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 Post subject: 2n7000 X 3 , Class E Part15 Transmitter Idea
PostPosted: Oct Sun 03, 2021 10:09 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 802
Three parallel 2n7000. Why? To increase efficiency.
Input gate drive capacitance is around 65pF.
The 0 to 5V 1MHz square wave should have high rise and fall times, 6nS is okay. Use 74AC04. A 1MHz TTL can oscillator should be able to drive the parallel gates of 74AC04.

I aimed at 70% efficiency but we are using three mosfets in parallel. Efficiency will rise to 80% quite easily if everything goes well.

80mW of carrier rising to 320mW with 100% modulation peaks. How does it sound, awesome right? An AF compressor is a must before feeding the audio to LM386 to maintain high average power. It should give a solid 200m to 400m meters range provided grounding is good. A 200ohm Low Pass Filter before the tapping at antenna coil is a good idea.

Not tested anywhere. It will work? Definitely. Use high Q inductors.


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 Post subject: Re: 2n7000 X 3 , Class E Part15 Transmitter Idea
PostPosted: Oct Sun 03, 2021 10:52 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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200ohm to 50K 6foot antenna matching.


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 Post subject: Re: 2n7000 X 3 , Class E Part15 Transmitter Idea
PostPosted: Oct Wed 06, 2021 9:42 am 
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Joined: Dec Tue 29, 2015 8:23 am
Posts: 286
Location: Australia
Dare4444 wrote:
Three parallel 2n7000. Why? To increase efficiency.

…

Not tested anywhere. It will work? Definitely. Use high Q inductors.

It probably simulates just fine - but those puppies have a tonne of spread in just about every relevant parameter e.g. threshold voltage, gain/transconductance, Rds, etc, even between ones from the same batch.

It'll probably work somewhat, but I'd want to see it built & thoroughly tested before believing the simulation results are accurate… :wink:


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 Post subject: Re: 2n7000 X 3 , Class E Part15 Transmitter Idea
PostPosted: Oct Wed 06, 2021 9:19 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 802
Rod FeC wrote:
Dare4444 wrote:
Three parallel 2n7000. Why? To increase efficiency.

…

Not tested anywhere. It will work? Definitely. Use high Q inductors.

It probably simulates just fine - but those puppies have a tonne of spread in just about every relevant parameter e.g. threshold voltage, gain/transconductance, Rds, etc, even between ones from the same batch.

It'll probably work somewhat, but I'd want to see it built & thoroughly tested before believing the simulation results are accurate… :wink:


It has not been stimulated in LtSpice.


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 Post subject: Re: 2n7000 X 3 , Class E Part15 Transmitter Idea
PostPosted: Oct Wed 06, 2021 10:01 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 802
Rod FeC wrote:
Dare4444 wrote:
Three parallel 2n7000. Why? To increase efficiency.

…

Not tested anywhere. It will work? Definitely. Use high Q inductors.

It probably simulates just fine - but those puppies have a tonne of spread in just about every relevant parameter e.g. threshold voltage, gain/transconductance, Rds, etc, even between ones from the same batch.

It'll probably work somewhat, but I'd want to see it built & thoroughly tested before believing the simulation results are accurate… :wink:


Somewhat? I even tested a BS170 delivering 350mW with 25mw in! Spice simulation done later showed 360mW. Very accurate I would say. Paralleling three reduces Ron so less heating. Class E one should work.


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 Post subject: Re: 2n7000 X 3 , Class E Part15 Transmitter Idea
PostPosted: Oct Fri 08, 2021 5:55 am 
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Joined: Dec Tue 29, 2015 8:23 am
Posts: 286
Location: Australia
Quote:
Paralleling three reduces Ron so less heating.

Heating isn't the issue. The assumption that 3 x 2n7000's / BS170's wired straight in parallel like that will share evenly & do much to increase efficiency/output power when driven in linear conduction (as opposed to driven hard in switching mode) is a big one - the parameter spread is so wide, even between same-batch parts, that it's unlikely even with the inherent PTC of mosfets.

More likely is that the one with lowest Rds(on) at the given Vgs will hog the majority of current, and the others will only contribute a little. The latter will increase slightly as the first one heats up - but, then, their Rds will rise as they heat up too; eventually reaching an equilibrium that's still dependent on their individual characteristics.

All I'm saying is it'd be worthwhile to see the assumption that they do share to a worthwhile degree tested before trusting in it.


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 Post subject: Re: 2n7000 X 3 , Class E Part15 Transmitter Idea
PostPosted: Oct Fri 08, 2021 6:11 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 802
You're right. Same batch same reel. 1ohm emitter resistors with 100n bypass should help. Don't know. It is just to squeeze 5-10mW more haha.


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