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Old 27th January 2015, 11:36 PM   #1
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Join Date: Aug 2013
Location: Munich
Default questions on simple mosfet regulator

Hello to all,
this is my first post at this forum, in the hope that the questions I have
are a piece of cake for you experts..

Sorry for my English in advance..

Before actually posting my questions I should possibly point out
that I'm a newbie to electronics..
The way I build things and have fun is playing around with simulations,
then possibly build the thing (hoping there's not too much smoke on power on),
listen to the thing (if possible) and again messing around until it sounds right
(or smokes again..).

Some things developed this way sound (to my ears) surprisingly good,
although I often learn later that e.g. in an amplifier normally one might apply some gnfb...

Anyway, this time I'm scratching my head for several days, but I still have the strong feeling there's something wrong about the voltage regulator.


The simulation results are quite good, most important to me has been the psr.

Click the image to open in full size.

(lower graph is the psr, upper phase)
The phase also includes the input filter L1/C1.

In the simulation this works quite well.

But...
The regulation seems to be very dependent on the exact values of R4..R7;
also minor changes in the temperature as well as the output current let the psr drop to 100dB or less.

This might be somehow theoretical, and in reality these values not be reached at all.

Still I'd like to understand what's going on.

So my guess would be, that I'd need to apply some feedback to Q1/Q2 in order to stabilize the gain of this amplifier unit, making it independent of minor changes in hfe as well as the values of R4..R7 ?

Anyway, at this point I'm lost: Q1 as well as Q2 do seem to not only amplify the changes in V(out).. anyway, I'm lost in calculating the amplification.
Would be glad if someone could help me out ?

Also would be glad if some could explain to me my circuit ,
especially what Q1/Q2 do besides amplifying the voltage changes of V(out)...
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