| Clipped |
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
:D |
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| richie00boy |
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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| Nordic |
| They would need to be rated in excess of the rail voltage, not? |
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| richie00boy |
| 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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| Conrad Hoffman |
| 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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| AndrewT |
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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| 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? |
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| AndrewT |
| 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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| Clipped |
| 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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| Clipped |
| 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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| Nordic |
| I think it is a translation issue, stronger, here, was maybe the wrong word choice... I think what he tried to say was "less likely to destruct"... |
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| Shaun |
| quote: | Originally posted by Clipped
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. |
It's not a question of "stronger"; more of "withstand". As posted before, read up on SOA- safe operating area. |
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| tomahauk |
| Clipped: i have few e-books about designing power amps, if u want i can send them on your e-mail |
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| culcanmar |
Tomahauk, if I may say so, I would be interested in those!
Grtz,
Martin |
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| big jim |
Hi
The ratings of the transistors must be atlease
10 % higher than the supply in your case
110 volts+
jim |
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| tomahauk |
| quote: | Originally posted by culcanmar
Tomahauk, if I may say so, I would be interested in those!
Grtz,
Martin |
so giveme your e-mail |
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| Clipped |
| quote: | Originally posted by tomahauk
Clipped: i have few e-books about designing power amps, if u want i can send them on your e-mail |
tomahauk...PM sent
thank you |
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| AndrewT |
Hi,
the output device Vce0 can match the Vrail+|-Vrail| exactly. Some amplifiers even use Vce0 that is less than 2*Vrail and they survive.
The maximum voltage across a device will be just less than 2*Vrail and if the rails sag on heavy current draw then even lower.
It is much more important that SOA, both short term and long term, should not be exceeded at operational temperatures.
Most devices that meet this series of SOA limits will comfortably meet the Vce0 limit as well. But, the +10% margin on Vce0 is not a requirement.
It also follows that choosing a device with a very high Vce0 margin, even exceeding +100%, does not ensure that the device will not be overloaded at lower voltages. |
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| big jim |
Hi
I dont want to get in to an argument but any good
text book on amplifier design will tell you that the
power transistors must or should rated atleased
10% higher than the supply
[andrewt ] some amplifiers even use less and they survive.
Could you list a comercial one |
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| Clipped |
anyone want to explain how 'N' channel amps work without a 'P' transistor?
or 'P' ? |
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| teemuk |
"anyone want to explain how 'N' channel amps work without a 'P' transistor?"
Generally, they don't. I assume you refer to "quasi-complementary" topology: The NPN transistor is actually a part of PNP Sziklai pair. Some amplifiers with interstage transformers might not need the other polarity transistor either because the transformer acts as a phase inverter. |
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| Clipped |
ahhh ok...there are a few car amps that ive been wondering about that only use 'n' devices (or so i thought) in the output channel...like rockford fosgate.
what actually happens when the transformer inverts the phase?
and why would a designer choose this topology? |
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| Shaun |
| Assuming a push-pull configuration: the N=channel devices on one rail may operate conventionally, but those on the opposite rail would need the help of an extra transistor stage (phase inverter if you will). This was mentioned by the previous poster (Sziklai pair). |
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| AndrewT |
| quote: | Originally posted by big jim
[andrewt ] some amplifiers even use less and they survive.
Could you list a commercial one? |
I have have attached an SOAR plot of a modified Sugden P128.
It shows the Vce for various loads.
The original used 2 pair of 2sj81/k225 rated at 120Vds0.
The supply rails run @ about +-70Vdc when biased to 150mA and rise to over +-80Vdc when unbiased, transformer regulation is atrocious. These voltages are 6% higher when the mains supply is @ 254Vac.
I realise these are FETs, but the important point to extract from the graph is the highest Vds is only just over 100Vdc. Similar measurements/modelling would come from a BJT stage. I believe that many manufacturers are aware of this and know that devices do not usually breakdown due to high Vce. Again look at the loadlines closest to the SOAR limits. Vds of about 60 to 70V is the problem area for reactive loads i.e. near the Vrail voltage when high currents can be passing. But, even lower critical Vds/Vce for lower value loads.
I do not have any other examples to hand but I have seen other schematics with similar Vce0 and high supply rails, they are not that uncommon. Even so it's not something I would advocate, but then I now design for SOAR having been enlightened by others on this Forum.
ps.
this FET amp is sold as an 8ohm and 4ohm capable design that is generally reliable from a reputable manufacturer. I have looked at the 2pair SOAR and I would never ask it to work into 4r0 (resistive) and yet it has been tested by the press in it's day into low loads and no reports of failure were declared. As a 4pair, I consider it to be a good 6ohm capable amplifier that could drive 4 to 8ohm as well as 8ohm. |
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| tomahauk |
| amp with only n-mos works similar to tube amp |
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| Shaun |
Transistor Vce0 is not only selected based on the power supply rail voltages, but can also be selected based on the maximum theoretical voltage that can appear across them. The latter can be significantly lower than the supply rails.
Whereas some manufacturers may choose to cut it fine (using under-spec'ed parts), as a DIYer one has the choice to build a circuit that is inherently more reliable. Why would one consciously choose to compromise the quality of a design you have full control over? |
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| AndrewT |
| quote: | Originally posted by Shaun
Transistor Vce0 is not only selected based on the power supply rail voltages, but can also be selected based on the maximum theoretical voltage that can appear across them. The latter can be significantly lower than the supply rails.
Whereas some manufacturers may choose to cut it fine (using under-spec'ed parts), as a DIYer one has the choice to build a circuit that is inherently more reliable. Why would one consciously choose to compromise the quality of a design you have full control over? |
Hi Shaun,
I completely agree with the sentiment in your posting. Why compromise reliability when it could be very easy to avoid the riskof unreliability?
| quote: | | (underspecified Vce0)...it's not something I would advocate |
but, using Vce0 as one of the important selection criteria does NOT guarantee reliability.| quote: | | I now design for SOAR |
SOAR design demands much more from the performance of the semiconductors and virtually guarantees that the Vce0 requirement is met.
Put this another way, there will be few if any transistors that pass the SOAR requirement and yet fail the Vce0 requirement.
I find that looking at current ability at Vce=Vrail voltage gives much more useful design information than selecting by Vce0 ever does. |
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| janusz |
Hi Tomahauk,
I'd be very much interested in getting those ebooks on amp design as well.
my email is: janusz.pradzynski@dpi.wa.gov.au
Thanks,
cheers / zdrowko |
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