Another interesting NPN

Here is another interesting NPN part that I found on someone's
RF blog.



http://www.datasheetcatalog.org/datasheets/400/74256_DS.pdf




$1.82 in quantity of 1 from Mouser. It looks like it is good into the VHF/UHF range
and should be able to make 1W of power without crazy amounts of current. It looks
like a 10dB gain kind of part

30m Oscillator (Night 8)

For tonight the theme was trying to get more gain with less current. (This is a theme over the past several nights.)



So I started poking around in Experimental Methods in RF Design... it is pleasure
reading for knack victims. As I started browsing through Chapter 2 intending to read
up on push-pull amps I stumbled across some interesting notes. It was almost a eureka
moment except that I don't understand exactly how to calculate the target RF power
levels yet.




So far I have been largely focused on creating power based upon the DC biased values.
That is important but it does not maximize RF gain and is not as efficient as I might
like it to be. I was getting ~14.6 dBm of gain at a cost of 35mA which is not brilliant.
In the book I noticed a design almost the same as I what I have been working with
but getting almost 20dBm of gain at 20mA.




The difference in the two designs was simple:


1. Don't drive the DC design nearly as hard... I was throwing current at the problem
in brute force fashion.

2. Think about RF.... use some AC coupled feedback to create the gain.




The schematic above is my first attempt to bread board out the idea in the book against
my world and collection of available parts. It looks a LOT like the previous designs
except it is a lower current design with AC coupling between the collector/base and
at the emitter.



Now that I own a nice power meter I ran some tests and captured data.







I measured my signal generator with my power meter, then took down some base values,
and then reran the same test with the 2N3904 amp circuit. I did a little spreadsheet
magic to calculate the gain and then built a graph with the results.



Highlights:

• 
  I can generate about 16-17dBm of gain with about 25mA of power. (That is +3dBm and
  -10mA from the previous tests with no AC feedback and strictly DC bias.)


• 
  The gain is pretty consistent with -50 to -10dBm of drive source.


• 
  The gain falls off very quickly at 0dBm of drive.


• 
  As the gain falls off the amp begins to consume a lot of current to produce a small
  amount of gain.


• 
  The 2N2222 and 2N3904 appear to be good up to about 0dBm of input signal and then
  loose their effectiveness. They willl be driving up the harmonic content instead of
  the fundamental signal. (I visually saw & measured this on my spectrum analyzer
  over the past several nights of testing.)

In theory I could probably tweak the resistor values to drive another 2-3dBm of gain
& use a transformer at the RF output to reduce the loading and create a better
match.



In reality I have a good predriver circuit and need to look at some other options
for the next gain stage. (Things like class AB or class C, push-pull amps, or simply
some different parts as I start to transition out of small signal components and into
power.)



Tons of learning tonight. I feel pretty positive about the results. I need to dig
in and better understand the concept of feedback in an amplifier. (35 minutes at the
work bench.  2 hours at the PC working the data and documenting the results.)



73 de NG0R



PS... a quick follow up note after the initial post


I think that the output impedance of this is ~156 ohms.

Ie = Ve / Re  or .053 = 3.66/68  (ohms law)

Zout = (Vcc-Vb) / Ie


155.6 = (12.61-4.36)/.053    




156 ohms would make sense... you see a lot of these circuits using a 4:1 transformer.
That will transform 200 ohms to 50 ohms which is in the ball park.




PPS... another thought from yesterday's reading & bench time


Rf * Re = Rs * Rl  or  Rf * Re = Zin & Zout


Rf & Re are the AC coupled components.
(updated below... Re is the total of Re + RE)




In my schematic above I probably should switch the 1K and 3.3K resistors between the
collector and base. That would set  Zin to 50 and Zout of 200 approximately. 
Something to test. (This is designed for 200 ohms at the moment.)




PPPS.... more math


Rf is the parallel feedback resistor (the AC path)


Re is the net of Re + RE  (Re is the DC bias + RE is the AC
degeneration)

Zin & Zout (Input and output impedance.)

