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Old 5th December 2006, 06:47 PM   #1
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Default putting transistors in series

Why is it so difficult to put transistors in series, like if you want it to be in a amp?

People have said by putting transistors in series is not that easy.

Can someone please explain that one to me please ?
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Old 5th December 2006, 07:19 PM   #2
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Default Re: putting transistors in series

Quote:
Originally posted by jacquesl
Why is it so difficult to put transistors in series, like if you want it to be in a amp?

People have said by putting transistors in series is not that easy.

Can someone please explain that one to me please ?
What do you mean by 'series'?

If you mean like in darlington configuration,
it is not so difficult or bad.
And in circuits/amplifiers using negative global feedback
this is never any big problem.

Click the image to open in full size.
Darlington configuration of 2 transistors.
http://www.hobbyprojects.com/junctio...gton_pair.html

HobbyProjects.com - Transistors Tutorials

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Old 5th December 2006, 08:26 PM   #3
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is this what you call series?
used to be used with low Vce transistors in70's
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Old 5th December 2006, 08:43 PM   #4
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Itís not Darlington configuration, itís more like darkfenriz have said in a way I think, not 100% sure
It like 2 or even more transistors put in series to handle more voltage, collector to emitter

Please correct me if Iím wrong
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Old 5th December 2006, 09:09 PM   #5
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Hmmm...

Like a cascode? Common emitter followed by a common base amp (in BJT terms)?
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Old 5th December 2006, 09:18 PM   #6
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That is how the output stage of the swtpc 250 watt amp was, back in '78. edit:..(wired in series due to low voltage zistors)

Here's a partial. Q17, 19, 21, 23 are the neg rail zistors.

Q21 and Q23 are held to half the rail to output voltage by:

R34 and R36 divide the total (note the three diode drops also are included). Q13 holds the bases of q21 and q23 at half that voltage drop.
Q15 is the soa limiter, if the drop across r40 is too high, q15 reduces drive. r44 goes to output node, and lowers the clamp threshold when the output rail is further from q15, this limits negative current max when output is positive. Same happens with pos rail.

Those 200 ohmers do get hot..

Cheers, John
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Old 5th December 2006, 09:20 PM   #7
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That arrangement is called a ladder.

I have not seen it used much in audio, since transistirs >100v are easy to find.

I used to use them in 2,000V power supplies ( crudely 10 x 250V transistors in a ladder).

The problem is getting them to share the voltage equally, which wasres quite a bit of power in the base to base resistors.
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Old 5th December 2006, 11:36 PM   #8
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Quote:
Originally posted by cliff
That arrangement is called a ladder.

I have not seen it used much in audio, since transistors >100v are easy to find.

I used to use them in 2,000V power supplies ( crudely 10 x 250V transistors in a ladder).

The problem is getting them to share the voltage equally, which waste quite a bit of power in the base to base resistors.
Yes, no problem find high voltage transistors today.
Most of them REALLY HIGH were developed for Video =TV, use.

---------------------------------------------------------------

Very nice TO92 for moderate high voltage for audio are:
2N5551, NPN, 160 volt C-E ..... 2N5401, PNP, 150 volt Collector-Emitter

... and we have also also these TO92
MPSA42, NPN, 300 volt, 0.625 Watt .... MPSA92, PNP, 300 volt, 0.625 Watt
http://www.fairchildsemi.com/ds/MP/MPSA42.pdf
http://www.fairchildsemi.com/ds/MM/MMBTA92.pdf

Quote:
MPSA42, NPN High Voltage Amplifier
This device is designed for application as a video output to
drive color CRT and other high voltage applications.
Spice Model
NPN (Is=34.9f Xti=3 Eg=1.11 Vaf=100 Bf=2.65K Ne=1.708 Ise=16.32p Ikf=23.79m Xtb=1.5 Br=9.769 Nc=2 Isc=0
Ikr=0 Rc=7 Cjc=14.23p Mjc=.5489 Vjc=.75 Fc=.5 Cje=49.62p Mje=.4136 Vje=.75 Tr=934.3p Tf=1.69n Itf=5
Vtf=20 Xtf=150 Rb=10)

MPSA92, PNP High Voltage Amplifier
This device is designed for high voltage driver applications.
Spice Model
PNP (Is=218.9f Xti=3 Eg=1.11 Vaf=100 Bf=99 Ne=1.307 Ise=218.9f Ikf=.2016 Xtb=1.5 Br=24.67 Nc=2 Isc=0
Ikr=0 Rc=7 Cjc=19.88p Mjc=.4876 Vjc=.75 Fc=.5 Cje=81.49p Mje=.3493 Vje=.75 Tr=516.9p Tf=1.395n Itf=1.5
Vtf=22 Xtf=270 Rb=10)
.

If we think TO126 capsule
(might be needed, Power= Volt x Current )
we have these 'video transistors'.
They are quite often used in Audio Amplifiers
MJE340, NPN, 300 volt, 20 Watt
MJE350, PNP, 300 volt, 20 Watt

Speaking of high voltage devices.
There are MOSFET N-CHANNEL, at least,
that can take extremely high voltages for very fast switching ... 700 Volt D-S and so


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Old 6th December 2006, 08:52 AM   #9
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Yea, Itís a ladder Iím talking about,Nice, I would like to se a diagram on how to switch like example 100v with two 50v Max transistors?
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Old 6th December 2006, 11:26 AM   #10
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The Dynaco 4XX used the "totem Pole" configuration. Note that in this design 2 xistors per rail were adequate for a 200 watt amp. Since the voltage across the xistors is cut in half their power dissipation is 1/4.
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