Re: Groundplane in poweramplifier PCB design

Wiebe Cazemier wrote:
On Monday 17 April 2006 04:27, Ken Smith wrote:


This is the amp I'm making BTW:
http://sound.westhost.com/project3a.htm. Perhaps you can describe it
in that context.

Wiebe, I just had a short look at the webpage. Even if the author says it is
an excellent amp, it doesn't necessarily need to be so. This construction
invites any instability and there are a few very weak points.
To bring good performance, the input pair has to be operated with equal
currents, which is almost impossible here. There will always be some offset
voltage at the output. When you trimm it away it will be again there when
the temperature has changed. This is due to the current source made with Q4.
which relies only on its Vbe. It is also dependent on the beta of the
transistor and the forward voltage of the green LED.
C3 will be always operated with changing polarity, this calls for trouble
too.
The worst is when there is an overload on the input and all the current goes
through Q1 and saturates Q4, especially without a load. The output goes high
and C5 will be pulled down by R10. Thus the current through Q9 increases,
and the voltage drop across it. The moment Q6 starts conducting it turns on
Q8 and the current will increase until the fuses blow, but by then it is too
late. No current limit. In this kind of topology it is impossible to
incorporate a simple current limit, because when you pull down the base in
the usual way, the other side starts conducting and shorts the supply rails.
So it is not the gnd plane, it is the principle of operation which results
in self-destruction, whatever you do.
There are many more details that can be improved, but IMHO it wouldn't make
sense.

--
ciao Ban
Apricale, Italy


.

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