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 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.

On 03/17/2017 01:02 AM, idzuan@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.
>

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.

On Thu, 16 Mar 2017 22:02:16 -0700, idzuan 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.

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": http://www.arrl.org/shop/
Experimental-Methods-in-RF-Design-Classic-Reprint-Edition/.  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.

-- 
Tim Wescott
Control systems, embedded software and circuit design
I'm looking for work!  See my website if you're interested
http://www.wescottdesign.com

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.

On 17.3.17 07:02, idzuan@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.
>

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

On 03/17/2017 01:30 PM, Tauno Voipio wrote:
> On 17.3.17 07:02, idzuan@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.
>>
>
> 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?
>

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

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.

Hi all, as suggested, i recalculated the input impedance, and instead of directly 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 that i can drive the transistor up to 12 volt. Thanks guys, i hope this will work.

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 i was a fake imported chinese product.

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.

idzuan@gmail.com wrote...
> idz...@gmail.com wrote:
>> I found old mosfet transistor IRF843 ...
>> My actual objective is to amplify the 27 mhz rf

 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