Rick Campbell's One Transistor Amp Article

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ssbothwell SWL:
hi, this is my first post here so forgive me if i do anything wrong. i'm new to ham radio and don't yet have my license. i've just been really busy lately and haven't gotten around to scheduling a test. i have read the ARRL license study guide and large sections of the ARRL handbook. 

i found the QST article by Rick Campbell KK7B called "Designing and Building Transistor Linear Power Amplifiers" on the ARRL handbook CD-ROM and thought it would be fun to try working through the methodology he presents. i'm not actually broadcasting anything or hooking it up to an antenna so i don't think i am breaking any rules doing this without a license.

i'm using a 2n3866 transistor which i found in a schematic for a low power linear amplifier in the ARRL handbook (page 13.24). i've gotten so far as building the LM317 bias circuit from Figure 4 in Rick Campbell's article and hooking up the transistor as in Figure 3 from the article. I am able to 'turn on' the transistor and i have gotten modest voltage gain on a signal from my function generator.

i am using a 50ohm dummy load as is recommended in the article. In the section titled Load Network (page 3) it says that with a 50ohm load the power output is a product of the supply voltage and that with a 12v supply i can get up to 1.44W of output. i assume that means i should be able to get ~8.5v P-P output across my dummy load (E^2/R=P).

i am sending a 7MHz signal at 100mV from my function generator into the transistor base along with a bias from the LM317 and i am powering the collector with 12V. my collector current and P-P voltage output to the dummy load seem to shift as the transistor heats up. i only have one multimeter so i cant look at bias voltage and collector current at the same time.

in any case the highest P-P voltage output i have achieved is 3.2V which i get when my collector current is ~30mA. the higher the collector current the lower my P-P voltage output. does that sound right?

also, will a heatsink significantly stabilize the collector current from my transistor? i don't have any to-39 heatsinks currently but i have one coming in the mail next week.

i was under the impression that i should be able to get much higher output from one transistor even without a matching network. how do i achieve the 1.44w output mentioned in Rick Campbell's article?

KA2DZT:
Welcome to the AM Forum.

I'm not familiar with the acticle you're working from, but did want to congratulate you for trying to build something.

Many hams today manage to get a license and then buy everything for a station.  They never build anything.  Most folks you'll find on this forum are builders.

I built my first radio at age 7, that was 58 years ago.  I was given (by a ham) a 1953 ARRL Handbook in 1953.  I still have the book and refer to it often.  I don't think I've mastered everything in the book but I'm going to stay on it until I do.

Keep up the good work.  I'm sure someone on here will come along with more help with your project.

The one thing I will mention,  1.44 watts on 40 meters??  I think you're going to need a bigger set.

And don't worry about doing anything wrong,  there are plenty of experienced hams on here that screw-up all the time (of course, I'm not one of them).

And I will note, that from reading your post, I can see that you have some knowledge of electronics.

Fred, KA2DZT

ssbothwell SWL:
thanks for the kind words. i'm really excited about getting more into this hobby. i do have a little experience with electronics, mainly audio stuff, but i have always wanted to get into radio. i recently decided to just go for it and have been reading as much as possible. rf electronics is a lot more complex then the kind of stuff i have done in the past!

i admire the the fancy new ham gear that is available for sale, but i really like the idea of building stuff myself. its nice to know exactly what is going on 'under the hood.'  :)

at some point, likely after i get my license, i plan to try building larger gear but for now i want to try building low power stuff to experiment with on the bench. i would like to know i can properly tune a single transistor stage before trying to add additional ones.

KF1Z:
Across your 50 ohm load you should see 20 volts Pk-Pk for 1 watt of output. ( with O'scope AFTER the DC blocking cap)

And are you measuring the the output voltage with a multi-meter? or with an oscilloscope?

If you are using a multi-meter, you likely aren't getting an accurate reading.


Are you using figure 3  in the article ?

Also,
The 2n3866 HAS to have a heatsink!
"We" use these devices for 1 watt  driver amplifiers in the SDR transceiver kits I build.

They can get very hot even with the to-5 heatsink on them.
They are good devices, but they are delicate!
\

ssbothwell SWL:
thanks for the info. i am measuring on an oscilloscope so i am getting Pk-Pk readings.

i have some heatsinks coming in the mail sometime next week and i don't mind burning out some of these transistors testing them before the heatsinks arrive. the main problem i notice with no heatsink is that as the transistor heats up, the collector current changes which causes me to have to constantly adjust the bias voltage to keep a steady power output. at this point its not a big deal because i am just testing out the circuit but hopefully a heatsink will fix this.


yes, i am following figure 3 in the article. the highest voltage i have achieved is 3v Pk-Pk which is what i get with a direct (no resistor) ground shunt from the emitter, and 90-100mA current on the collector.

The only discrepancy i notice between my circuit and that of the articles is the RF Chokes. the article does not say what inductance values to use for the chokes and so i put a 15uH one on the bias input and a homemade one made of 8turns on a FT82-43 (http://toroids.info/FT82-43.php) on the power input (like in the schematic). these are the only inductors i have available, although i have additional FT82-43s and FT37-43s that i could wind if necessary.

when testing a transistor i plan to use for the first stage of amplification, what sort of signal input should i be using? i have a nice HP function generator i got from a swap meet which i have been using. i set it to 100mV sine wave input at whatever frequency. is that the appropriate voltage i should be using to simulate the output from a transmitter's oscillator?

also, if i wanted to drive a transistor i plan to use in a different stage, what would i use? do i just use E=sqrt(P*R) [where P is whatever power i plan to input into the transistor] to determine the voltage of the function generator?

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