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Old 16th April 2007, 09:44 AM   #1
Clipped is offline Clipped  Thailand
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Default starting to study amp designs

starting to study amplifier circuits and i would really appreciate clarification of a couple of little things to expedite my learning.

say you have a rail voltage of +50 and -50 which equals 100v
would the output transistors have to be rated at 50v or 100v ?

and

i would like to try designing a mosfet amplifier....but how do amplifiers that work with strictly N channel devices work without having to use a P channel device? and vice versa.

and would a transistor working closer to its maximum voltage rating be stronger than working at half its maximum voltage rating?

thanx for putting up with my basic questions

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Old 16th April 2007, 10:01 AM   #2
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They would need to be rated to 100V as there will be 100V across the 'off' transistor when the output is swinging hard towards the opposite rail.

Read up on quasi complementary design, there is a lot of information out there.

A transistor will be stronger the lower the voltage it works at in relation to it's rated voltage.
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Old 16th April 2007, 11:29 AM   #3
Nordic is offline Nordic  South Africa
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They would need to be rated in excess of the rail voltage, not?
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Old 16th April 2007, 11:34 AM   #4
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Well as with most component ratings, it doesn't meant it will instantly explode when you hit 101 volts on a 100 volts transistor. Yes ideally it should be rated higher, but the emitter resistors will add in some drop to the 'seen' voltage as well as the fact just mentioned, so they should be fine.
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Old 16th April 2007, 02:32 PM   #5
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The breakdown voltage is also tied to base drive and current through the device. Look at SOA curves on data sheets.
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Old 16th April 2007, 05:28 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
when is the voltage on the supply rails equal to +-50Vdc?
If the voltage goes higher due to less quiescent current or due to higher mains supply voltage then the output transistors will see this high voltage.

Look up the ESP site
http://sound.westhost.com/articles.htm
for good design information.
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Old 16th April 2007, 10:42 PM   #7
Clipped is offline Clipped  Thailand
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Quote:
Originally posted by richie00boy
A transistor will be stronger the lower the voltage it works at in relation to it's rated voltage.
sure about that?
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Old 16th April 2007, 10:50 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Clipped
quote:
Originally posted by richie00boy
A transistor will be stronger the lower the voltage it works at in relation to it's rated voltage.


sure about that?
yes as the voltage is dropped the permissible current rises to maintain the same limiting power.
If the device was above the start of second breakdown then one gains twice by reducing voltage.
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Old 16th April 2007, 10:50 PM   #9
Clipped is offline Clipped  Thailand
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Quote:
Originally posted by Nordic
They would need to be rated in excess of the rail voltage, not?
for most amps ive seen yes....


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Old 16th April 2007, 10:54 PM   #10
Clipped is offline Clipped  Thailand
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Quote:
Originally posted by AndrewT
yes as the voltage is dropped the permissible current rises to maintain the same limiting power.
If the device was above the start of second breakdown then one gains twice by reducing voltage.
what if the ps design was current limited?

so a transistor rated at 100v will be stronger at 10v than 50 v?

something about that just sounds funny,i better read up, because i always thought it to be the opposite...seems like it would be more efficient the higher the voltage goes.
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