The EN369FN mosfet companion CBTricks errors in the drawing and the correction.

[PsiDOC]

Hi all. Was playing with mosfets over the weekend when I noticed something a bit strange going on with the information on CBtricks with regards the EN-369FN.
Whilst the information below does contain some maths and Ohms law it does clearly prove that the EN-369FN drawing and in particular the value of R2 is infact not just incorrect, but it's waaaay off!

The example being used is a bog standard Magnum 257 that uses the EN369DR and EN369FN companion parts on the driver and final mosfets. Below is the schematic of the driver and finals mosfets and their biasing resistors and below that the EN-369FN and DR nabbed from CB Tricks .com

Now for the calculations:

Worked driver and final PA bias examples using a Magnum 257 as our example:
All examples are taken with RV11 and RV13 at their midpoint which is 10k and 250k ohm respectively.

Keep in mind that we would expect to bias the driver mosfet at about 3.1 - 3.3 volts and the finals at 3.7 volts.

Driver biasing example - Please refer to red box 1 on the above schematic.
Transmit line voltage is the usual 8 volts DC.

First we calculate the current:
current total = transmit line voltage / (RV13 midpoint + R233 + [R2 of the EN369DR]) =
So current total = 8 / (250000 + 270000 + 320000) = 8 / 840000 = 0.000009523A or 9.523 uA (Micro amps)

Now we can calculate the bias voltage across the EN-369DR
bias voltage across the EN36DR = current total x [R2 of the EN369DR]
So bias voltage = 0.000009523 x 320000 = 3.04 volts.
3.04 volts is a nice driver bias voltage to start with and we can tweak it from there.


Final biasing example - Please refer to red box 2 on the above schematic..
Please note before we start the transmit line voltage is now 7.4 volts as we drop 0.6 volts across transistor Q5
First we calculate the current:
current total = transmit line voltage / (RV11 midpoint + R239 + [R2 of the EN369FN])
So current total = 7.4 / (10000 + 10000 + 230000) = 8 / 250000 = 0.0000296A or 29.6 uA (Micro amps)

Now we can calculate the voltage across the EN-369FN
bias voltage across the EN369FN = current total x [R2 of the EN369FN]
So our bias voltage = 0.0000296 x 230000 = 6.808 volts
Hang on a moment! That’s not good at all, actually this is very BAD. 6.808 volts is almost double what we should be biasing the mosfet at and will destroy it very quickly!

So what’s wrong here? We know full well that the Magnum was never designed to run at 6.8 volts bias, or else they’d be popping finals left right and centre.

The answer to that question is R2 in the EN369FN in the drawing is incorrect.

If we were to decrease value of R2 in the EN369FN to 20k and then redo the maths.

Final biasing example with R2 in the EN369FN at 20k ohms
First we calculate the current:
current total = transmit line voltage / (RV11 midpoint + R239 + [our new R2 of the EN369FN]) =
So current total = 7.4 / (10000 + 10000 + 20000) = 8 / 40000 = 0.000185A or 185 uA (Micro amps)

Now we can calculate the voltage across the EN-369FN
bias voltage across the EN36FN = current total x [our new R2 of the EN369FN]
So bias voltage = 0.000185 x 20000 = 3.7 volts

3.7 volts = A perfect bias!

As you can clearly see from the above simple maths the EN369FN should not be 230k ohms as per the drawing, but should be 20k ohms.
If you use the following drawing edit it will not only work, but work as intended.

Psi

 

[melaza]

100K / 1W

 

[PsiDOC]

100K / 1W

Ok let's run with that.

Final biasing example with R2 in the EN369FN at 100k ohms
First we calculate the current:
current total = transmit line voltage / (RV11 midpoint + R239 + [our new R2 of the EN369FN]) =
So current total = 7.4 / (10000 + 10000 + 100000) = 7.4 / 120000 = 0.00000616A or 6.16 uA (Micro amps)

Now we can calculate the voltage across the EN-369FN
bias voltage across the EN36FN = current total x [our new R2 of the EN369FN]
So bias voltage = 0.00000616 x 100000 = 6.16 volts

6.16 volts. :blink: Are you sure you want to run with that? Hope you have some spare Mosfets handy.

 

[Robb]

So; are the other biasing devices wrong - also?

 

[PsiDOC]

No robb.
The EN369DR I have proved as above is correct in the driver section, and looking at the EN1230 if you were to bias it with a 3.9k resistor to the 8v tx power line it would bias properly at 3.66v.
Whilst I have not found any radios that use an EN1230 for mosfet biasing from the factory the CRE8900 is very close with r1 being 330 ohms and r2 being 3.9k so we can quite safely say it is only the EN369FN that's incorrect.

Psi

 

[sonoma]

the cobra 29 uses the en1230, I have done 2 of them so far and they seem to work just fine. I make my own as the chart listed and have not had a problem with either of them so far.

 

[PsiDOC]

That's excellent news Sonoma. I had a very good idea it was correct. I just couldn't find any application data to work from.
Seeing as you're here and I cannot find any application information for the EN1230. Can I ask you what is the size resistor you use to the +8v tx line? I am guessing 3.9 - 4.7k?

Psi

 

[sonoma]

here is a link to all the install pages for the cb tricks site listed. I use the page here for the application of the mosfet in the 29.

http://www.cbtricks.com/miscellaneous/fet_papers/ekl/index.htm

 

[melaza]

I used D1 and R2, Gate voltage = 2.67. 100K / 1W + 1N4148. DK 7W and swings 30.


Make a change to that RV11 to 100K.

 

[Robb]

Sooo . . . The EN369 needs a 20k ohm resistor at R2 to make this device bias properly?

What happens if you use a 100k ohm VR in place of R2?
Would that make the bias voltage adjustable?

 

[Robb]

The reason I asked the last question, is so that one device could be used for each different application. Or even a different device, such as a 13N10.

Thoughts?

 

[LeapFrog]

3.7 volts = A perfect bias!

3.7 Volts is somehow, not, too much? What's the limit, guess I'll have to fry a few to figure it out.
Edit: or is this bias for a driver?

 

[sonoma]

this is a old one brought back. could not remember posting on this then saw the date. 4 years ago.

 

[LeapFrog]

Yes, old indeed; my bad.
Still curious, because I've read 2.7-3.2 Volts for the IRF520N

 

[LeapFrog]

VGs on voltage is listed in the datasheet for the device, usually 3.5-3.7 Volts at the Gate pin is okay for most of the common devices used, IRF520, ERF2030, FQP13N10..