Re: Groundplane in poweramplifier PCB design
- From: Wiebe Cazemier <halfgaar@xxxxxxx>
- Date: Mon, 17 Apr 2006 12:06:22 +0200
On Monday 17 April 2006 04:27, Ken Smith wrote:
Don't forget the thermal stabilization for the bias. The device(s) used
for this need to be at the same temperature as the power transistors.
Sudden changes in the output power can lead to sudden changes in device
temperatures so those components need to be on the heat sink if you aren't
using PCB copper to conduct the heat.
It's not a darlington pair setup, it's a compound pair. In such a setup, the
bias servo needs to be in thermal contact with one of the driver transistors
(not the class-a driver of course). If you connect it to the main heatsink,
the quiscent current will only get higher as the temperature increases.
Will it matter (much) BTW if that track has a gap in it?
Yes, it matters about any gap. Think of it this way:
When a magnetic line force passes through a conductor it makes a tiny
voltage right at the part it goes through. Any current that this voltage
may cause is always such that it opposes motion of that line of force.
Now imagine a ring of copper, with 101 lines of force going down through
the center of it. If any of these lines of force try to get away, they
will cause a current in the copper ring that slows their departure. If
you try to slip another one in, a current will be created to slow that
down too.
The result of this is that the magnetic field can't change as quickly as
otherwise. This is how the copper reduces the high frequency changes in
the field.
OK, I'll see if I can make some adjustments to make the circle complete.
What do you mean, the two sides of the speaker? The voltage feedback for the
long tailed pair input stage is taken care of on the PCB, if that's what you
mean.
Ideally, the power amplifier puts a controlled voltage onto the speaker.
In real life, you are putting the voltage onto the terminals the speakers
get wired to.
Lets say, your power amplifier has a pair of connections for the input
signal on one side and another pair for the speaker on the other. (This
would be a simple mono amp) You want the voltage on the speaker to be
some number times the voltage on the input. You don't want any of the
voltage drops in the internal wiring to get into the picture. Using an
op-amp symbol in some ASCII art, I think makes the idea clear:
R1 R2
(in-) -+--/\/\/----+---/\/\/----
! ! !
[?] ! !
! ---!-\ !
GND ! >-----+-- (Out+)
---!+/
!
!
(in+) ---/\/\------+-----/\/\---+-- (Out-)
R1 R2 !
[?]
!
GND
The two mystery components represent the wiring. If the R1's are the same
and the R2's are the same, a voltage drop in the wiring won't change what
gets applied to the speaker.
I'm still not sure what you mean. The only voltage feedback this amp has, as
far as I know, is that it's output is fed back to the long tailed pair through
a resistor.
This is the amp I'm making BTW: http://sound.westhost.com/project3a.htm.
Perhaps you can describe it in that context.
.
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