EMRFD Message Archive 1738
Message Date From Subject 1738 2008-06-23 08:42:52 bkopski A TRANSFORMER-LESS AND FEEDBACK-LESS WIDEBAND AMPLIFIER Here is a simple, good performing wideband amplifier stage intended
ultimately for cascading. The ugly constructed amp is shown in a
PHOTO. The schematic is in an accompanying FILE and the GAIN and RL
behavior over frequency is shown in a posted PHOTO.
This circuit permitted a good opportunity to experiment with a
peaking inductor in the collector of an amplifier transistor. The
trial and error value (~0.26 uH) was arrived at experimentally while
watching live swept performance as in the PHOTO. The result is a BW
> 200 mHz. as seen in the magenta and orange overlay traces in thePHOTO.
The performance traces were obtained using an aging Wavetek sweep
generator and the QST / QEX AD8307 power meter as the detector with
the live data being displayed and then captured on an inexpensive
($325) OWON 5022S DSO. The latter USB interfaces with a pc which
is how I'm able to share the screen capture.
In that this is an amplifier approach that does not employ feedback,
the circuit displays relatively good input and output return loss
performance each of which is relatively little affected by the other
termination. Admittedly, I'm not sure just how good my HB bridge is
over the full 200 mHz sweep range but I am confident of the gain
data. My SA is limited to 100 mHz but the displayed output
distortion products are about -50 dBc at an output level of +3 dBm
within that display range.
The plastic PN5179's were definitely the best performers among all
the common HR/HB transistor choices I have and they are cheap!
(Try Mouser cat. # 512-PN5179 @ $0.07 in quantity of 10.) These
are "hot" performers and in fact the 10 ohm resistors in the
transistor bases are necessary to prevent amplifier VHF/UHF
1745 2008-06-24 10:56:00 ve7ca2 Re: A TRANSFORMER-LESS AND FEEDBACK-LESS WIDEBAND AMPLIFIER Hi Bob:
Very interesting work that you are doing with WB amplifiers. I
developed a single stage 2N5179 feedback amplifier based Wes's cct
with the additi
1746 2008-06-24 11:58:31 bkopski Re: A TRANSFORMER-LESS AND FEEDBACK-LESS WIDEBAND AMPLIFIER Hi Markus and Group,
I've just re-checked - yes - the photos are indeed posted - as is the
circuit .pdf file. The 3 photos I refer to can be found under K3NHI
in the Photos area. They include the amplifier assembly, the swept
gain, and the swept return losses.
You are right that an inductor in the feedback path of Wes' circuit
will cause a gain boost with frequency - up to some point. I have
done this and Wes has also commented on this to me - reporting his
simulation of it. I have found both in the FBA case and in the
feedbackless amplifier that an inductance value can be found that
renders an essentially flat gain much past where roll off begins
without an inductor. This is illustrated in one of the photos of
mine. "Too much" inductor will result in a peaking effect and this
can be many dB. All this is best observed in a swept system and
while point by point data will show the same thing, it is a bit
tedious to say the least! I have also found that the RL's can vary
wildly with an inductor in the FBA case. In fact, this is one
driving reason why I came up with this new amplifier - I need fairly
good RL's and minimal interacti
1747 2008-06-25 12:03:11 ve7ca2 Re: A TRANSFORMER-LESS AND FEEDBACK-LESS WIDEBAND AMPLIFIER Thank you Bob for your further comments. I found the photo's and files.
I have put on my "to-do list" to build up your cct and check it out.
Good work, and again, thanks for sharing your experimentations.