EMRFD Message Archive 3218

Message Date From Subject
3218 2009-06-19 06:48:54 chrisethompson First Transmitter
I'm trying to progress from a competent kit builder to an experimenter and it's proving to be difficult than expected... After several different dead ends (circuits in progress that are not yet working) I have gone back to basics and constructed the First Transmitter from Ch 1 with the intention of really understanding how it works.

Of course, it does not quite work at the moment. It oscillates well but the buffer did not drive Q3. There was no signal on Q3's base. I removed the 100ohm base resistor (which provides load in the negative part of the cycle as I understand it) and Q3 was then driven with 4Vpp. Hmm, so I tried 470ohms, which gave me about 2Vpp and 1K which gave me 4Vpp. This gives me 23.8Vpp at the output of Q5 across a dummy load - or 1.4W (if my calcs are correct).

So what is going on here? Why do I need a higher load impedance?

Note that I have NOT used 2N3904. I used BC547s because they were in the junk box.

3227 2009-06-20 21:00:02 Wes Hayward Re: First Transmitter
Hi Chris,

I'm sure that Rick and Bob are with me when I say that we are pleased that you are making the transition from building kits to some design and homebrewing. It sounds like you are asking the right questions and making the right measurements. You didn't say what part you were using for Q5. It must be something other than a BC547. As I recall, I didn't get more than a couple of watts from that MJE181.

The 100 Ohm resistor in the base of Q3 is a load to prevent excessive reverse voltage on the base of Q3. It may help with amplifier stability. Changing to a high load is consistent with what you might need to do to get the output up.

Another thing to try is to move the point where drive is extracted from Q1. As shown in the book, drive for Q2 is extracted from the base of Q1. You might get better results with the 100 pF cap on the emitter of Q1.

The one thing that I noticed when I downloaded a data sheet for the BC547 was that the pinout on the BC547 is just the opposite of a 2N3904. In fact, I wondered if there was a misprint, so I looked at sheets from several manufacturers. If you ended up with the parts in backwards, they might still work. However, the reverse base-emitter breakdown might get you in trouble. Also, the beta would probably be extremely low. I recall an old friend (w7dra) purposefully using some transistors connected in such a backwards fashion just to obtain the very low beta. He used them in regen receiver applications and found that he got smoother regenerati
3228 2009-06-21 12:33:21 Ed - K9EW Re: First Transmitter
Hi Chris,

Just came across your "First Transmitter" email, and wanted to offer any
assistance that might help. I just built the circuit in EMRFD (with
2N39034's), and I can make any measurements in my circuit as a reference if
that would help. I did not build the 2W PA (yet). The "As-Built" circuit
only produced about 100mW at 12.6V, and I had to reduce the driver emitter
resistor from 150 ohms down to 68 ohms in order to get sufficient drive to
Q3. I then tweaked the output impedance match on Q3 (the shunt C and series
L that go to the low-pass filter), and now get about 275mW.

Let me know if I can make any measurements that would be helpful, and keep
us posted on your progress. I do want to get up to the 2W output

ed - k9ew

3229 2009-06-21 12:36:50 Ed - K9EW Re: First Transmitter
Here's a picture of the circuit. I added a tuning cap to warp the crystal,
and a transistor to key the B+ with a standard key-down input.

3230 2009-06-21 14:22:19 chrisethompson Re: First Transmitter

Thanks for the reply. It's a fantastic book btw and I am really enjoying working through it.

Luckily I had noticed that the pin out of the BC547 was different, so I have them in the right way around. I used a 2N3866 for Q5 - it was what I had to hand. I'm realizing I need to pull it's datasheet and compare it's characteristics.

I'll experiment with taking the signal from the emitter of Q1, I didn't try that. I also saw the document that you put out with test voltages from a DVM, RF probe and Scope. I'll compare my results and see if I learn more from that too.

I built the output filter and I now get a clean sine wave output, but the power is again cut in half. Does that mean half of the power I was measuring was in the harmonics? Is that sort of reduction normal? Or have a built the filter with the cutoff too low (or some other error..) I don't have a signal generator to characterize the filter, but I can make a simple oscillator to measure the coils and see if I wound them correctly.


3231 2009-06-21 14:28:55 chrisethompson Re: First Transmitter
Thanks Ed,

Frustratingly I'm now on a business trip and not in front of the workbench. I hope to get back to this shortly. I did build the PA, though I used a 2N3866.

The picture did not come through. You probably need to post it into the Photos secti
3233 2009-06-21 22:03:22 Rick Re: First Transmitter
Hi Chris et. al.,

Just to add my 2 cents worth--I use that circuit to introduce RF Design to my Portland State University engineering students. Some of the problems I assign are: Increase power to 1 watt output without adding another stage; reduce power supply voltage to 9 volts while maintaining 1/4 watt output; add a frequency doubler to obtain 14 MHz output with the 7 MHz crystal...etc.

It's an ideal circuit to learn non-kit radio techniques. As you learn about the circuit you can easily test your ideas.

My students use a flashlight bulb as a load, so it's easy to detect the bright ones.

Best Regards,

Rick kk7b