On Tue, 11 May 2004 16:41:22 +0200, "Marc Battyani"
wrote:

But my problem for the impedance matching is that I don't know the amplifier
output impedance.
It's a basic class-E amplifier (A self + an IRL510 MOSFET)
Is there a simple way to measure it or to compute it ?

Thanks,

Marc

Hi Marc,

For actual specifics of your device, go to:
http://www.irf.com/product-info/data...ata/irl510.pdf
The answer to your literal question requires more details of the
circuit, as the device shows a drain-source resistance that is common
of many finals transistors found in 100W amateur transmitters which
typically exhibit a 50 Ohm source Z at rated power.

This exhibited characteristic Z is a consequence of Z multiplication
through conventional transformer theory. These designs also have the
benefit of low pass filters aiding a smooth transition of Z from the
device Z on the order of 0.5 to 1 Ohm to the System Z of 50 Ohms.

If your amplifier lacks in these regards, it will suffer in efficiency
in spite of the claims of designs of its class. You may wish to
consider driving a small antenna directly to aid in matching. This is
because very small antennas (once the reactances are balanced out, an
important consideration) exhibit a much smaller load resistance that
corresponds to your device's natural output Z. However, to maintain
efficiency, you will need to be scrupulous about all Ohmic sources of
loss that come from construction issues.

73's
Richard Clark, KB7QHC