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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 29th Jan 2021, 10:37 am   #21
livewireless123
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Default Re: IRF510 FET bias

I found this in my pile of old mag cuttings if it's of any interest.
the PW Chatter box.PW aug 1991, (also on the world radio site)
Looks like the PA is run in class C without bias so would need good low pass filtering.output is claimed as 10W. I mean to have a go at this when I can get round to it but would probably try push-pull audio with a pye westminster
mod' transformer .regards Mike g3zii
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Old 29th Jan 2021, 3:51 pm   #22
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Default Re: IRF510 FET bias

Hi Mike... That is the basis for my current design Thanks ..... I have rejected the Logic gates driver as it requires a 50/50 squarewave..... this is done by using a Bistable, being driven from a X2 oscillator. This is inconvenient... as my RF source is a DDS vfo. The VFO is also used for the receiver with an IF offset of plus 455 Khz...thus I can only have one offset.... If I used 4 Mhz with a 2 mhz offset on the display, I cannot have an IF offset of 455 Khz as well....... so.... I will stick to the fundamental drive at 2 Mhz .. ish.
I have made an auto transformer for the audio with several taps...... which need optimising...another job to do.......
I cannot get 10W out of this thing.... no matter how I tried.... DC input is 800 m/A @ 13.5V = 10.8W. My RF output is 4W.. see above
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Old 29th Jan 2021, 5:36 pm   #23
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Default Re: IRF510 FET bias

I think 10W carrier output (40W PEP fully moduated) is asking rather a lot from a single IRF510 with a 12V supply, even as a Class C final amplifier.

They will do about 5 or 6 Watts PEP SSB from 12V in Class AB.
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Old 29th Jan 2021, 6:25 pm   #24
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Default Re: IRF510 FET bias

The IRF510 wouldn't be my go-to FET in this instance either. it has a nastily-high Ciss which means you need a drive circuit that can really shove some current into that input-capacitance to ensure it switches-on promptly [and an equally-good circuit to drain the capacitance so it switches-off promptly]!

If you want to stick with historic 1980s power-FETs, the TO3-cased VN67AJ would be my go-to; only 33pF input-capacitance, so driving it is easy.
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Old 29th Jan 2021, 8:12 pm   #25
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Default Re: IRF510 FET bias

Quote:
Originally Posted by Wendymott View Post
I have rejected the Logic gates driver as it requires a 50/50 squarewave..... this is done by using a Bistable, being driven from a X2 oscillator. This is inconvenient... as my RF source is a DDS vfo
The DDS module has complementary squarewave outputs, once you have carefully set the duty ratio pot on the board. I agree this is not a true symmetrical square wave though you can get it close
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Old 29th Jan 2021, 11:27 pm   #26
Wendymott
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Default Re: IRF510 FET bias

I do wonder if that "reported" 10W was RF out..... more likely DC input... as all you have said..The IRF510 is not capable of 10W rf out....... well in my experience anyway.
The eventual transceiver will be modular anyway so I can look at the final again later.
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Old 30th Jan 2021, 10:36 am   #27
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Default Re: IRF510 FET bias

ISTR 10W DC input was your limit for topband AM/CW anyway back in the '70s?
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Old 30th Jan 2021, 10:04 pm   #28
Wendymott
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Default Re: IRF510 FET bias

Yes Chris.. Thats what I was working towards...... but I understand its increased now... but without checking the new regs Im not sure so I wont quote it.
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Old 31st Jan 2021, 10:01 am   #29
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Default Re: IRF510 FET bias

The full licence above 1.850MHz is 32W PEP Output.
At 100% AM Mod that is a carrier power of only 8W.

You could get away with 10W Carrier so long as the mod is kept to 80%.
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Old 4th Feb 2021, 11:24 am   #30
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Default Re: IRF510 FET bias

Some possible ideas here from rad com TT 2000. they are for 50 Mhz so would obviously need bigger coils for TB and they use higher psu volts.
Note the bit about using lower impedance drive to offset the high internal C of the fets. hope the diagrams are readable. regards, Mike.
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Old 4th Feb 2021, 11:55 am   #31
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Default Re: IRF510 FET bias

Thanks Mike.... all noted...... but at the moment it is on the "back burner"..I have decided to upgrade my Eddy EC10..... as an interim project...... but I will... as Arnie says "be back"
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Old 4th Feb 2021, 12:47 pm   #32
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Default Re: IRF510 FET bias

This is for a frequency near top band. The relevant bit is around Q3. The RF section is a shortened version of the July and August 1979 Practical Wireless VMOS Top Band Transmitter.

I'm pretty sure back issues of PW were available here: https://worldradiohistory.com/#UK

but it looks like they have been removed. (Maybe a good reason for that, but it seems small minded to me.)
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Old 4th Feb 2021, 1:40 pm   #33
Wendymott
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Default Re: IRF510 FET bias

Hi Ian. Many thanks for your input.... I have this article and that is what my modulator is based on...... The original question was.... what the bias should be set to for a single IRF510.......no one actually answered it.... but never mind...in the future.. I will answer it..I have had lots of pointers of different circuits and have tried most of them. One single IRF510 will not give anymore than 4W RF into 50 R.... for a DC input of 12W. But as I say above.. its on the back burner for a few days.
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