[sdiy] simple class A bipolar power amps?

[René Schmitz]

Hi Anthony, Harry and all,

Harry Bissell Jr wrote:
> MOSFETS would have the advantage of ability to be
> paralleled and have inherent current sharing without
> emitter resistors (necessary waste of power...).
> Beside
> thermal runaway, they would deliver less current as
> they get gotter so sharing is enhanced.

In a class A care must be taken still. MOSFETs have a
region where they run away, theres a certain Id where
the Id starts to increase with temperature. In a class
AB or B power amp you might get away with setting your
quiescent current below that.

> Often overlooked is that although they do not need
> static current to drive the gates (as bipolar need for
> the base)... but they need really high peak currents
> for the gate capacitance...

Indeed. Its worse even with tubes, particularly triodes.
These will have a dynamic input capacitance which is way
(mu times) higher than the static capacitance.

With regard to the actual problem of a class A bipolar:
The simplest would be to use an emitter follower, and bias
its emitter to Ub/2. You need to use a low emitter resistor
capable of dissipating as much power as the transistor....

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159