EMRFD Message Archive 5399

Message Date From Subject
5399 2010-11-08 20:28:13 w7zoi BD139 RF Power Amp.
Hi all,

A couple of days ago I received an email from Steve, G0FUW. Steve had built the "Updated Universal QRP Transmitter" originally described in QST for April, 2006, with vital follow-up data in Tech. Correspondence for July, 2006. The rig originally used a Panasonic 2SC5739 in the PA, with another as a driver. But as luck would have it, the QST piece hardly made it into print before the part was discontinued. Steve was in the same boat with the rest of us; he could not find the 2SC5739s. So he had used the BD139 instead with good results. This reminded me that a year or two back I had purchased some of them to test just for this application. (See posting #3861.)

I found the parts in the semiconductor shoe box. I then dug into another corner of the junk box and located the original transmitter. Upon opening it, I discovered that it had some other parts in the PA and driver, the KSC2690A, another inexpensive plastic medium power part. That part had been a disappointment.

Here is a summary of the results that I obtained for RF power at 7 MHz. In all cases, the same part type was used in the PA and driver. The rig was the transmitter pictured in QST. Vcc was 12.3 volts for these measurements.

Part Power Out
KSC2690A 2 W
2SC5739 7.6 W
2SC5788 7.6 W
BD139 6.15 W

The 2SC5788 is the same part as a 2SC5739, but without a mounting hole. It's also nearly extinct.

After the measurements at 7 MHz with the BD139, I changed the band determining parts and did measurements at 14 MHz. The result there was 4.75 W.

I have not tried the BD139 on any of the higher bands, but these results suggest that it should work. One might have to hit the base a bit harder to get the design value of 4 W at 28 MHz with this particular transmitter. The price tag is amazingly low for this part and they are readily available in the usual catalogs.

The pinout for the BD139 is opposite that of the 2SC5739.

73, Wes
5401 2010-11-08 21:56:53 Ashhar Farhan Re: BD139 RF Power Amp.

in these parts, it has been a pretty common transistor in qrp rigs. BD137
and BD139 have been used quite commonly. they were probably designed as
driver transistors (you can get a matched pair as well with BD136 and BD140,
if i remember correctly)

the lore has been that it performs much better with a very slight bias (even
for class C). though, i haven't personally checked this. it has a high Ft,
and a proclivity to give up the ghost on heating up. i tried using it as a
driver at 14MHz but it was not behaving properly (there's probably too much
capacitance between base and emitter, i couldn't confirm then because i
hadn't built the carlson bridge by then).

- farhan (VU2ESE)

5402 2010-11-09 05:16:26 Bob Re: BD139 RF Power Amp.
Hi Farhan -

Interesting post, but what's a "carls
5404 2010-11-09 06:37:41 tetranz Re: BD139 RF Power Amp.
Hi Wes and all

I think post #3861 was a reply to me about the same subject. As a result of that, I can report that I've been happily using the BD139 in the Updated Universal QRP Transmitter design on both 80 and 40m.

Here's my latest effort using the RF stages from that design combined with the simple NE602 / LM386 direct conversi
5405 2010-11-09 09:51:58 Tim Re: BD139 RF Power Amp.
5406 2010-11-09 10:03:24 Tim Re: BD139 RF Power Amp.
5407 2010-11-09 13:15:47 aa7ih_ray DDS-60
Hi All,
In a recent post, Ross mentioned that he is using a DDS-60. Interesting!

I'm tinkering with a design for a digitally controlled RF Sweeper. The oscillator I selected is a DDS-60 being controlled by an 87C52. Failing to find any software for the 8051 Microcontroller Family, I wrote my own. This software is still in a very preliminary stage but does a great job of setting frequency value for the AD9851.

I've been evaluating my DDS-60. I've found numerous problems.

*78M05 which is the regulator for the Reference Oscillator and DDS chip is too close to these parts and generating too much heat. So much, in fact, that adjacent components get finger burny-hot. Years of switch-mode regulator design experience taught me that once you pump heat into a PC board, it's very hard to extract it in a controlled manner.

*The 30 MHz Reference Oscillator is a 100 ppm part, meaning that the frequency of the oscillator can be off +/- 3 kHz and still be in spec. This makes the DDS output off frequency a proportional amount. If the oscillator was stable, an offset could be inserted, but it's not. The hot Vreg makes the oscillator frequency relatively unstable, trumping the offset notion. Reference Oscillator and DDS output frequencies remained in the 70 to 80 ppm range with outputs from 1.8 to 60 MHz but not really good enough for any application in my shack. It's difficult for me to imagine that the raw DDS-60 could or should be used in any transmitter or receiver application.

*A leveled output is probably too much to expect from such a simple circuit. Some sort of leveling circuit will be needed before the DDS-60 can be useful to me.

*One might hope that the output stage, an AD8008 Op-Amp, could be immune to 12 V power supply ripple. It's not! The ripple and noise out of an unregulated wall-wart appear on the output. Blah! A well regulated 12 V supply seems to cure this problem.

Thanks to the TEK Country Store, I have a 25 MHz TCXO, or should I say, what I believe to be a 1 ppm TCXO. So far, a four day study on my bench has confirmed that the oscillator stays within 20 Hz. This will be a good candidate to replace the truly awful, crummy 100 ppm 30 MHz part. Only a slight software tweak required.

I shall replace the 500 mA 78M05 with a 7805 with heatsink. (Yes, I'm well aware that the current through the regulator is on the order of 130 mA.)

Now what I need is a good broadband AGC circuit. Any suggestions?

Ray, AA7IH
5408 2010-11-10 01:53:39 victor Re: DDS-60
Hi Ray,
I have built just such a scalar network analyzer with a DDS-60.
I changed the voltage regulator to a 7805 which is standing upright from the PCB and attached a medium size heath sink to it. It certainly helped with the heath problem.
I discarded the amplifier at the DDS output and the output was taken directly from the DDS output low pass filter. I even added a 6dB attenuator to stabilize the output impedance of the generator.
My reason for discarding this amplifier is that when using this RF sweeper with a AD8307 broadband RF detector, having harmonics at -30 to -40dB down (typical for the output amplifier) will limit the dynamic range of the measurement (it depends on the frequency response of the DUT - Device Under Test).
In my case the DDS spurs limit the measurement dynamic range, which in my experience is in most of the cases at least 60dB.
I did use a RF leveler by detecting the RF level with a simple diode (Schotky) detector connected to an integrator (with a reference dc level at its input) and used a FET connected in parallel to the resistor that controls the DDS output level. It works beautiful, now I get a flat RF level from 400KHz to 60MHz.
I used a graphic display designed by S53MV (he designed it to functi
5409 2010-11-10 18:46:36 Ashhar Farhan Re: BD139 RF Power Amp.
bob, sorry :)

it is the caron bridge. it is one you had recommended to us last year.

- farhan

5412 2010-11-11 15:38:58 aa7ih_ray DDS-60
Robert, Victor and Glen, Thank you for responding to my DDS-60 post. Your suggestions are very helpful and much appreciated. I think I have enough info now to move forward. Thanks again.
Ray, AA7IH