Re: Class AB common source mosfet power amplifier

On Sun, 11 Nov 2007 18:37:54 +0000, John Devereux
<jdREMOVE@xxxxxxxxxxxxxxxxxx> wrote:

John Larkin <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> writes:

On Thu, 08 Nov 2007 15:37:54 +0000, John Devereux
<jdREMOVE@xxxxxxxxxxxxxxxxxx> wrote:

Fred Bloggs <nospam@xxxxxxxxxx> writes:


OK, I have been simulating away but am now all spiced out!

I am trying to come up with a linear (i.e. not switched) power mosfet
amplifier that works with the output device drains tied together. This
is mainly so they can be bolted to the same heatsink without using
insulators, although an extended voltage swing would also be good.

I.e. generally like this:

. V+ --------------------------------------------
. |
. [R] (optional)
. |
. |<
. ------|| P-Channel
. |-
. Drive |--------------------[LOAD]--->GND
. |-
. ------|| N-Channel
. |>
. |
. |
. [R] (optional)
. |
. V- --------------------------------------------


[...]

Here's the basic idea of a "perfect" class AB signal splitter, similar
to what Win posted, and sort of like what I use in my ultralinear NMR
gradient drivers. It really has zero crossover distortion, and
quiescent bias is independently adjustable from splitter gain.

http://img85.imageshack.us/my.php?image=psplitay8.jpg


The idea of taking signals from an opamp's power supply pins has been
around for decades.

Sorry for not replying till now - I wanted to simulate it.

It seems to work embarassingly well for resistive and capacitive
loads, while inductive loads make it oscillate. Slowing it down,
e.g. with capacitors across the opamp current sensing resistors, can
make it stable again. I will play with it some more.

Are the diodes for supply sequencing?


Yup. Keeps the transistors from frying if the HV supplies aren't up. I
like to add a couple of led's in those paths, too, just for fun.

If you want to close an overall loop on output current, inductive
loads (the kind I get!) can resonate and wreck loop stability. So loop
dynamics, and maybe some load damping, have to be tweaked.

John


.

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