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long range RF amplifier by parallel connection of 3904
Is it possible to make high watt fm transmitter for long range using 2 or 3 low power 3904 transistor connecting in parallel as an out put RF amplifier?
It is illegal in most countries to use a transmitter on a broadcast band without a license and without using an approved circuit.
An unapproved circuit will probably have its frequency wander and cause interference with a real radio station.
The harmonics might disrupt police, ambulance and airplanes radio communications.
I know, transmitting long range is illegal but i think it's not illegal to share and get idea about transmitting long range RF. It means, it is also illegal to learn about how FM station can transmit their signals as far as much. I'm a person in support of RF cops and my question was not about ligel or illegal. I just want to learn how can i amplify low watt RF signal into high watt using low watt transistor 3904, just like general power amplifier. Sorry my ideal !
You do not learn by causing illegal interference.
You learn about RF power amplifiers by analysing working approved circuits and reading the datasheets for the powerful output transistor.
you have to take into account that the junction capacitances are 8pf for Ccb and 4pf for Cce. paralleling two transistors will give you an input capacitance of 16pf, which has a reactance of 99 ohms at 100Mhz. this reactance reduces gain by providing negative feedback through the capacitance.
You can talk all you like about junction capacitances. Paralleling two transistor may have worked, despite all the theory. But unfortunately it doesn't.
You can talk all you like about junction capacitances. Paralleling two transistor may have worked, despite all the theory. But unfortunately it doesn't.
That depends on the transistors..... There are plenty of examples of transmitters operating from HF <3MHz to low microwave bands say 1200 MHz using paralleled transistors
And to colin55. Dunno where you got that idea from. Paralleling appropriate transistors for the freq of nearest pretty much does, at least close to, double the output of a transmitter
It's been done commercially for years!!
My 1.8 to 50 MHz transmitter is using transistors that on their own will produce ~55 - 60 Watts out will happily run at 100 - 110 Watts parallel
And again my 144 - 148 MHz amplifier runs a pair of transistors rated at ~ 65 - 70 each Will easily produce 120W in parallel
To the OP .... The transistor you have commented on is neither a hi power device nor is it likely to operate at100MHz very well. But there are plenty of other choices that will
I'm searching that- how can I transmit as far as much using 3904 with more watt RF output. Because I cannot find any high power RF transistor like 3866. So I'm searching alternative idea. Show me a circuit like this which can transmit more than 2/3 km.
I'm searching that- how can I transmit as far as much using 3904 with more watt RF output. Because I cannot find any high power RF transistor like 3866. So I'm searching alternative idea. Show me a circuit like this which can transmit more than 2/3 km.
As has been said by several of us, the 3904 type device is no good for what you are trying to achieve
Have you done an eBay search for the 2N3866 ? There's sure to be someone selling them. They are a old device but a good low power one, you will get ~1/2 a Watt or so at 100MHz.
The may be Mitsubishi power block modules on that few that can produce up to several10's of Watts at that freq
I have specifically looked as my interests are either lower or much higher in freq when the modules abound.
Have a look on the Mitsi. site for the RH..... series of modules.
I have had no need to use discrete transistors in that area for many years since the power modules became readily available. Many of the only require 50 to 100mW to get full output. There are many MMIC's that will easily produce up to 200mW at 100 MHz
The sensitivity of the receiver has A LOT to do with the range of a transmitter.
The supply voltage for the transmitter also has A LOT to do with the range of a transmitter.
Objects in the path that block radio signals also has A LOT to do with the range of a transmitter.
My FM transistor uses a 2N3904 RF amplifier with an average current of about 54mA from a brand new 9V battery. Its range across a large river valley is more than 2km to my very sensitive home FM tuner and very sensitive FM car radio. After 1 hour, the battery voltage has dropped to 7.5V and the range is cut in half.
The range is down the street to my cheap Sony Walkman FM radio. The range is across the street to a cheap clock radio and a cheap radio from The Dollar Store.
The good transmitter and receiver antennas were one vertical whip, 80cm long. The Sony walkman uses its earphones cord as its FM antenna. The clock radio uses its mains cord as its antenna. The whip antenna on the radio from the Dollar Store is only 12cm long.
The radio from The Dollar Store still works 5 or 6 years later with its original two button battery cells. It picks up strong local FM radio stations and sounds awful. It uses a TDA7088 FM radio IC.
You will find that 54mA is far too much for the 2N3904 to handle at 100MHz. The max is about 15mA.
You will get the max output with about 10 - 15mA at 6v to 9v supply.
Our Voyager MkII FM transmitter achieves 600m to 1km with 9v supply and about 10mA.
Your transmitter uses the low power oscillator to drive the antenna without a power amplifier like mine has. It shows a voltage of only 2.0V across its oscillator's high value emitter resistor so its average current is only 2V/470 ohms= 4.3mA, not 10mA. With an average power that is very low.
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