Fail to amplify 27mhz rf using mosfet IRF843

I found old mosfet transistor IRF843, using 20k and 70k resistor, i manage to set the voltage at transistor gate to 3.9 volt and using 1uF to channel the 27mhz rf signal to the transistor gate, i manage to get an audio from 8 ohm speaker connected to the transistor drain. I'm using about 5.3 volt pow er supply from transformer. The audio is loud and clear. The mosfet gate ca n received from 2v to 4v, as documented in the technical paper.

My actual objective is to amplify the 27 mhz rf to at least 1 watt output. I've seen someone using IRF84x series to get more than 100 watt of rf power , but i wonder why i couldn't do so with this transistor. I replace the spe aker with coil, in hope that some rf signal to be emitted. The rf signal ca me from modulated local oscillator perhaps around 1-3 miliwatts power. It c an be heard in shortwave radio, but not far enough.

Reply to
idzuan
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The input likely isn't well-matched to the transistor's input impedance, the transistor's output likely isn't well-matched to the load impedance, and the transistor likely isn't biased correctly for class A operation.

Reply to
bitrex

Wow. Simple problem statement, many complicated answers.

The first quick answer is that because of the amount of capacitance in that transistor, you should only expect to be able to build a wideband amplifier that has about 10 or maybe 20dB of gain at 27MHz. That means you need to put more like 100mW in to get 1W out. Theoretically you could get more gain if you used tuned circuits on the input and output, but you'd run a very real risk of building a power oscillator instead, and the tuning would drift with temperature as the transistor warmed up.

The second quick answer is that because of that thing's voltage rating, you could probably get 5W out of it (with 1/2W in), but you'd have to be Really Good at making heatsinks.

The long answer is book length, and a good book for you if you can afford it is "Experimental Methods in RF Design":

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I don't have the book, but I'd be very surprised if they don't talk about using that sort of FET to build RF power amplifiers.

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Tim Wescott 
Control systems, embedded software and circuit design 
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Reply to
Tim Wescott

The IRF843 has as gigantic input capacitance, around 1500p, and the gate to drain C is probably in the hundreds of p. Miller effect will make that even worse.

With just feeding the signal direct "wideband" like an audio amplifier to that transistor without any kind of power-transfer matching I'd be surprised if he got any gain out of it at all at 27 mHz.

Just with the Ciss and a 50 ohm source alone the insertion loss is already like 30dB.

Reply to
bitrex

The question does not parse well.

If 27 mhz means 27 MegaHertz, it should not run a speaker at all. If 27 mhz means 27 milliHertz, it will be below the lowest audible frequency.

What is the ultimate goal of the OP? Maybe an illegal CB power amplifier?

--

-TV
Reply to
Tauno Voipio

Sounds like at the moment he's running AM modulated RF into the MOSFET and it's just acting as a rectifier

Reply to
bitrex

e to set the voltage at transistor gate to 3.9 volt and using 1uF to channe l the 27mhz rf signal to the transistor gate, i manage to get an audio from 8ohm speaker connected to the transistor drain. I'm using about 5.3 volt p ower supply from transformer. The audio is loud and clear. The mosfet gate can received from 2v to 4v, as documented in the technical paper.

. I've seen someone using IRF84x series to get more than 100 watt of rf pow er, but i wonder why i couldn't do so with this transistor. I replace the s peaker with coil, in hope that some rf signal to be emitted. The rf signal came from modulated local oscillator perhaps around 1-3 miliwatts power. It can be heard in shortwave radio, but not far enough.

Hi all, as suggested, i recalculated the input impedance, and instead of di rectly matching the transistor gate with 1uf capacitor, i firstly add 10mm, 25 turn coil (30 swg) and its starting to produce some result. I got some rf signal within 20 meter radius just by using 3 inch antenna. As suggested , i want to try putting 0.01uf capacitor in place of 1uf, hoping that this will reduce the transistor internal gate capacitance and add heatsink, so t hat i can drive the transistor up to 12 volt. Thanks guys, i hope this will work.

Reply to
idzuan

e to set the voltage at transistor gate to 3.9 volt and using 1uF to channe l the 27mhz rf signal to the transistor gate, i manage to get an audio from 8ohm speaker connected to the transistor drain. I'm using about 5.3 volt p ower supply from transformer. The audio is loud and clear. The mosfet gate can received from 2v to 4v, as documented in the technical paper.

. I've seen someone using IRF84x series to get more than 100 watt of rf pow er, but i wonder why i couldn't do so with this transistor. I replace the s peaker with coil, in hope that some rf signal to be emitted. The rf signal came from modulated local oscillator perhaps around 1-3 miliwatts power. It can be heard in shortwave radio, but not far enough.

In my country, the legal limit is only up to 5 watt for 27Mhz. About the rf signal turning into audible sound, when i match 47nf capacitor from oscill ator circuit ground to the other circuit ground containing the mosfet trans istor, it successfully turn the rf signal into audible audio, when i change the coil connected to the transistor drain and replace with a 8ohm speaker . I saw a lot of example people using mosfet to amplify rf signal. I hope u sing this old mosfet that i found will also works. Its very hard to find rf power transistor at my place, and if there is, it will be very expensive. The last time i buy one rf power transistor 2SC1972, it should work out to emit up to 18watt power, but i got cheated. It was a fake transistor, the l abel are carefully rubbed of and printed again with new label. I found it t he hardway, after a while wasting my time figuring out, why it doesn't work as it should. I'll guess i was a fake imported chinese product.

