Thanks to Caaarlo for pointing out my long input lead problem.
I've redone the MPS2222 build plus built a second one using
2SC5551. I've deleted the old picture and sweeps from the folder
and put new pictures showing the short input leads and the input
coupling transformer with the shortest leads I could use. I've
added a lot of info to the folder description so be sure to click
on it so it all shows up. Reducing lead length indeed lowers the
input impedance at the 30 MHz high end, but has no effect on loop
amp actual gain. It's interesting that the MPS2222 has a slightly
lower input impedance than the 2SC5551 version. (The same
calibrated-out input transformer was used for both loop amp
sweeps.)
/g/loopantennas/album?id=296351
73,
Steve AA7U
On 7/11/2024 8:14 PM, Caaarlo via
groups.io wrote:
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On Thu, Jul 11, 2024 at 09:41 PM, Steve Ratzlaff wrote:
The
transistors run out of gain-bandwidth product at 30 MHz as you
can see
from the sweep (the gain is rolling off). Caaarlo's loop amp
should have
similar gain since both used the same transistors.
Steve,
Thank you for the compliments.
I have just measured gain as per your request, and mine is
almost flat:
1MHz 33.15dB
7.1MHz 33.51dB
30MHz 33.50dB
At 1MHz I am sure that the lower gain is due to 1uF input
capacitors used, which add some reactance hence reducing input
level, output level and measured gain. Some 10 or 100uF should
lift the gain to the same level as the other frequencies.
Regarding your gain rolling off, it is not a problem of
transistors, but a problem of ugly-build as you just named it.
For being such a low impedance input, those input capacitors,
their legs, and everything in between your generator and the
emitters,? are inserting an important amount of reactance in
series with the actual input of the amplifier: the emitters.
So you basically have a voltage divider consisting of a large
parasitic inductance and the smaller input impedance, reducing
the actual power delivered to the input hence reducing output.
Parasitic inductance is not a problem when load impedances are
much higher, but when they are as low as in LZ1AQ amp, in the
order of 2-4 ohms, they become a serious threat to performance.
A quick calculation:
those extended legs are probably 25mm for each capacitor, total
50mm, that is an inductance of about 52nH
At 1MHz the reactance is 0.3 ohms
At 30MHz it skyrockets to 9 ohms, which is at least twice the
resistive component at the emitters.
I would strongly recommend to shorten all input paths as much as
possible, because what you are experiencing has nothing to do
with the transistors.?
73
