Re: [KX1] Variation in driver transistors (e.g. 2SC2166)

Posted by Don Wilhelm-4 on Feb 10, 2011; 2:11am
URL: http://elecraft.85.s1.nabble.com/KX1-Variation-in-driver-transistors-e-g-2SC2166-tp6010045p6010305.html

  Chip,

My experience with the KX1, K1 and K2 indicates that you will find
little variation in power gain, even when the hfe of the PA transistors
vary considerably.  There are design parameters such as the collector
load resistance that swamp the variations - and the PA output circuit is
designed for that load resistance.

Take a look at Experimental Methods in RF Design - the section on
Practical Power Amplifiers, and you will find that the most important
consideration is in the formula R(load) = Vcc (squared)/2 times Power
Out.  So the load resistance is the predominate factor in determining
the power output capability of the amplifier - as long as the PA
transistor can stand the required current and voltage for that power.

73,
Don W3FPR

On 2/9/2011 8:39 PM, Chip Stratton wrote:

> Thanks Leigh&  Don,
>
> I'm not as concerned about maxing output, though getting 4 watts on 20
> meters (like I'm getting on 80 meters) would be nice. I do like to
> experiment a little, so trying the 2SC5739 would be interesting.
>
> I am interested getting some idea about part to part variability. I have a
> lot of experience with LEDs and the variability they exhibit coming off the
> assembly line, and am wondering if other semiconductor devices - like the
> 2SC2166 - show similar variability in efficiencies?
>
> Thanks,
> Chip
> AE5KA
>
> On Wed, Feb 9, 2011 at 7:22 PM, Don Wilhelm<[hidden email]>  wrote:
>
>>   Leigh,
>>
>> Elecraft is currently shipping KX1s with the 2SC2166 again.  I have
>> built 4 in the last month and all 4 have that transistor.
>>
>> The thermal pad provides better heat conduction and should not be
>> eliminated if a plastic tab transistor is used - only the shoulder
>> washer is eliminated.
>>
>> 73,
>> Don W3FPR
>>
>> On 2/9/2011 7:21 PM, Leigh L. Klotz Jr WA5ZNU wrote:
>>> Here's some info to read:
>>>     http://elecraft.365791.n2.nabble.com/KX1-Q6-question-td662532.html
>>> and here, in this erratum superseded by the revision E manual which shows
>>> the changeover:
>>>     http://www.elecraft.com/manual/KX1_Owners_Manual_Errata_Rev_D-1.pdf
>>>
>>> It seems that Elecraft went from a 2SC2166 to a 2SC5739 for RoHS
>> compliance.
>>> A side benefit is that the new part eliminates a thermal pad, shoulder
>>> washer, and thus a potential for a short from the KX1 build.
>>>
>>> Some have reported improved output on 20M with this part, albeit with the
>>> usual warnings about too much power causing you trouble later if you
>> don't
>>> have a great match to your antenna, a strong possibility under the KX1
>>> normal operating conditions.
>>>
>>> Leigh/WA5ZNU
>>>
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