[Lowfer] Lowfer Final Transistors-Switching Time

[Bill Ashlock]

 "Bill, where can I find your pp output circuit you mentioned today?  I don't have LOWDOWN that far back."

Don't feel alone, Lee, I never DID see the LowDowns with my articles because I was not a subscriber at the time. (Hint, John? :) This should get you a schematic of that circuit with some explanation:   Defining and Refining the Lowfer TX Loop Antenna  BTW, check the plots of current gain at increasing current in the data sheets for the 2N4401/4403 and the MPS6602/6652. The MPS6602/6652 pair has higher current gain at currents in excess of 200ma. It is possible, however, that the MPS6602/6652 is really the same as the 2N4401/4403 but is (or was) computer selected for this parameter. Also it IS important to balance the two halves in order to reach the highest possible output power at 1W DC input power. You should see a small glitch in the leading and trailing edges of the square wave output where the cross-over in transistor conduction occurs. This should fall half way on these edges. Invariably the 2N2222/2907 pair will be way off in my circuit unless the balance pot is rotated to one extreme.

Bill





> From: pulsarxp at embarqmail.com
> To: lowfer at mailman.qth.net
> Date: Mon, 2 Aug 2010 20:52:35 -0500
> Subject: Re: [Lowfer] Lowfer Final Transistors-Switching Time
> 
> I looked everywhere for 2N6601 and 2N6651 transistors.  Nowhere to be found. 
> These seem more unobtainable then MPS6602 and MPS6652.  However, you can 
> easily buy zillions of 2N4401 and 2N4403 transistors  for plus and minus 4 
> cents each.  I just ordered 200 each from Mouser plus a few NTE128 and 
> NTE129.  I also ordered a few Zetex ZTX653 and ZTX753.  I heard they worked 
> good, but getting educated on here regarding turn on and turn off times plus 
> saturation figures don't make the Zetex ones look good on paper.  However, 
> we will see!  I bought a few of the Zetex anyway.  I'm trying to source some 
> MPS 6602 and MPS 6652 now.  I might get lucky but have not been given a 
> price for them to date.
> 
> Bill, where can I find your pp output circuit you mentioned today?  I don't 
> have LOWDOWN that far back.
> 
> Lee, w0vt
> pulsarxp at embarqmail.com
> 
> 
> 
> 
> 
> >
> >
> >
> >
> > 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 curre
> > nt 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
> >
> 
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