Fail to amplify 27mhz rf using mosfet IRF843

On Fri, 17 Mar 2017 15:44:02 -0400, bitrex wrote:

> 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...;-)

Yup.  Now that I have time to think back far enough, people were getting 
reasonable amateur-radio type performance out of those things in the 80 
meter (3.5MHz) band -- but that's a hair lower frequency than 11 meters.

Should be good suitable parts from DigiKey or Mouser, if he's in the US.

-- 
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