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AM with the BS170 Mosfet?




 
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Author Topic: AM with the BS170 Mosfet?  (Read 1877 times)
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k9jri
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« on: December 23, 2012, 05:26:23 PM »

I have several kits from Hendricks QRP Kits that Steve (KD1JV) designed around the BS170 MOSFET. All of them use (3) BS170s in parallel running from 9 - 12 vdc in a CW only transmitter design.

Can these designs be run on AM with a modulated power supply for the final devices?  While the Retro-75 uses an IRF530 it appears to be a similar design with a much more robust MOSFET.

73 - Mike - K9JRI
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steve_qix
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« Reply #1 on: December 26, 2012, 10:41:57 AM »

In theory, any MOSFET can be used for AM.  You have to de-rate the device as follows:

1) Take the maximum VDS, divide it in half for a safety factor.  Example: with a 900 V MOSFET, this value is 450 V.  With a 200V MOSFET, this is 100V.

2) Figure the worst case maximum peak repetitive drain voltage under steady state RF conditions.  For instance, with class E, the worst case condition is 3.5X the applied DC.   This gives the maximum applied modulated DC that you can use.  Example:  take the 450V from step 1, divide by 3.5 - you get around 130V.  Another Example:  Take the 100V from step 1, divide by 3.5 = 28V.  Important:  If you think you can get away without doing step 1 (in other words, you think you can operate reliably without the de-rating from step 1), don't bother.  You will have repeated failures because things happen, like T/R switching anomalies, antennas falling down, mis-tuned loads, etc.

3) Now figure the unmodulated DC.  This will depend on what percentage of positive modulation you desire.  If you only desire 100% positive (NOT a good idea), you can divide the maximum DC (from step 2) by 2.  If you desire something like 180% positive (this leaves a lot of headroom), divide the voltage from step 2 by somewhat less than 3.  Example 130 / 3 = 43.3 V.  I use 45 volts DC at carrier.  Other Example from step 2: 28.5 / 3 = 9.5V (use 10 V).

An easier shortcut, at least for class E is to take the V D-S max, and divide by 20.  This gives the carrier voltage, assuming almost 200% positive modulation and a reasonable safety factor.

As can be seen, low voltage MOSFETs are not a win for AM unless you want to run really low power.
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kb3ouk
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« Reply #2 on: December 26, 2012, 11:30:28 AM »

The Retro 75 is running 12 to 15 volts on a 100v 14A device. It uses a transformer coupled modulator so on peaks it is running about twice that voltage (the thing can only do about 90% positive). The BS170 is only rated for 60v 500mA.
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« Reply #3 on: December 26, 2012, 12:06:39 PM »

The Retro 75 is running 12 to 15 volts on a 100v 14A device. It uses a transformer coupled modulator so on peaks it is running about twice that voltage (the thing can only do about 90% positive). The BS170 is only rated for 60v 500mA.

I think the class of operation for the retro 75 is C or D (don't remember).  This results in a somewhat lower RF peak voltage, and coupled with the low positive modulation capabilities, it probably works reasonably reliably even with that low voltage device.
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