Push-pull amp

I have some push-pull questions so I am going to build a model something like this:






I want to play with a simple push-pull NPN amplifier running in class AB.  I
have big stash of PN2222 so I want to use it because it is on hand. (I know that it
is a crummy RF part... but this is an R&D activity for some personal growth...
I will move on to real parts once I have a bit more experience.)



PS... the I have posted some updates in red to my own
questions below.


 

Q1. What size inductor do I need for L1?

In this phase L1 is not critical to choke the RF from the
power supply.



If you use an inductor, pick a value that is going to be at least 150 XL ohms at the
lowest frequency and a self resonant frequency higher than the highest frequency you
intend to use.


 

Q2. I would guess that L1 could be replaced with a resistor since it is really controlling
the current at the collectors.   I need to understand why the example has
an inductor in there since it is not a tuned circuit.

150 ohm resistor would probably be fine.



If you use an inductor, pick a value that is going
to be at least 150 XL ohms at the lowest frequency and a self resonant frequency higher
than the highest frequency you intend to use.


 

Q3. How do I determine the output impedance? (It might be helpful to know if I want
to feed a 50 ohm filter.)

In theory it should look something like:







Hmmm... more reading before I can visit the work bench.

30m Oscillator (Night 7)

My Boonton 4200 power meter showed up today.




I went back and remeasured the circuit on the meter. I originally thought that I was
getting 10-12 dB of gain. With the Boonton I am measuring about 14.6dB of gain from
the circuit with about 35mA of consumed power.



I tried several PN2222 components and the range was all over the board. Gain was pretty
close but the power consumed was pretty wild by a factor of 3x. (I threw the the ultra
wild part away.) I also substituted a 2N3904 into the circuit. It had almost exactly
the same gain and power consumption model.



It seems like I should be able to get this much gain with 1/3 of the power consumption.
I am not sure how to do that as balancing the base against the emitter and collector
for peak gain and minimal consumption is pretty challenging. I suspect that I can
tweak the resistors a bit more but this would be easier with a dual channel power
supply with digital feedback to rough out the voltage and current values while watching
on the oscope & power meter.




---Hmmm I am still kicking myself for not buying that bench power supply that I was
eyeing during the hamfest last Saturday. I might have to resolve that via eBay or
the up coming  Midwinter Madness hamfest in Buffalo.

NPNs for making power

I looked up what the Elecraft folks are using in their K2 to create RF at some real levels. (I love my K2.)

 

PreDriver: 2N5109 ($1.87 at Mouser)

 

Driver: 2SC2166 ($3.95/12.70 at RF Parts)

-- The 2SC2166 is rated for 3-4 watts of power at HF freqs

 

Finals: 2SC1969 ($9.95 at RF Parts)

-- Running a push-pull pair

-- The 2SC1969 is rated for 10-14 watts in class AB.

-- I can confirm that a 2SSC1969 is good for 7-10 watts running by it's self.

 

My K2 will generate 10-14 watts of power with 2-3 amps of power. At rest it draws
about 300mA.

 

Interesting stuff as I work thru how to make some RF power.  I will have
to search on eBay for some of these parts as the high end parts are probably a buck
a piece or so compared to the normal sources.

30m Oscillator (Night 6)

Last night I was messing with the modeling tools and getting weird answers that did not make sense. Ultimately it was time to go to the bench and put some parts on the breadboard.




Initially I stated out with with the values listed in night 5 schematic. I was not
happy with the power levels. I chatted with N0FP a bit and we came up with some different
values to push up the gain. That presented a new issue... my circuit got hungry. At
one point it was consuming 131mA to get 18dB of gain.  N0FP suggested that maybe
I should put my circuit on a diet.



After pondering this a bit I changed the values to those listed on the schematic/picture
above. Now I am getting 10dB of gain and consuming 35mA of power. This is better but
still seems kind of hungry for the power levels that I am creating in class A.



I would like to see 10dB of gain at 6mA. I will need to ponder this some more. It
might also be time to grab a 2N3904 and compare the results against the PN2222.



