EMRFD Message Archive 1835

Message Date From Subject
1835 2008-07-26 21:15:11 yo8can Push pull amplifier
Try also using BD139(137, or 135) transistors, I've used one to a
radiotelephone project operating at 28MHz and it worked well (1W/BD)
ar 12Vdc. I've tesetd also push pull schemes using BD transistors in B
or AB calss for SSB. In oder to get near flat gain in the 1,8-29MHz
range, an individual simple selective attack & feedback circuitry is
needed for each BD. It worked well by me attacking each base through a
560pF capacitor in parallel with a 22 ohm resistor, and the feedback
is closed between each collector and base being made by a 560...820ohm
resistor (equal for each BD)in series with a 4,7nF dc blocking
capacitor. To rise the gain more at high frequencies, an inductor is
required in series with each feedback resistor, consisting in 23-30
turns (equal to each BD) of 0,2mm dia enameled Cu wire wounded on a
4mm dia. 2,2k/0,5W resistor body which also shunts the coil in order
to prevent instability. To get more power, the high enough impedances
of BD allows dirrect parallel connecting of more BD's, only each
emitter should be individually connected to ground through a 1 ohm or
less balast resistor. The Collectors and Bases may be in parallel
connected. The common attack & feedback circuitry must then be
addapted, dividing the rezistor values and multiplying the capacitor
values also. Use BD's with low H21e, high values for h21e don't work
better in HF, even worse. Try using pre-matched transistors like:
BD139-06 or BD139-10 (meaning h21e is 40...65 or 65...130), not
BD139-16 or unspecified h21e, (which may be also 250, works bad).
I tried this because here in YO genuine 2SC2166 are very hard to find
and the fake ones don't work in HF.
Romi, YO8CAN
1840 2008-07-27 11:35:07 yo8can Re: Push pull amplifier
I wold like to write something more about using cheap bipolar
transistors in HF power linear (SSB) amplifiers. A special designed,
expensive transistor for HF linear power amplification, it keeps its
H21e constant in a wide Ic variation, up to very high collector
currents. Cheap bipolar don't, meaning that in order to preserve
linearity and to avoid splatter generation, the power must be
carefully limited less then they could deliver in non-linear
applications, like in class C (for FM). By means, a BD139 can be
drived up to develope 0,5 A in collector in class C, but because its
H21E drops fast when Ic rises more than o,3A, in SSB applications, the
peak collector current should be no more than this value, meaning that
the average collector current should not be over 0,15A increased. That
means that we should not claim more than 12Vx0,15A=1,8W input per BD,
when supplied at 12Vdc, meaning the output power in AB clss is limited
around only 1,8 x 0,33= 0,6W output for a single BD. So when using
transistors like those ( by means 2N2218) we should first study the
diagrams in the data sheets (H21e versus Ic) to avoid nonlinearity in
SSB applications.

1841 2008-07-27 11:35:08 yo8can Re: Push pull amplifier
An other possibility to build a cheap transistor PA is to use 2SA1141
transistors. They are pnp, not expensive, and the data sheet
specifies Ft=90MHz, Ic=10A at maximum 100Vdc or so. I've tried a pair
in some test push pull scheme, and they worked well enough at the
lower HF bands and I think they will work up to 14MHz. I could not
test them at "full power" only at 4 W because I hadn't enough drive
power there. The power gain was somewhere around 12dB (to get 4W
output was necessary to drive them with 1W). The hardware I've used
was not the best, there where significant losses in the driving
circuitry, so I think that they may work better. The transistors I've
had weren't genuine, even on them is written "NEC", they are fake made
in China or so, but they cost about 1Euro/piece. Genuine ones I
couldn't get here, they worked much better then the 2SC2166, also
fakes. The signal looked good at the oscilloscope, much better than
using IRF's, which distort badly the sinusoid without LPF and are
unstable. This thing dishearten me using those MOSFET's and trying
bipolars, like those or more BD139's.
The base driving circuit must be as short as possible, the driving
transformer must be as close as possible to the bases, in
"compensation", the collector circuit may be a little "longer" without
significant power loss. In order to get maximal gain the emitters must
be wery good RF grounded, else gain losses will appear. A wide band
1842 2008-07-28 05:05:09 Allison Parent Re: Push pull amplifier