Output stage on FM-10c

[KB0TTL]

Other FM transmitters such as the FM-30 MP3 transmitter have a variable RF output of up to 25 miliwatts which is a darn sight better than the 8 miliwatts which the fm-10c puts out. My question is this: Are there any transistors out there that would give the FM-10c more output power if substituted for Q1??? In theory??? This would be an experimental mod that I would like to try, especially if it enabled my fm-10c to put out 25 miliwatts!!

[radio8z]

A well designed amplifier circuit is almost independent of the active device used, especially at milliwatt levels, since the operating parameters are basically set by the passive components and the supply voltages. This is not the same as souping up an old hot rod with a bigger engine. Substituting another transistor will not help. Try it if you want but you will be wasting your time and money.

Your transmitter is already capable of supplying enough output power to exceed the FCC limits (which are on field strength and not power) in the US.

If your transmitter supplies 8 mW. to the antenna, and you are using a dipole or a similar antenna, you are already about a thousand times above the legal field strength.

Neil

[ June 06, 2007, 06:37 PM: Message edited by: Neil 8Z ]

[DigitalJunkie]

There's alot of info on the FM-10 online.

http://www.repairfaq.org/filipg/LINK/F_FM10.html

Here's a simple amplifier stage you can add to boost the output a bit. Be careful! 70mw is potentialy illegal with a half-decent antenna.
http://www.repairfaq.org/filipg/LINK/F_FM10.html#FM_016

[radio8z]

Yes, and quoting directly from one of the links you posted:

Here is a rough compilation of information about the Ramsey FM-10, and other BA1404 Stereo FM broadcasters. Some of the modifications may make your BA1404 based broadcaster illegal to use on the open airwaves in the US and Canada. Also it has been brought up that the stock Ramsey FM-10 kit may exceed FCC power limits when used with a proper antenna.

There are no power limits for part 15 FM in the US.

I also wonder what they meant by "open airwaves"...are there closed airwaves?

Neil

[ June 06, 2007, 08:14 PM: Message edited by: Neil 8Z ]

[kb3cs]

if you violate Part 15 FCC Rules and Regs with an over-the-field-strength-limit transmitter, you will likely lose your Part 97 FCC license. Proceed with caution KB0TTL!

[KB0TTL]

I was not intending to use such a circuit on the open airwaves, but rather to do some oscilloscope testing into a dummy load. Actual application of this project would come into play in South American countries which do not fall under the same jurisdiction. In DeVry, we learned that there are many types of transistors out there, some which provide different amounts of gain from one to another, even when in the same configuration. I was just checking to see wether or not anyone knew of a transistor that may boost the power up a milliwatt or two. Who knows, there may actually be one out there! Even the website you provided states that the transistor in this amplifier can be substituted with others which would yeild different results, thus somewhat proving my point!

See my site at www.freewebs.com/franitzasab and use the guestbook to discuss this and other RF theory! Christian Radio Project.

[radio8z]

KB0TTL,

Since you are aware of possible FCC problems I can offer some suggestions.

From your DeVry studies, you may recall that for a class A amplifier the gain is the ac collector resistance (including the load) divided by the dynamic emitter resistance (re'). You can lower the re' by increasing the Q point current (change the bias). Since I don't know what circuit the FM-10 uses I'll let you be the judge on this approach. Note that if the fT of the transistor is high enough then the circuit gain does not depend on the hFE. That's what prompted me to advise that swapping the transistor wouldn't help. You might also read about the Miller capacitance and see if that can be reduced somehow (such as selecting a trans. with a low Ccb), since for high gain amps. this usually sets the dominant corner frequency.

For a class C circuit, the transistor will make a difference, yet again, I do not know what the FM-10 uses. So if this is running class C, a part swap may help. Select one with a low emitter bulk resistance.

Maybe the easiest way to gain some milliwatts is just to increase the supply voltage. The power will go up about proportional to the voltage squared but, as you know, damage may occur.

Neil

[ June 08, 2007, 01:29 AM: Message edited by: Neil 8Z ]

[KB0TTL]

Then perhaps the best thing to do would be to obtain some BA1404's and just breadboard one of these things on a solderless breadboard (in case something does fry). This would then allow for an easy swap of Q1, as well as change of parts values to cope with increasing the voltage. Perhaps an NPN RF power transistor such as one found in the finals of a CB or junk 2-meter rig would suffice for higher voltages as well. The main thing would be to make sure that there is enough resistance present to keep the BA1404 from frying, yet little enough resistance so that the BA1404's output is still enough to drive Q1. I have a bench supply that goes up to 15vdc at 3a which would work fine for such a test. I have built a few of these from kits, one was an original fm-10, one was an fm-10a. I believe one of the manuals even states that you can use up to 15v, however I am not sure of this. The manual for the fm-10a states that up to 12v can safely be used. I would think 15v (3v difference) would make little difference but I could be wrong. Sometimes logic gates such as the ones present in the BA1404 may not tolerate the difference. Also the electrolytic capactiors should probably be replaced with ones of a higher voltage tolerance! As you had mentioned (and you are right) the different classes of amplifiers differ in response to different transistors, thanks for the reminder.

[DigitalJunkie]

I wouldn't use a solderless breadboard for RF work,the stray capacitance and inductance from the connections will cause lots of issues.
One possibility might be to build it over a piece of copper-clad PCB,to use as a ground plane,and use the "dead-bug/Manhattan" construction style.

[radio8z]

DigitalJunkie is right about the solderless breadboards. They are just not suitable for high frequency work. I have used "dead bug" construction (google this) for projects up to 800 MHz. with success.

If I recall, the BA1404 has a low operating voltage. A few designs I have seen use three or four diodes in series, forward biased with a series resistor to drop the high supply voltage down to the 2 or 3 volts required for this chip. This operates pretty much like a Zener diode.

Neil