Remarkably simple device, Dan. Do you think it's a possibility that the phasing issues Jerry mentioned could be what's limiting it to below 18MHz?
Remarkably simple device, Dan. Do you think it's a possibility that the phasing issues Jerry mentioned could be what's limiting it to below 18MHz?
Last edited by NQ6U; 07-24-2012 at 01:19 PM.
All the world’s a stage, but obviously the play is unrehearsed and everybody is ad-libbing his lines. Maybe that’s why it’s hard to tell if we’re living in a tragedy or a farce.
I am seeing a tuned circuit on the input circuit. What would happen if you took that 220 pF cap out of there? It does not seem like it would be really broadbanded tuned like that.
I don't know what the inductance is on the primary winding, but it seems like there is an attenuator on the input with the pi circuit of resistors. Maybe that attenuation could be decreased if the amplitude drops on the lower bands with the 200pF taken out, but it should not roll off at the higher freq bands. If you need the cap in there, it might be best to switch out the cap for the higher bands and use matching caps to tune to the proper band between the inductance of the primary and the reactance of the cap. Ideally, the XL and XC will match and that is impossible between 1.8MHz and 30MHz.
The IRF MOSFETS were intended for use in switching applications like motor control or power supplies, where operating frequencies are low. Also, it was important to keep the channel resistance low when on to minimize waste heat, which limits lifetime and requires expensive heat sinking or a larger package design.Originally Posted by mw0uzo
The high-current capability required a fairly wide channel, even for VMOS, TMOS, etc. designs. Put a metal gate across that, and you have a nice capacitor. That capacitance forms an RC circuit with any gate series resistance which limits frequency response. A related parameter, gate charge, provides a useful guide. The bigger it is, the harder it is to move it back and forth to vary channel conductance. The larger the gate charge, the higher the gate current (i.e., drive) required to modulate the channel conductance.
73,
I don't think so, I would have thought any troublesome phase shifting would have occurred at higher frequency. But that's just a hunch, some hidden inductance or capacitance could cause it. (hmm quite likely!!!)
Yeah the gain of the amplifier is a lot at low frequency, plus the input SWR is all over the place, so attenuator is necessary to reduce gain and provide decent input SWR.
The cap was put in to increase gain at 14Mhz, as it was rolling off quickly. With the arrangement shown, the output power is fairly level across 160-20m, with the switch for 20m.
Yes. Reason for choice is low cost. IRF520N's were the cheapest, lowest input capacitance devices I could find, before getting to the point of headbutting the wall in frustration.
There is clearly a lot of capacitance somewhere in my design as the gain rolls off very early and more drive is required. It is probably what we expect, the mosfets, and tucked away in the bias circuit is a 33k/2n2 RC network which rolls off at 2.2Mhz. I did reduce R significantly, but then encountered some stability problems and lost a mosfet, so it was returned to its previous value.
I also experimented with the gate resistance, removing them gave a good rise in gain. Unfortunately, I also experimented with the output transformer ratio at the same time... and lost some more mosfets, and because of this silliness could not isolate the cause of failure. I now know the failure to have been down to the output transformer ratio and excessive current peaks through the mosfets (I was trying to get as much power as I could at 14V).
Last edited by mw0uzo; 07-24-2012 at 04:09 PM.
(Heh, i'm still surprised I got any power out of it at all, let alone 400W!!! :D)
Limitations of the device aside, Dan, the type of experimentation you are doing is truly in the spirit of amateur radio.
All the world’s a stage, but obviously the play is unrehearsed and everybody is ad-libbing his lines. Maybe that’s why it’s hard to tell if we’re living in a tragedy or a farce.
thx Carl :) Make one!
Last edited by mw0uzo; 07-24-2012 at 06:15 PM.
Remembering i made one with 20 2SC1969's i had cheap, broadbanded input and output using ferrite bars.
Did pull some serious current on 14.4 volts but did work up to 10 meters then, must have the remnants still laying around in the junk box somewhere...
Good try and for the higher frequencies i would opt for combining sets and combining these outputs again.
That prevents the limitations of the devices you run into now, the IRF 520 should work on 10 meters...
"If the Republicans will stop telling lies about the Democrats, we will stop
telling the truth about them." - Adlai Stevenson (1900-1965)
“I’m not liberal/conservative, I’m anti-idiotarian.”
At some point in the last 20 years, the left moved to the center, and the right moved into a mental institution
So perhaps a 2(2+2) design with splitter and combiner to avoid multiplying up the gate charge/capacitance?
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