[Lowfer] Lowfer Final Transistors-Switching Time

[Bill Ashlock]

Hi All,

I spent a lot of effort to determine the best complementary output pair when I was experimenting with TX loop antennas for my loop articles that appeared in the LowDown some 7 years (?) ago. The main requirement at that time was that efficiency at a 1W DC input had to be as high as possible. Normally a few percent loss in power would not result in much loss in transmitted distance covered but I didn't want to add another variable in any of the very close loop comparisons I was making. I looked at the 2N2222/2907, the 2N4401/4403, the 2N6601/2N6651 and a number of others - in fact any  switching transistors that I could get my hands on. Turned out the 2N6601/6651 was a cut above the rest in the output configuration that appears in my second article. This circuit contains two additional base diodes from the popular Kohler circuit to reduce the collector conduction at saturation and thus reduce the power-eating turn-off transition where the transistors see both voltage and current at the same time. I found that with the right transistors the efficiency of this circuit to be very close to 100% as measured both from the RF and DC powers and the increase in case temperatures from 0 power to a continuous 1W. The second place pair turned out to be the 2N4401/4403 but the current gain at upper current levels did not hold up as well as the 2N6601/6651. The popular 2N2222/2907 was acceptable in most cases but did not balance all that well in my circuit with the 2N2907 requiring more base drive than the 2N2222. BTW, I found that using a Vcc in the area of of 24 volts to be preferable over approx 12V because the collector current is 1/2, and you will see faster switching times and less falloff of current gain at the upper current levels.

The last time I looked for a source for the 2N6601/6651 I was disappointed to find that ON Semiconductor (previously a part of Motorola) did not manufacture them. I found some other sources but the data sheets didn't exactly give me the feeling that these were the same device. I would appreciate knowing what luck others have had in finding these.

Bill Ashlock

> From: weazle at hurontel.on.ca
> Date: Sun, 1 Aug 2010 21:51:03 -0400
> To: lowfer at mailman.qth.net
> Subject: Re: [Lowfer] Lowfer Final Transistors-Switching Time
> 
> Hi Andy, Lowfers,
> 
> That's some useful data that you posted.  I can't find it right now, but I recall from several years back that Bill Ashlock had found a
> pair of transistors in the TO-220 package that worked nicely and where readily available.  What package are the 4401/4403's? 
> 
> 73, J.B., VE3EAR
> 
> LowFER Beacon "EAR"
> 188.830 kHz. QRSS30
> EN93dr
> 
> 
> 
> 
> 
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