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

The ft of the '2222 is 250 to 300 MHz.? How could that account for gain slump at 30 MHz?

Dave - W?LEV

On Fri, Jul 12, 2024 at 9:23?PM Steve Ratzlaff via <ratzlaffsteve=[email protected]> wrote:

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