I never did figure out whether the root cause of failure was oscillation or thermal. None lasted long enough :-( After the second failure on the tower, I got tired of climbing and roasted the next 2 on the bench. I ran all at 5v; slow learner, I guess. Boy, did they get hot. Go with the 3.3v through the speced 22 ohm resistor. Only .165w versus .5w and the performance difference is trivial. BTW I did the bias-T thing with these new ones. No detectable gain difference.
Ed Krome K9EK
Sent from my iPhone
On Dec 8, 2020, at 7:33 PM, Zach Metzinger [email protected] wrote:
On 12/08/20 17:35, Ed Krome wrote:
I’ve built a couple of PGA103 preamps using W6PQL board. They work great Sure hard to beat for performance/$. Follow it with a band pass filter if necessary. FWIW there is an optional “stabilizing” circuit shown in his schematic. I highly recommend it. I also run them on 3.3v; hardly any performance difference but they sure run cooler at 50ma vs. 100ma Several years ago, I built four PGA103 amps on my own boards without that stabilization circuit, running 5v@100ma. Heatsinking wasn’t good. Every one died. Round file. But these new ones at lower voltage on a board with much better heat dissipation and the input circuitry are doing just fine. I haven’t done noise figure characterization yet. Gain (at 3.3v) is ~20db.
Good to know! I have a board for a custom LNA which has coax-powered bias and relays that default to "through" when +12 isn't on the coax.
My 70cm LNA uses this board and the MAX2640 LNA, which requires an input bandpass to knock down the cellular and FM stations. That's followed by a 435 MHz SAW filter, then a MAR-3+ gain block, and finally dual anti-parallel PIN diodes (BAR64) to kill the RF from the rig during TX before the relay switches over. Without the PIN diodes, I killed one SAW filter (before adding gain block) and one MAR-3+ during full break-in CW operation.
I plan to put the PGA103 on this same board, and I'll include the stabilization circuit. Might fire it up on the bench w/o and see if it breaks into oscillation.
--- Zach N0ZGO
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