Hi, I want to buy some non inductive emitter resistor for a power amplifier project. I've seen value from .1 Ohm to 1 Ohm. Is there a rule to choose this value. .22 Ohm and .33 Ohm look like to be a very standard value. But why some use .1 Ohm and others use 1 Ohm. The value I'm looking are for an approx. 200w power amp.
Thanks.
Thanks.
Hi,
based on 8pairs into 8ohm then I think 0r39 would work.
A highish value to force some balancing action between the 8pairs.
When all eight are paralleled then the effective emitter reisistor is about 0r05 in each half.
I think you will find that a single pair of emitter resistors are in the range 0r0 to 0r33. When paralleling the outputs you find that the designer increases the values as the pairs multiply.
250W from +-65V supply rails might not happen. Even 200W from PSU rails at that voltage will require a very stiff supply.
based on 8pairs into 8ohm then I think 0r39 would work.
A highish value to force some balancing action between the 8pairs.
When all eight are paralleled then the effective emitter reisistor is about 0r05 in each half.
I think you will find that a single pair of emitter resistors are in the range 0r0 to 0r33. When paralleling the outputs you find that the designer increases the values as the pairs multiply.
250W from +-65V supply rails might not happen. Even 200W from PSU rails at that voltage will require a very stiff supply.
Hi, thank you very much. I can buy those on ebay
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7575512230&rd=1&sspagename=STRK:MEWA:IT&rd=1
but they are .47 Ohm, can't find any .39 Ohm
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7575512230&rd=1&sspagename=STRK:MEWA:IT&rd=1
but they are .47 Ohm, can't find any .39 Ohm
Hi,
A good read of Doug Selfs "Power Amplifier Design" is recommended.
Basically it depends on the accuracy and stability of the biasing,
but .22 is a good value for most designs. 0.1 is better but only
with a good sized heatsink and accurate biasing such that thermal
runaway is not an issue.
As AT says generally multiple output pairs the values are typically
increased to end up with the same effective emitter resistance,
though theorectically I can't see why this is done.
(Possibly there are stability issues, that is as you add pairs to
the output stage current gain is increased, which is pegged back
by increasing the emitter resistances, but then again 4 output
pairs are going to need an additional pre-driver stage, which
complicates stability issues no end, compared to no pre-driver.)
With 4 pairs and 0.47 you will end up with an effective resistance
of 0.12, so there is no reason at all why it won't work well, especially
if the pairs are there to dissapate power and swing decent current
into decent impedance loads. If the pairs are there in an attempt
to swing very high currents into low impedance loads then possibly
0.22 would work better.
/sreten.
A good read of Doug Selfs "Power Amplifier Design" is recommended.
Basically it depends on the accuracy and stability of the biasing,
but .22 is a good value for most designs. 0.1 is better but only
with a good sized heatsink and accurate biasing such that thermal
runaway is not an issue.
As AT says generally multiple output pairs the values are typically
increased to end up with the same effective emitter resistance,
though theorectically I can't see why this is done.
(Possibly there are stability issues, that is as you add pairs to
the output stage current gain is increased, which is pegged back
by increasing the emitter resistances, but then again 4 output
pairs are going to need an additional pre-driver stage, which
complicates stability issues no end, compared to no pre-driver.)
With 4 pairs and 0.47 you will end up with an effective resistance
of 0.12, so there is no reason at all why it won't work well, especially
if the pairs are there to dissapate power and swing decent current
into decent impedance loads. If the pairs are there in an attempt
to swing very high currents into low impedance loads then possibly
0.22 would work better.
/sreten.Hi,
just had a look at your Ebay link.
They are described as non inductive, but they look like conventional wire wound (WW) to me.
It is possible they have contra wound halves to cancel the inductance, but I think this would be unusual for this type.
How many output pairs in each of your comparison amps?
just had a look at your Ebay link.
They are described as non inductive, but they look like conventional wire wound (WW) to me.
It is possible they have contra wound halves to cancel the inductance, but I think this would be unusual for this type.
How many output pairs in each of your comparison amps?
There is a very simple rule for that. Voltage on this resistor shold be between 0.5 up to 0,75 V for the maximum current that goes through this resistor.
R=U/I (Ohm)
P=U*I (W)
For example:
I=2A and U = 0.66V
---> R=0,66/2=0,33 Ohm
---> P=0,66*2= 1,32W
In this case you can use the following resistor --> 0,33 Ohm / 2W
R=U/I (Ohm)
P=U*I (W)
For example:
I=2A and U = 0.66V
---> R=0,66/2=0,33 Ohm
---> P=0,66*2= 1,32W
In this case you can use the following resistor --> 0,33 Ohm / 2W
3rd harmonic cancellation by emitter degeneration is well known for low power circuits, but will only work in class A and most probably not with power transistors. BTW: the correct value is 13mV/Ie (e.g. 0R26 for Ie=50mA).
E.g. Fig.10 on p.323 in
W. Sansen, Distortion in elementary transistor circuits, IEEE Trans. Circuits Syst. II, vol. 46, pp. 315--325, Mar. 1999.
But I would also be interested in reading the patent.
Cheers,
Gerhard
E.g. Fig.10 on p.323 in
W. Sansen, Distortion in elementary transistor circuits, IEEE Trans. Circuits Syst. II, vol. 46, pp. 315--325, Mar. 1999.
But I would also be interested in reading the patent.
Cheers,
Gerhard
This question pops up regularly...
There is an optimum voltage across the emitter resistors, so the resistor value depends on the chosen bias current (or the other way around).
http://www.diyaudio.com/forums/showthread.php?postid=247900#post247900
ostie01 said:
There is an optimum voltage across the emitter resistors, so the resistor value depends on the chosen bias current (or the other way around).
http://www.diyaudio.com/forums/showthread.php?postid=247900#post247900
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