[Amps] Input Impedance of RF MOS amp

[Karl-Arne Markström]

It is most likely that the KL300 is optimized for the 27 MHz range, as being
a CB amplifier. This usually means that the magnetic core area and number of turns in the input transformer may be insufficient  to handle lower frequencies.

The amplifier also lacks the compensation networks that are put into
wideband solid-state PA's to smooth out the gain variations over the frequency range.
These networks also helps to keep the input impedance reasonably constant.

In summary, I believe that the KL300 will need a rework according to the relevant
Philips and Motorola application notes covering wideband bipolar amplifier design.

73/

Karl-Arne
SM0AOM




----- Original Message ----- 
From: "Stefan" <ea5fy at ure.es>
To: <amps at contesting.com>
Sent: Thursday, May 05, 2005 4:06 PM
Subject: [Amps] Input Impedance of RF MOS amp


> Dear collegues
> 
> Recently I was lucky enough to buy two KL300 
> (http://www.rmitaly.com/scheda.asp?IDGr=1&cat=0&tipo=94) amplifier at a flea 
> market for a reasonable prize (2USD each) and I got them to work again. I 
> have checked the devices with my FT817 and they work rather well (I know, it 
> does not have any output filters but I only wanted it for my FT817 to give 
> it a punch, out there in the "wilderness" when carrying the appropiate 
> battery).
> 
> These devices are claimed to be useful for the whole HF band but I have 
> noticed a quite non uniform output power over the HF bands. I think the 
> reason is because my rig folds back its ouput power by noticing an unmatched 
> input impedance (the antenna is tuned or even a dummy load). Therefore I 
> have the feeling that this amplifier does not have a flat input impedance 
> (50 ohm). How can I flatten this impedance or what is done ususally to solve 
> this problems. I suppose that the input impedance is the input impedance of 
> the transistors (class B push-pull) plus the resistance put between the 
> transistor's base and emitter (10ohm here).
> 
> I would appreciate your suggestions and solutions. On the above web is not 
> only the amp itself but also its very simple diagram.
> 
> Thanks for your help.
> 
> Kind regards,
> 
> Stefan
> EA5FY 
> 
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