how to increase FM transmitter output power?

[Ziddik]

hi

is it possible to use the oscillator transistor(ie.2n3904,bc547,549) of a low power(100m or 300m range) Fm transmitter in darlington mode to get more output power? Is darlington mode possible on RF circuits?

 

[ronsimpson]

Darlington is a slow amplifier. Not usually used in RF.

 

[Ziddik]

a darlington mode is involved with 2 transistors to get higher gain than 1 transistor can provide..

Here for example..
**broken link removed** the emitter of first transistor is fed to the base of second transistor so it acts as just a one transistor with higher output gain

 

[ronsimpson]

I know that. A darlington is slow. Too slow for FM. You will not get watts through a 2N3904.

 

[carbonzit]

Reread what he wrote: a Darlington is a slow amplifier. As in S-L-O-W. As in not suitable for RF.

 

[ronsimpson]

hi

(100m or 300m range) Fm transmitter?

how many threads on this?

 

[Ziddik]

I know that. A darlington is slow. Too slow for FM. You will not get watts through a 2N3904.

i dont want any watts out from 2n3904, the real question is, is it possible to use a DARLINGTON MODE

 

[carbonzit]

i dont want any watts out from 2n3904, the real question is, is it possible to use a DARLINGTON MODE

That question has been answered, TWICE.

If you don't get it, you really are stupid.

 

[Ziddik]

Reread what he wrote: a Darlington is a slow amplifier. As in S-L-O-W. As in not suitable for RF.

don't u ever post your own comment?

 

[Ziddik]

That question has been answered, TWICE.

If you don't get it, you really are stupid.

i hav never got any helpful reply from u ignorant man..why dont u go and play with a 1.5v battery and some Leds? ..u r the real stupid as u never ever post ur own comment but repeating on one's comment

 

[Ziddik]

Why is a darlington transistor becomes slow?

 

[colin55]

You possibly mean two transistors in PARALLEL not Darlington.
No. Two small-signal transistors in parallel does not increase the output power in your particular circuit.
The only way to increase the power is to pass the signal to what is known as a LINEAR AMPLIFIER.
Here is a circuit with an amplifying stage after the oscillator:
**broken link removed**

 

[Ziddik]

You possibly mean two transistors in PARALLEL not Darlington.
No. Two small-signal transistors in parallel does not increase the output power in your particular circuit.
The only way to increase the power is to pass the signal to what is known as a LINEAR AMPLIFIER.
Here is a circuit with an amplifying stage after the oscillator:
**broken link removed**

thats a nice circuit but i have made it and it works just fine.. About 300meter of range

 

[ronsimpson]

Why is a darlington transistor becomes slow?

The input transistor can turn on the output transistor but it has no way to turn off the output transistor (fast). To turn off, it starves out the output transistor. There are capacitance in all transistors. The output transistor's base capacitance will keep it on for a long time. It needs something to actively pull down the base to turn it off.
There is also a B-C capacitor in both transistors. When the transistors are turned off the C-voltage starts up. This causes current to flow in the B-C capacitor back to the bases and turns back on the transistor. The input transistor has very high input resistance so it is easy turned back on by this "miller" capacitor. The output transistor has nothing to pull it's base down so it may stay on for a long time.
Long time might be uS but in the RF world a uS is forever.

 

[audioguru]

The FM transmitter with the short range will have a much farther range to a very sensitive FM radio in an expensive stereo or in a good car radio. Also the antennas must be good and aligned at the same angles.

The circuit has low output power because its supply voltage is very low, its oscillator provides a weak input to the output transistor and the 150k base resistor provides low base current.

You asked about using a darlington transistor to boost the output power. Then why didn't you look at a datasheet?
Here is the specs on the datasheet of a small darlington transistor. At 1kHz its minimum gain is 20,000 but it is only 1.5 at 100MHz over a narrow range of current. So its output power is almost the same as its input power and it is useless.

 

[Ziddik]

The input transistor can turn on the output transistor but it has no way to turn off the output transistor (fast). To turn off, it starves out the output transistor. There are capacitance in all transistors. The output transistor's base capacitance will keep it on for a long time. It needs something to actively pull down the base to turn it off.
There is also a B-C capacitor in both transistors. When the transistors are turned off the C-voltage starts up. This causes current to flow in the B-C capacitor back to the bases and turns back on the transistor. The input transistor has very high input resistance so it is easy turned back on by this "miller" capacitor. The output transistor has nothing to pull it's base down so it may stay on for a long time.
Long time might be uS but in the RF world a uS is forever.

thanx ron.. This information is very helpful

 

[Ziddik]

The FM transmitter with the short range will have a much farther range to a very sensitive FM radio in an expensive stereo or in a good car radio. Also the antennas must be good and aligned at the same angles.

The circuit has low output power because its supply voltage is very low, its oscillator provides a weak input to the output transistor and the 150k base resistor provides low base current.

You asked about using a darlington transistor to boost the output power. Then why didn't you look at a datasheet?
Here is the specs on the datasheet of a small darlington transistor. At 1kHz its minimum gain is 20,000 but it is only 1.5 at 100MHz over a narrow range of current. So its output power is almost the same as its input power and it is useless.

thanx guru.. I got it now.. N I must try 2n2219a or something else

 

[Ziddik]

the main problem is high power RF transistors are not available in my local shop.. They have got only 2n3866 and that expensive 2n2219a..

 

[em_iran]

hey,guys,come on!,get off one`s high horse!
it's not dificult to amplify FM power at all,you only need a vhf amplifire transistor ,I would recommend you to use 2SC2101,it has high gain & suitable for FM mobile radio output stage,first you should preamplifire your oscilator output in order to getting power to at least 0.5W(for more than 5w output),then hand it on to P input,for Vcc=12.5 V and Pi=0.5 the output power will be about 7W!you can find details on relevant dataseet,





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[WTP Pepper]

OK. You have been told a Darlington pair are is not the answer. However if you choose a BRF91/BFR96S transistor in the output stage then you may get the range. These are cheap medium power transistors that are certainly up for the job.

You do not need watts of power to cover 300m with the right aerial. 50mW would be OK. A 2N3866 would be OK in an output stage.


WTP