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Fail to amplify 27mhz rf using mosfet IRF843

Started by Unknown March 17, 2017
On Fri, 17 Mar 2017 11:55:36 -0400, bitrex wrote:

> On 03/17/2017 11:28 AM, Tim Wescott wrote: > >> 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 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.
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 http://www.wescottdesign.com I'm looking for work -- see my website!
On 17.3.17 20:23, idzuan@gmail.com wrote:
> On Friday, March 17, 2017 at 1:02:22 PM UTC+8, idz...@gmail.com wrote: >> 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 8ohm speaker connected to the transistor drain. I'm using about 5.3 volt power supply from transformer. The audio is loud and clear. The mosfet gate can 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 speaker 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 oscillator circuit ground to the other circuit ground containing the mosfet transistor, 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 using 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 label are carefully rubbed of and printed again with new label. I found it the 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.
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
On 03/17/2017 03:20 PM, Tauno Voipio wrote:

> 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. >
What inspection (at least in the US)? 1 watt PEP on ~27 MHz is certainly less than 4. <https://en.wikipedia.org/wiki/Title_47_CFR_Part_15>
On 03/17/2017 03:25 PM, bitrex wrote:
> On 03/17/2017 03:20 PM, Tauno Voipio wrote: > >> 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. >> > > What inspection (at least in the US)? 1 watt PEP on ~27 MHz is certainly > less than 4. > > <https://en.wikipedia.org/wiki/Title_47_CFR_Part_15>
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...;-)
On 03/17/2017 03:30 PM, bitrex wrote:
> On 03/17/2017 03:25 PM, bitrex wrote: >> On 03/17/2017 03:20 PM, Tauno Voipio wrote: >> >>> 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. >>> >> >> What inspection (at least in the US)? 1 watt PEP on ~27 MHz is certainly >> less than 4. >> >> <https://en.wikipedia.org/wiki/Title_47_CFR_Part_15> > > 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...;-)
<http://www.commlawblog.com/2011/06/articles/enforcement-activities-fines-forfeitures-etc/fcc-fines-man-for-not-having-an-unobtainable-license/> 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.
On 03/17/2017 03:09 PM, Tim Wescott wrote:
> On Fri, 17 Mar 2017 11:55:36 -0400, bitrex wrote: > >> On 03/17/2017 11:28 AM, Tim Wescott wrote: >> >>> 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 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. > > 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... >
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...;-)
On 17.3.17 21:36, bitrex wrote:
> On 03/17/2017 03:30 PM, bitrex wrote: >> On 03/17/2017 03:25 PM, bitrex wrote: >>> On 03/17/2017 03:20 PM, Tauno Voipio wrote: >>> >>>> 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. >>>> >>> >>> What inspection (at least in the US)? 1 watt PEP on ~27 MHz is certainly >>> less than 4. >>> >>> <https://en.wikipedia.org/wiki/Title_47_CFR_Part_15> >> >> 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...;-) > > <http://www.commlawblog.com/2011/06/articles/enforcement-activities-fines-forfeitures-etc/fcc-fines-man-for-not-having-an-unobtainable-license/> > > > 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.
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)
On 03/17/2017 04:37 PM, Tauno Voipio wrote:

>> 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. > > 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).
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...
On Friday, March 17, 2017 at 1:02:22 PM UTC+8, idz...@gmail.com wrote:
> 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 8ohm speaker connected to the transistor drain. I'm using about 5.3 volt power supply from transformer. The audio is loud and clear. The mosfet gate can 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 speaker 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 legal 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 strong point :) But you have to do, what you need to do. I have to check again with FCC here, the CB regulation. Not many home build radio enthusiast and hardly any CB'ers here. Last time, there was flood disaster and people got 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.
On 18.3.17 02:47, idzuan@gmail.com wrote:
> On Friday, March 17, 2017 at 1:02:22 PM UTC+8, idz...@gmail.com wrote: >> 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 8ohm speaker connected to the transistor drain. I'm using about 5.3 volt power supply from transformer. The audio is loud and clear. The mosfet gate can 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 speaker 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 legal 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 strong point :) But you have to do, what you need to do. I have to check again with FCC here, the CB regulation. Not many home build radio enthusiast and hardly any CB'ers here. Last time, there was flood disaster and people got 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.
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