power amplifier(c class) in FM transmitter

fm transmitter amplifier

Hi people
I have a fm transmitter(it works in 100 mhz) it has a c-class rf amplifier in last stage(see to figure)
**broken link removed**
now I have some question about this:

1)how do it works?
2)how can i measure output power and reach to maximum power in output?
3)how can i match it to half-wave dipole(73+j42.5) with a 75 ohm coaxial as transmission line?

very very tanx

hem passagen transmitter

I don't think it will work as you have drawn it. The inductor is a DC short across the base-emitter junction so the transistor will never turn on. You need a capacitor between the inductor and the base of the transistor. During the peak of the cycle of the input signal, the voltage must be larger than Vbe. This pulses the transistor on at the 100MHz rate. The LC in the collector should be tuned to 100MHz and the stored energy in the inductor will fill in the pulsed waveform and make it look more sinusoidal. You can then use a PI network at the output to match it to your antenna.

class c amplifier fm

Your amplifier it will work perfectly in Class C.
To bias a transistor for Class-C operation, it is necessary to reverse bias of base-emitter junction. External biasing is usually not needed, because is possible to force the transistor to provide its own bias, using an RF choke from base to ground.

class c power as fm amplifier

tanx man
other question:
i wanna to measure value of inductor that is a 5 turn of enameled wire
i do a series RLC resonance test but i think that is not correct

drive a class c amplifier

farshid-ar said:

Hi people
I have a fm transmitter(it works in 100 mhz) it has a c-class rf amplifier in last stage(see to figure)
**broken link removed**
now I have some question about this:


2)how can i measure output power and reach to maximum power in output?
3)how can i match it to half-wave dipole(73+j42.5) with a 75 ohm coaxial as transmission line?

very very tanx

Click to expand...

2) Put 75 Ohms 1W resistor between right side of output capacitor (27pF) and ground, then attach anode of a diode (the best would be germainum one, AAxxx) to the same spot then one end of analogue instrument to the cathode and another to ground. Tune L and drive circuit for highest voltage reading on the instrument.
3) half-wave dipole impedance should be 75 Ohms so no impedance matching should be necessary. You can however atach balun to it but it is not so necessary.

amplifier c

one question:
what is role of C2 and L1(rfc) exactly in this circuit?

c class rf amplifier

please help me

class c power amplifier

C2 is just a coupling cap and together with L1 work like a high-pass-filter ..
On top of that, L1 provides DC reference for the transistor's base ..

Regards,
IanP

power amplifier rf class c

Hi Farshid-ar,

You can find good theoretical and practical info at many places on the internet for Class C amplifiers, see links like:

http://hem.passagen.se/communication/clc.html and

http://hem.passagen.se/communication/pa313.html

and an interesting discussion on the formula that helps calculate the output impedance at given power supply voltage and output power needs:

**broken link removed**

(If this links breaks down into two lines then copy and paste them into your browser.)

Also a practical amplifier schematics is here:

**broken link removed**


Now on your questions: The role of C2 and L1 is answered, my addition is that L1 helps to "discharge" the base-emitter junction when the transistor should be switched off. There are circuits where a low value shunt resistor is used instead of the rfc choke, this dissipates from the input power of course but helps to stabilize the amplifier and reduce parasitic oscillations.

Regarding the measurements of VHF air cored coils needs good instrument capable of accurate reading in some 10s to 100s nanoHenry range and this is not easy to find. For single layer solenoids there are pretty good formulas to arrive in the ballpark by calculations and then you can tune the coils by spreading or squizing their turns by hand and watch the change in the output level (preferably selectively at your working frequency). There are many pages on calculating air core coils, see for instance links:

http://www.ee.surrey.ac.uk/Workshop/advice/coils/air_coils.html or

http://www.moshier.net/coildoc.html

Finally I can see a C=10nF capacitor in parallel with the L at the collector, I guess it is a misprint? 10nF is much too high value at 100MHz frequency.

Regards
unkarc

rf choke rfc fm

Hi

Is a simple circuit and it will work..........provided you have enough drive at the input to make the Class C happen.

If you are using low power things , as I suspect, you will not succeed in Class C. Then it is better to make the amp in class A with adding a base resistor for proper Class A setting.

rgds
from Mike

fm transmitter ampilifier

unkarc said:

Finally I can see a C=10nF capacitor in parallel with the L at the collector, I guess it is a misprint? 10nF is much too high value at 100MHz frequency.

Regards
unkarc

Click to expand...

very very tanx
yes it is 10pF and not 10nF

100mhz fm power amplifier

thanks

a/c powered fm transmitter

tnx for your replies
i have a other question?
I need to simulate it by proteus or orcad(or spice)
what do I do?

fm transmitter class c amplifier

Hi Farshid-ar,

All I can advise is you have to use a circuit simulator that includes a good radio frequency model of your active device (bipolar transistor or power MOSFET) you intend to use at 100MHz.

Two useful links (out of many) on the simulation of Class-C amplifiers:

http://www.intusoft.com/articles/classc.pdf
http://www.eznec.com/Amateur/Articles/Class_C_Matching.pdf

By the way, Intusoft has a useful page on Spice based topics:

http://www.intusoft.com/

By the way, if you have access to using proteus or orcad, you could first make sure whether they include your active device or are able to import/use Spice models you can find on the internet like here: http://www.intusoft.com/slinks.htm

Here is link also to a Spice based software, the limited version is free or you may choose a 30 day long free trial version too:

**broken link removed**

but I DO NOT know how good (or bad) this software is! Some comments on its full version usage is here:

http://www.beigebag.com/testimonials2.htm

If you are "brand new" to (mainly Spice based) computer simulations, look for tutorials on the internet, there must be many papers on it.

Good luck,
unkarc

fm . a class c amplifier

very very tanxs man