Reply to
idzuan

e to set the voltage at transistor gate to 3.9 volt and using 1uF to channe l the 27mhz rf signal to the transistor gate, i manage to get an audio from 8ohm speaker connected to the transistor drain. I'm using about 5.3 volt p ower supply from transformer. The audio is loud and clear. The mosfet gate can received from 2v to 4v, as documented in the technical paper.

. I've seen someone using IRF84x series to get more than 100 watt of rf pow er, but i wonder why i couldn't do so with this transistor. I replace the s peaker with coil, in hope that some rf signal to be emitted. The rf signal came from modulated local oscillator perhaps around 1-3 miliwatts power. It can be heard in shortwave radio, but not far enough.

In my country, the legal limit is only up to 5 watt for 27Mhz. About the rf signal turning into audible sound, when i match 47nf capacitor from oscill ator circuit ground to the other circuit ground containing the mosfet trans istor, it successfully turn the rf signal into audible audio, when i change the coil connected to the transistor drain and replace with a 8ohm speaker . I saw a lot of example people using mosfet to amplify rf signal. I hope u sing this old mosfet that i found will also works. Its very hard to find rf power transistor at my place, and if there is, it will be very expensive. The last time i buy one rf power transistor 2SC1972, it should work out to emit up to 18watt power, but i got cheated. It was a fake transistor, the l abel are carefully rubbed of and printed again with new label. I found it t he hardway, after a while wasting my time figuring out, why it doesn't work as it should. I'll guess it was a fake imported chinese product.

Reply to
idzuan

Pleasse, that's MHz, not mhz. You're aware the MOSFET's datasheet says its input capacitance is 1300pF, right? That's j4.5 ohms at 27MHz.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Blarf. I did the math in my head and slipped a digit. What's a factor of 10 between friends?

Yes! He's screwed! Needs newer parts...

--
Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

You still did not tell what you're attempting to make: - a transmitter power amplifier, - a receiver with loudspeaker amplifier, - something else.

The CB band is not for self-built devices. I doubt that you have enough competence to pass an official inspection.

For self-built equipment, get the training and licensing for an amateur radio operator.

--

-TV
Reply to
Tauno Voipio

What inspection (at least in the US)? 1 watt PEP on ~27 MHz is certainly less than 4.

Reply to
bitrex

Disregard, you're correct. I was thinking of micropower AM broadcasting, and that looks like 100mW is the limit.

I was never big into CB, anyway...;-)

Reply to
bitrex

Looks like you can't get a "license" to operate on the CB bands, you are deemed to have one by possession of an industry-made CB transmitter. But the power output has to be limited to 4 watts.

So it seems that to operate home-made equipment for that band of any power level it has to be inspected, while if one were just to obtain a ham license you could use any of the assigned amateur radio bands with any equipment you wish without inspection, so long as you followed the guidelines on power output, modes, spurious emissions, etc.

Weird. Seems like a pain-in-the-ass band to operate on, at least for a home builder who wanted to keep things legal.

Reply to
bitrex

Could probably get something with giving up on the wideband hope and the appropriate input and output matching circuits and biasing it right and maybe have to neutralize the Miller capacitance etc. and it definitely starts to become a "why am I doing this?" kind of project...;-)

Reply to
bitrex

Right.

I have used 30 years of my engineering career to guard the RF bands here.

The 28 MHz ham band (10 meters) is not far from the CB band. The basic idea of CB is not operate like ham radio, but for simple communication with certified equipment and power levels not intended for long-distance communication (though occasionally it may happen).

--

-TV (licensed ham since 1960)
Reply to
Tauno Voipio

The conclusion the article draws is silly. Seems like they're arguing that he was justified in breaking the law, because he "couldn't get a license" to do something that was illegal to do.

Um, duh. I've been pestering the CIA to re-instate my "license to kill" for ages now, and they keep turning me down! Assholes...

Reply to
bitrex

e to set the voltage at transistor gate to 3.9 volt and using 1uF to channe l the 27mhz rf signal to the transistor gate, i manage to get an audio from 8ohm speaker connected to the transistor drain. I'm using about 5.3 volt p ower supply from transformer. The audio is loud and clear. The mosfet gate can received from 2v to 4v, as documented in the technical paper.

. I've seen someone using IRF84x series to get more than 100 watt of rf pow er, but i wonder why i couldn't do so with this transistor. I replace the s peaker with coil, in hope that some rf signal to be emitted. The rf signal came from modulated local oscillator perhaps around 1-3 miliwatts power. It can be heard in shortwave radio, but not far enough.

Actually i want to build a cbradio. But since TV mention, even though its l egal to transmit under 4 watts, but still the equipment is not certified. I 'll guess i couldn't escape taking HAM exam. Taking exam is not one of my s trong point :) But you have to do, what you need to do. I have to check aga in with FCC here, the CB regulation. Not many home build radio enthusiast a nd hardly any CB'ers here. Last time, there was flood disaster and people g ot stranded on the hills without food and also all telephone line went off, i was thinking, if we could built a cheap cb kits, then it can help people in trouble, since the good CB equipment was expensive.

Reply to
idzuan

The point with the exam is to ensure that you have the necessary competence to prevent you and your equipment from harming you or the RF environment around you.

To build a sensible radio, you need the knowledge and skills anyway.

--

-TV
Reply to
Tauno Voipio

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