| Centauri |
That is basically one of the most common Mosfet configurations around, and does work quite well. Thermal compensation isn't required as Mosfets have a negative temperature coefficient, meaning the current reduces as heat increases thereby reducing the heat.
Cheers
Graeme |
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| B.I.G |
| i thought only lateral mos-fet`s have negative temperature coefficient :xeye: |
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| Centauri |
According to IRFP240 datasheet, "on" resistance increases with increasing temperature, so therefore more heat less current.
Cheers
Graeme |
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| Mr Evil |
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. |
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| jam |
The output stage works quite well, but if I were using thr IR devices I would replace the trimpot wuth a VGS multiplier.
Jam |
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| Upupa Epops |
| It will be work well only with lateral devices :cool: . |
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| Workhorse |
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 |
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| BobEllis |
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/show...6108#post576108
| quote: | Originally posted by Nelson Pass
[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.
:cool: |
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| Leolabs |
| To any newcomer who want to build this circuitry,be careful with the layout,unless u want to build a power oscillator:bigeyes: |
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| Centauri |
| quote: | Originally posted by Mr Evil
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. |
Yes, you are quite correct - not a good choice of Mosfet.. :(
| quote: | Originally posted by Leolabs
To any newcomer who want to build this circuitry,be careful with the layout,unless u want to build a power oscillator:bigeyes: |
There do appear to be a few issues in both schematic and layout.
Cheers
Graeme |
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| Zero Cool |
| quote: | Originally posted by Centauri
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 |
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| jacco vermeulen |
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. |
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| B.I.G |
| so the layout is ok ? if i build it how can i see if it oscilates without a scope ? |
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| jacco vermeulen |
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 ! |
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| B.I.G |
so how will oscilations look :xeye: i haven`t used a skope before ....
i just got a scope software and it has a sine wave generator :D |
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| BobEllis |
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. |
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| darkfenriz |
source resistors are 0.33
this may be OK
otherwise use a Vbe (or Vgs) multiplier as mentioned before
regards |
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| jacco vermeulen |
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! |
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| SM2GXN |
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
:D
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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| Mr Evil |
Regarding layout: It mostly boils down to knowing that there are stray impedances. Where on a circuit diagram two components are joined by a line, in reality it's actually a resistance and inductance. Also there are capacitances between components. You need to be aware of what effect these will have and design to minimize their effects.
For instance, even a very small inductance in the source or drain of a MOSFET can send it into oscillation, so you must make sure that those tracks are as short as possible. Also place supply bypass capacitors as close to the circuit as possible (there don't seem to be any in the amp posted here, but I would recommend using some anyway, just to be safe), again to minimize the effects of inductance.
Grounding is a major area of concern. You may have a track which you call ground, but since it will have a finite impedance, any current flowing in it will raise its potential above what you would like it to be, which can inject hum and noise into anything grounded there. Using a star-ground and keeping high current and noisy grounds separate helps a lot.
Other things to consider include heat and EMI, ease of construction, fault conditions etc, but whole books could be written on the subject. |
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| SM2GXN |
Thank you for reply Mr Evil.
Think I know what you mean, there is both capacitive and inductive behavior of a pcb and coupling between components on a PCB. I was looking at the pcb of the amp in this thread and stargrounding was used as well as short tracks but no bypass caps on the psu rail. As you said, this is a real big subject and it can not be explaned even in a pile of A4 sheet. I'll try to find some books about how to route pcb:s
Since this thread started with thermal compensation on mosfet amps I will stop here. Anyhow, it would be interesting to post a schematic in a new thread and let some experts make their own layouts and post them as well, just to see the difference.
Bjorn |
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| B.I.G |
| ye it would be a good ideea for some of the gurus to make a layout and see the diferences ... but i don`t know if they have time :rolleyes: |
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| SM2GXN |
Hello B.I.G!
Sometimes I see comments like: this board will not work it will oscillate or be noisy. I know that if you don't have proper knowledge in fundamentals about components involved and what makes inductive and capacitive components on a pcb it will not be an easy task to explane and can not be compressed in a few rows of text.
We could choose any amp maybe two, one with BJTs and one with FETs and just to make it simple enough a small number of components so it can be explaned as simple as possible.
There are lot's of books written on almost any subject wich comes up on DIY so why not pcb design?
Excuse me for repeating my self a little and for being off topic but since the question came...
Am I way out of....?
Bjorn |
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| jacco vermeulen |
Bjorn,
you are so right.
Give us a title, please. |
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| SM2GXN |
Thank you jacco vermeulen!
Sorry for doing this again :xeye: I mean using B.I.Gs thread for another purpose even tough for a short while.
It's hard to come up with something good about the title.
Maybe a title for a new thread would be: PCB routing for audioamps?
It must be clear that it is in no way any sort of competiton.
Bjorn |
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| Mindaugas |
This modification works in spice simulator (ltspice).
It uses Mosfets in second differential stage.
Any comments from more experienced people are welcome. |
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| ezis666 |
I have built those amplifier.Works good.With little change-Mosfets are IRF640/9640, (at 4 ohm-two pairs) T3,4=MJE350, T5,T6=MJE340, T1,T2=2N5401. At V+/- MUST 100n 250V(not in shematic), P1=470R Fixed, power supply-65V.
I can recommend this shematic, it is cheap and works good and stable. here is photo of my PCB variant: |
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| ezis666 |
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