Fail to amplify 27mhz rf using mosfet IRF843

Started by Unknown March 17, 2017
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