Lots of learning going on... slowly moving the ball forward.

SO2R box from K1XM

SO2R = Single operator two radio

 

While my interest
my hard core contesting has diminished over time and as I become more committed to
building & testing I still enjoy some of the innovation that comes from the contest
community. The K1XM box is kind of neat. The K1XM box is need because they are trying
to establish a open source protocol/standard for how to talk to this type of interface
box.

http://www.k1xm.org/SO2R/index.html





The image & text is from the K1XM web page... go visit... it
is good reading.




I have been pondering building own mic, key,
speaker, etc kind of relay/switch box for more than a year based upon of of own requirements.
My box is just a concept with the details written down in my "idea book." For now
(ok... back in December 2009) I decided to reduce the amount of clutter on my desk
by focusing my primary station on my Icom
IC-756, Elecraft K2 and
a Small Wonder Labs PSK Warbler
for 80m.



Eventually I will probably build a matrix switching box but it will be a little ways
down the road. It is fun to read about how other people solve these types of problems
in the mean time.

30m Oscillator (Night 5)

I was not happy with the power levels from the RF gain stages. So last night I started slowly tweaking them until I decided to remove them and come up with another plan.




It will not likely put out this kind of power
at these levels... but it is a learning tool to figure out what is going on.




For tonight (time permitting after the family responsibilities) I think that I am
going to build a simple test fixture and use my signal generator to drive it with
a 0 dBm source. I did some modeling of the NPN at different bias levels. In my original
testing I was noticing some clipping of the sine wave on the oscope which looks like
I was running out of current on the negative peak. I would like to drive more gain
in the class A stage and maintain a healthy sine wave in this early stage.



I am hopping that I can use the test fixture to try to drive the NPN much harder and
get some gain out of this part. (Granted that the PN2222 is not a spectacular component
but I am well within it's rated power levels.) Originally I was trying to be too safe
and the result was low power levels. I will try to spank it a bit to see what happens.
(The PN2222 are cheap parts so I can afford to let the smoke out of a couple of them
if needed.)

30m Oscillator (Night 4)

I found some bench time after the family shared a movie Saturday night and went to bed.






I did a bunch of testing and made some changes.

• 
  I tweaked the gain stages slightly. Q3 and Q4 seem to operate more efficiently with
  a 100 ohm resistor instead of the 10 ohm resistor. I validated both values with the
  spectrum analyzer and I got a bit more gain of the fundamental frequency and less
  impact on the harmonic frequencies.


• 
  In talking with N0FP earlier in the day I discovered that my output at 50 ohms was
  loading the final transistor. I played with using a matching transformer but was unhappy
  with the result. (I think that I was getting tired and should retest the process with
  the transformer again.)
  
  
  


• 
  I also tried changing the output from a common-emitter to a common-collector. The
  common-collector design has a lower output impedance and was a better match to the
  50 ohm load. It was a not a perfect match @ 4:1 but it was pretty decent and reduced
  the amount of loading. (In the end I left it as a common-emitter design for now.)


• 
  I then relocated the low pass filter from after Q2 to it's new home after Q4. I tried
  using a 4:1 transformer from the collector to the input of the filter but was running
  into some flaky issues. I experimented again with common-emitter vs. common-collector
  to try to get a better match. In the end the common-emitter design worked pretty well
  so I left it alone.

Results:

• 
  Power is now running about  3.2mW or ~ +5dBm. (I referenced the spectrum analyzer
  against my signal generator and then compared the final output of the transmitter.)


• 
  My harmonics are down 55dB against the fundamental frequency. (It would be legal to
  put on the air as is.)

Next steps:

• 
  Look at some more options to increase the power into the 100-200mW range.


• 
  Add a key (RCA or 1/8" jack) so that I can try to make a QSO with it.
  
  

Bifilar transformers

Here is a nice description of bifilar transformers.

http://www.qrp.pops.net/xmfr.asp






Here is another very nice article about transformers including in a push-pull NPN
design.

http://www.oselectronics.com/downloads/Broadband%20Transformers.pdf