80 meter class E amplifier input impedance question

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80 meter class E amplifier input impedance question

Postby KC3OL » Sat Feb 15, 2003 6:52 pm

I would like to eliminate Q1 and drive T3 directly. Does anyone have a suggestion for a tuned circuit on the primary of T3 that would act as an impedance match?

Ted, KC3OL
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Postby frank carcia » Sat Feb 15, 2003 8:27 pm

Ted,
Bob is doing the direct drive method. Maybe he has a schematic on his site.
I would think the input is a large amount of C so a series inductor might work. A solid state rig wants to see 50 ohms so might take an additional cap at the input. Just monitor
your gate voltage while playing with matching networks to make sure the Gates don't
get blasted. Today I got my second batch of 20 FETs from Steve and plan to do another
push pull design for 75 Meters. I may also do direct drive and was thinking about the input during the ride home. I'm thinking of a 1:2 step down with resistor loading on
the gates. I like broad band drive myself. Good luck fc
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No problem with driving the final gates directly

Postby steve_qix » Sat Feb 15, 2003 8:51 pm

No problem at all driving the gates of the final RF MOSFETs (hey, this is starting to sound like vacuum tubes!).

I did this for a long time with my transmitters. It was only when I started building mutiple transmitters and transmitters on different bands that I started standardizing my input requirements - in my case, to 5 watts.

This necessitated the driver MOSFET. However, this is no reason whatsoever that you need to have it.

Just insert a matching network between the primary of T3 and the 50 ohm RF driver. an L network should do it - maybe an input series inductor and a parallel variable capacitor.

I also do the driver FET because my yard sale slop-bucket transmitter does not have much output capability, and it gets very hot when operating at near full power.

Regards,

Steve WA1QIX
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