开云体育

Testing note..

How I arrive at the issue of gain vs frequency.

One was on paper using the well know EMRD feedback amp spreadsheet.
Every try said not possible unless the device has a FT of greater
than 600mhz THe simple test is we need a gain of 39 (~16db at 3mhz)
and at 30mhz we need a device Ft of not less than 1170mhz based on
the Ft/F rule of thumb.? We can however get away with a bit less with
only a small decrease in gain.

The other was a mod to the board and measure:
I removed the IRF510s and added a trans former to go from the gate
connection to 50ohms (1:1), added 10db 1W pad for safety (protect the
Rigol). and to keep the mosfet out of the test.

Lifted C1 to isolate the TX amp from the LF and hooked a piece of
coax with a.1uf cap at the and to the input of Q90.??
RV1 set to 100% so we see total stage gain.
Enabled TX mode with out Raduino, jumper T/R to 12V..
Set up RIgol DSA 815Twith tracking generator for -20dbm output from 3
to 30 mhz. I didn't do a screen capture though I should have.
( I used lower drive as I'd seen gain compression in driver and
pre-driver at higher output levels at higher frequencies. this is
device HFE related at higher currents. this makes the system look
worse.)

The result was about 45DB gain at 3mhz falling to 34db at 30mhz. A
drop of nearly 10db. The slope of the curve was such that at about
7mhz we were already dropping by 2db. Power out of the driver for
that setup was .255 Watt at 3mhz falling to .023W at 30. We could get
more power by cranking up the trancking generator output at lower
frequencies but gain compression at higher power levels limited us
at?14mhz and above.

Adding emitter caps and inductors to the feedback helped but the
result was still well short.? The net effect is the curve was flatter
to about 13mhz then resumed sloping down.

Why is this.? The 2n3904 has a FT of 300 (nominal) and the rule of
thumb is ft/f=Beta BEta is the maximum attainable gain for the
stage.? So at 3mhz the result is 100, lots of gain and its tamped
down with feedback to 39 (~16db).? ? At 30mhz the result is 10! 10 is
10DB of gain...? ?Paralleling transistors is supposed to help that
but the result is typically a 3db improvement at higher frequencies.
So for 4 stages at approximately 16db per stage at 3mhz showed?48db
(Not allowing for transformer losses) but close to the result of 45
DB gain.? ?However at 3mhz we are supposed to get 30db and did get 33
so we are doing better than predicted?but well down from 3mhz.

Adding the various caps and inductors in the feedback to the q90,
pre-driver, driver made the curve flatter but at 14mhz we are now 3db
down and would end up at 35db down at 30mhz.? Clearly the 2n3904s
were not cutting it.? Its hard to get more than? a beta of 20 out of
two in parallel and we need an effective beta of at least 39.

This is why higher frequency transistors that work at higher currents
are being pursued. Devices with FT greater than 650 mhz gets use
within about 3-4db without circuit tricks. 3Db flatness means at 3mhz
say 10W and 30mhz 5W.? Adding tapered feedback helps some so we
should see near 7-9W at 30 and so far my results confirm that using
2n2369 for Q90, and the pre-driver one 2n3866,? and driver two
2n3866.? ?All of those have an Ft of 600mhz or so and power handling
suitable for the stage they are in. The problem is 2n3866 and 2n5109
are not cheap.

NOTE: if as was, we get about 4W (at 30mhz) from the IRF510s with the
measured drive it confirms the MOSFET is being starved!.

Putting the irf510s back in and grabbing the 50W 30db power
attenuator agrees nicely as the IRF510s at 3mhz were netting about
16db of gain and at 30mhz a respectable 14db of gain.?

Conclusion is we need uniform gain and the ability to deliver more
than .5W (about .7 would provide good headroom).? ?The IRF510 has
enough gain.

Now I wait for lower cost parts.

Allison