1. BITX20@groups.io

Re: RD16HHF1 in the uBITX #ubitx

Henning Weddig
 

Gordeon, please have a lok on the "famous" WA2EBY amp. He is using inductors in series with teh gate-bias resistors. My explanation is that this inductor of unknown value together with the gate-souce capacitance of the IRF 510 forms a parallel resonator, i.e. cancelling out the high gate capacitance. This may be the reason why he gets a flat(er) frequency response as shown in part 2 of his paper.   


In addition: the driving transistors hav e arelativly low ft, they must roll off.
LTSPICE simualtion of the bidi amp show a gain reduction of about 1 dB at 45 MHz, but for the driver stages in teh transmitter section things my be a bit different.
Simulation will schow!

BTW: I can confirm that the audio preamp in the receiver clips in reallity (on my BIOTX40) at output voltage swings as LTSPICE predicted.
Henning Weddig
DK5LV

Am 15.02.2018 um 18:58 schrieb Gordon Gibby:

​The old fashioned solution for capacitive loading was simply to put an inductor in parallel tuned to the desired frequency, to cancel it out.   


In my ancient vacuum tube rigs, they do that in the driver and final amplifier stages, and I get about the same power all the way from 80 meters through 15 meters, and in some rigs through 10 meters.    


Maybe they were on to something?   Perhaps there is a way to take that technique and apply it to semiconductor systems with some semiconductor switches to adjust component choices?



From: BITX20@groups.io <BITX20@groups.io> on behalf of Jerry Gaffke via Groups.Io <jgaffke@...>
Sent: Thursday, February 15, 2018 12:41 PM
To: BITX20@groups.io
Subject: Re: [BITX20] RD16HHF1 in the uBITX #ubitx
 
As reported here:  https://groups.io/g/BITX20/message/40938
an LTSpice model of Q90, Q911,Q912, Q92,Q93,Q96,Q97
shows that with the IRF510 gates clipped free of the circuit,
the signal available to the final in volts is about 77% at 30mhz what it is at 7mhz.
Power available is down by the square of the voltage, or about 60%.
LTSpice is probably assuming worst case 2n3904 parameters.
This result will not be fixed by going to RD16HHF1's.

With the IRF510 gates tied back in, the drop in signal became much more pronounced
due to capacitive loading by the IRF510's.  They need to be driven harder for good results at 30mhz.

Jerry, KE7ER

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