC3116 suggestion, as they appear to be designed for high speed switching, not audio.
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read in this case Lumba's comments about
http://www.diyaudio.com/forums/soli...bd159-gbp-transit-frequency-ft-unknown-2.html
in this case also this thread could be of interest:
http://www.diyaudio.com/forums/solid-state/3853-bf469-470-471-472-vs-mje340-350-better-2.html
 
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soekris

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Telstar,

I will shortly post the whole design, incl. PCB layout, a fully developed one with power supply and protection. I will probably also offer assembled boards (surface mount), but will use the Fairchild FJA4313OTU/FJA4213OTU in production.

The OnSemi NJW1302/NJW3281 are just a little better but also cost twice as much.

But that's for another thread....

Soren
 
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maouna

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Joined 2008
hi.. which type of transistor is better to use as vas?
bd829-bd830 , 2sa1306-2sc3298 or 2sd669-2sb649?

bd829-bd830 have an ft of 250mhz ,are applicable at television circuits,but the datasheet doesnt make any refference of the cob of the specific transistor.
 
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maouna

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Joined 2008
there are many who claim that using a high Ft transistor as Vas, affects stability and you have to apply a higher value compensation capacitor to the Vas transistor.

i am using 100pf in my amp with +-36 volt power supply and 2sc3298 as vas and i am completely sastisfied with the performance.
if i replace it with 2sa1406 or its complement,which is said to be suitable for vas,what improvements there will be?
in a few words what would be the benefit of replacing with a high Ft ,low Cob transistor?
 
homemodder

homemodder

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maouna said:
there are many who claim that using a high Ft transistor as Vas, affects stability and you have to apply a higher value compensation capacitor to the Vas transistor.

i am using 100pf in my amp with +-36 volt power supply and 2sc3298 as vas and i am completely sastisfied with the performance.
if i replace it with 2sa1406 or its complement,which is said to be suitable for vas,what improvements there will be?
in a few words what would be the benefit of replacing with a high Ft ,low Cob transistor?
Click to expand...

Lower Cob will allow you to use a smaller miller cap, there isnt much benefit in very high ft as such but it comes down to the fact that very low cob transistors have high ft, RF transistors have cob of less than 1pf. Rf transistors do sometimes get used for audio, there are a couple of comercial designs using them in audio.
 
tiefbassuebertr

tiefbassuebertr

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villain said:
Do you know any high voltage BJT's for integrated circuit other than discrete devices?
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I don't understand this question. Mean you integrated OP-Amps like AD797 but for high voltages?

Because computer monitors and television sets with picture tubes are no longer in production, I guess, that also all BjT devices with high Vce (like the mentioned types from post #1) are obsolete and no longer available in a relatively short time.
This means, that must be by the developing of high voltage amplifier stages (VAS) other solutions realized.
Perhaps basiclly a topology realized already by the Threshold 800A output stage - go to post #14 about
http://www.diyaudio.com/forums/pass-labs/5426-800a-schematic-2.html
Four pcs. of an good known small signal device in the new outline with 45V Vce like
http://www.onsemi.com/pub_link/Collateral/BC807-25WREV0.PDF
in serial connection delivers 180V Vce
 
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tiefbassuebertr

tiefbassuebertr

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Bob Cordell said:
The MJE340/350 have been used as drivers quite a bit (as opposed to pre-drivers), in both Darlinton and Triple output stages for amplifiers of modest power. However, beware of the SOA of drivers. Many people overlook this and smoke the drivers before the output transistors go. Remember, under high-current conditions, beta of the output transistors may droop, maybe to 20 or lower, putting a significant current burden on the drivers.

I usually like TO-126 pre-drivers and TO-220 drivers.

Cheers,
Bob
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Which types from currently production are the best choice?
one good example for not too high supply voltage I have found at post #12 about
http://www.diyaudio.com/forums/parts/253580-bc546-type-to126-2.html#post3869496
 
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