would this circuit work ? i see no thermal compesation ..... so wouldnt it blow the output devices ? i see the author recomending irfp240/9240 that are not lateral devices..... 
http://users.swing.be/edwinpaij/ampli_mosfet_simple.htm
http://users.swing.be/edwinpaij/ampli_mosfet_simple.htm
But that's resistance when fully turned on. Under quiescent conditions they exhibit a positive temperature coefficient. You can see this in the graph of Vgs vs Id, which shows zero tempco at about 15A, positive below, negative above.
You can get away with no thermal compensation if the devices are very well cooled and the source resistors are large, but I wouldn't be happy doing it myself, especially with only a resistor to set the bias.
You can get away with no thermal compensation if the devices are very well cooled and the source resistors are large, but I wouldn't be happy doing it myself, especially with only a resistor to set the bias.
Reply
Hi everybody,
I think there is need for something little more on switching vertical mosfets.
These mosfets do need thermal biasing compensation only when there is a high bias [Id~100-200mA]is implemented , but when a low biasing [Id~10-20mA]is used there is no need for any thermal compensation.
Similarly when the mosfets are subjected to heavy loads i.e. large current drawing loads, their RDS starts increasing due to the phenomena of positive temp. coeff. and the current starts decreasing which ease in paralleling and current sharing.
Secondly , the effect of applied rail voltage is very evident in terms of Vgs~Ids criteria.
regards,
kanwar
Hi everybody,
I think there is need for something little more on switching vertical mosfets.
These mosfets do need thermal biasing compensation only when there is a high bias [Id~100-200mA]is implemented , but when a low biasing [Id~10-20mA]is used there is no need for any thermal compensation.
Similarly when the mosfets are subjected to heavy loads i.e. large current drawing loads, their RDS starts increasing due to the phenomena of positive temp. coeff. and the current starts decreasing which ease in paralleling and current sharing.
Secondly , the effect of applied rail voltage is very evident in terms of Vgs~Ids criteria.
regards,
kanwar
workhorse, I think it's the other way around - temperature coeficient is negative at small values of Ids, but once a certain threshold is reached, it becomes positive. Lightly biased MOSFET stages need thermal compensation, but once the bias gets high enough they so not.
From this thread: http://www.diyaudio.com/forums/showthread.php?postid=576108#post576108
From this thread: http://www.diyaudio.com/forums/showthread.php?postid=576108#post576108
Nelson Pass said:[snip]
In Class A with a good sized bias, the temperature coefficient
gets less, and as the current increases it swings the other
way. This point occurs at a value somewhat higher than where
Class A amplifiers are usually biased, but you can use much
smaller Source resistance as the current goes up, not only
from less temperature coefficient, but also because the resistive
voltage drop of the resistor becomes a larger part of the bias
voltage and is linear.
Centauri said:
Yes, you are quite correct - not a good choice of Mosfet..
There do appear to be a few issues in both schematic and layout.
Cheers
Graeme
the old Hitachi 2SK134/135/J49/J50 would work well here.
Please exapnd as to what is bad with the layout? I am new at making my own circuit boards and im always looking for tips.
Zc
I have the idea that Edwin Paij uses established circuits for his homepage.
The ones i saw when i looked at his site were familiar ones, think i saw an Anthony Holton then.
The circuit you posted originally used BUZ's in my opinion, or Hitachi's like ZC mentioned.
Elektor stuff, even the PCB layout looks Elektor signature.
Others on diy with more authority than me may confirm that, such as Hugo.
The ones i saw when i looked at his site were familiar ones, think i saw an Anthony Holton then.
The circuit you posted originally used BUZ's in my opinion, or Hitachi's like ZC mentioned.
Elektor stuff, even the PCB layout looks Elektor signature.
Others on diy with more authority than me may confirm that, such as Hugo.
You can download single channel scope software from a guy in Russia. With Google it is easy to find, many audio people have download links on their homepage.
Build a signal generator and connect it to the entrance of the amp.
Sinus generator circuits are plenty around too, from 50 cent stuff to complex.
Feed the output signal through a high resistance resistor to the input of your computer sound card, the scope program shows the signal.
For the real thing you will need a scope !
Build a signal generator and connect it to the entrance of the amp.
Sinus generator circuits are plenty around too, from 50 cent stuff to complex.
Feed the output signal through a high resistance resistor to the input of your computer sound card, the scope program shows the signal.
For the real thing you will need a scope !
Using your sound card input directly you probably won't see any oscillation, since the frequency response of most sound cards isn't high enough.
With a scope oscillation will look like a signal at a (usually high) frequency that you are not driving the amp with. It may be large, it may just make your sine wave signal look squiggly.
With a scope oscillation will look like a signal at a (usually high) frequency that you are not driving the amp with. It may be large, it may just make your sine wave signal look squiggly.
Bob is right,
amplifiers oscillate at (very) high frequencies.
To fully test an amplifier you need an oscilliscope.
The kind of amplifier you posted will not do a mHz signal, a 20 mHz scope is not needed .
Secondhand single-channel scopes that can test an amplifier circuit up to a few 100 kHz you can find on the web for less than 50 Euro. Sometimes for 25, with probes, and manual.
Scope software with a sinus generator is good enough for checking the output signal to see if its still a smooth looking sine wave, but it is freeware.
Be very very carefull with attaching output voltage on soundcards!
amplifiers oscillate at (very) high frequencies.
To fully test an amplifier you need an oscilliscope.
The kind of amplifier you posted will not do a mHz signal, a 20 mHz scope is not needed .
Secondhand single-channel scopes that can test an amplifier circuit up to a few 100 kHz you can find on the web for less than 50 Euro. Sometimes for 25, with probes, and manual.
Scope software with a sinus generator is good enough for checking the output signal to see if its still a smooth looking sine wave, but it is freeware.
Be very very carefull with attaching output voltage on soundcards!
Hi all!
I'm a bit curious about what Leolabs said about being careful with the layout of the pcb.
I've made i big number of pcb:s and luckily not any of them turned into oscillation.
I'm not educated in any type of electronic cad soft so probably many things I've done is more or less close to total disaster
My question might be a start to a new thread?
What should I be concerned about when routing a board?
Let us take the amplifier in this thread as an example.
I do realize that the answer is not simple.
I'm new to diyaudio and this is my first post.
Bjorn
I'm a bit curious about what Leolabs said about being careful with the layout of the pcb.
I've made i big number of pcb:s and luckily not any of them turned into oscillation.
I'm not educated in any type of electronic cad soft so probably many things I've done is more or less close to total disaster
My question might be a start to a new thread?
What should I be concerned about when routing a board?
Let us take the amplifier in this thread as an example.
I do realize that the answer is not simple.
I'm new to diyaudio and this is my first post.
Bjorn
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