Hi,

Recently I mentioned to Roy that a transistor can be used as rectifier
if its B-E voltage breakdown was high enough to withstand it. Greg
Mayman remarked that it needed a squarewave drive at the bases to make
it work. I tried working out some variations but the simplest I found
was to have a transformer with extra taps. Here's an example:

:                     .----Rb1---.
:                  T1 |          |
:                     (         B|
:                  || (         ---
:          L1 o--. || (________/   \__
:                | || (       C     E |
:        primary ) || (          Q1   |
:                ) || (___            |____.____o +DC out.
:                ) || (  _|_          |   _|_
:          L2 o__| || (  ///     Q2   |   ~|~ C1
:                  || (_______      __|   _|_
:                  || (      C \   /E     ///
:                  || (         ---
:                     (        B |
:                     |          |
:                     `----Rb2---'

The circuit is basically a full-wave rectifier and it works this way,
imagine the Q1 transistor is receiving a positive voltage at its
collector, then the base receives an even higher voltage with its
current limited by resistor Rb.

The base is forward biased and collector current will flow to the
emitter and charges up the filter capacitor. When the collector voltage
goes negative the base voltage is even more negative so that the
transistor won't turn-ON in reverse and the capacitor keeps its charge.

The operation is the inverse for the Q2 transistor so that the circuit
will rectify both half waves with a very low voltage loss which is the
transistor C-E saturation voltage of about 0.1 volts. More transistors
could be paralleled to lower VCEsat for higher currents. Perhaps this
circuit could be adapted to FET transistors for an even lower loss.

Alternatively instead of a transformer with extra taps, a second smaller
transformer with a higher voltage could be used to drive the bases.
Comments welcomed! 

 Mike
 ****

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