"Frank Gilliland" wrote in message
...
On Sat, 6 Jan 2007 20:35:49 -0500, "Jimmie D"
wrote in
:


"Frank Gilliland" wrote in message
news
On Sat, 6 Jan 2007 15:40:50 -0500, "Jimmie D"
wrote in
:

snip
Two transistors in paralell only have the gain as one.


His transistors are in push-pull, not parallel.




Same thing no more gain. One would have to be driving the other to have
any
more gain, It will just handle more power.


I rushed the last answer because I was in a hurry to get out the door.
Anyway, your statement is true if you are talking only about current
gain. Power gain, OTOH, is double in either configuration.

For example, if a transistor with a current gain of 10 is fed an input
of 100mA the output will be 1A. Two transistors in parallel will split
the input current at 50mA each, with an output of 500mA each or 1A
total. No additional current gain.

But you are forgetting that when two transistors are in parallel the
input and output impedances are reduced by half. If you take the input
signal and convert its impedance to one-fourth of what is fed to a
single transistor you can maintain 100mA input to each transistor
using the same input power, with an output having -twice- the current
at half the impedance of one transistor. The result is double the
power gain.

Brian's amp should be pushing 18dB. If it isn't then he screwed up the
matching of the transistor input and output impedances. That's one
reason why I want to see his schematic.



I must admit I never built a single transistor PA before SO I will have to
look at the numbers again. Granted the push pull amp is more efficent than
the single ended amp but Im still not sure about the gain. Il have to crank
the #s when I dont have a snoot full of cold medicine. Please correct me if
I am wrong but you are saying that two amps biased for say classB operation
that a 2 transistor anp will have 6 db more output than a 1 transistor amp
with the same drive power.

He probably does have an impedance matching problem , almost all amps of
this type do to some degree. The impedance
transformation of the transformers can almost always be improved upon by the
inclusion of an adjustable L or Pi network in the output of the amp.. I know
the last amp I tested had an output impedance of about 35 ohms. A matching
network added to this amp increased power out, reduced harmonics and made
the transistors stop popping.