# how to i amplify radio wave?

Discussion in 'Wireless & RF Design' started by alok maurya, May 12, 2016.

1. ### alok maurya Thread Starter New Member

May 12, 2016
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would i able to amplify raido wave with 2N2222 transistor?

2. ### Papabravo Expert

Feb 24, 2006
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It depends on the frequency. What frequency did you have in mind?
The transition frequency is in the range of 250-300 MHz. In order to be useful as an amplifier it must be used at a much lower frequency.

For transistors, the current-gain–bandwidth product is known as the fT or transition frequency.[3][4] It is calculated from the low-frequency (a few kilohertz) current gain under specified test conditions, and the cutoff frequency at which the current gain drops by 3 decibels (70% amplitude); the product of these two values can be thought of as the frequency at which the current gain would drop to 1, and the transistor current gain between the cutoff and transition frequency can be estimated by dividing fT by the frequency. Usually, transistors must be applied at frequencies well below fT to be useful as amplifiers and oscillators.[5] In a bipolar junction transistor, frequency response declines owing to the internal capacitance of the junctions. The transition frequency varies with collector current, reaching a maximum for some value and declining for greater or lesser collector current.
--- Wikipedia on Gain-Bandwidth Product, https://en.wikipedia.org/wiki/Gain–bandwidth_product#Transistors

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3. ### shteii01 AAC Fanatic!

Feb 19, 2010
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What Papa is telling you is that Bandwidth Product is 250-300 MHz when Gain is 1. When you increase gain, you shrink the bandwidth. If you try to get gain of 10, your bandwidth shrinks to 25-30 MHz. If you do gain of 100, your bandwidth will be 2.5-3 MHz. I can see this working for AM radio, I don't see how it will work for FM radio.

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4. ### Papabravo Expert

Feb 24, 2006
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Since a 2N2222 is not characterized as an RF transistor you will have to determine the relevant parameters of the device in order to do a competent design. Ordinarily this would be done with a batch of parts and several pieces of test equipment. Maybe you could rent the equipment but it will take you a while to learn how to use it.

5. ### alok maurya Thread Starter New Member

May 12, 2016
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I have a working FM transmitter on a raspberry pi2. I'm running my FM raido at 103.3MHz, and the antenna that I connected to the GPIO4 is a 20cm copper wire.

More data:(checked with a multimeter between GPIO4 and pin6-ground)

• DC voltage: 1.2V
• AC voltage: 1.8V
• Current: 18.4mA
I manually checked the distance by tunning cell phone fm to 103.3MHz. it was around 10 meter.

which transistor to use to amplify 103.3MHz radio signal?

6. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Consider what you are trying to amplify. You will have an antenna, a signal will arrive to this antenna, it will "excite" electrons and induce a current, which in turn will produce voltage. How much voltage will it produce? A few millivolts? How much gain do you need? Do you need to amplify a few millivolts into volts? Then you need gain of 1000. Is 2N2222 able to provide gain of 1000? No. If I recall correctly its maximum is 300 or so.

If I recall my software radio class correctly, when we talked about receivers, they use LNA after antenna. LNA is Low Noise Amplifier. So when you do your research, search for LNA and like Papa suggested RF Amplifiers.

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Aug 21, 2008
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8. ### Papabravo Expert

Feb 24, 2006
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This is not really a good analogy. How much gain is the output stage providing? It is typical for a transmitter output stage to have a much lower gain than a receiver input stage. I would expect that a 2N2222 would have a maximum gain of 2.5 to 3.0 at 103 MHz. If that is what you need then fine. Also in the above example you have no clue on the input impedance or the output impedance of that circuit fragment. These would be important things to know to design a proper circuit. You could simulate the circuit and test it or you can breadboard it and test it. In either case I believe you are going to be disappointed.

9. ### alok maurya Thread Starter New Member

May 12, 2016
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Thanks guys, I have less knowledge about electronics. I will try to amplify with 2N2222.

one more question how do i calculate the gain of my fm transmmiter? and how do i amplify it to 3 gain using 2N2222?

here what i'm thinkingwill it work?)

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10. ### DickCappels Moderator

Aug 21, 2008
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I can't answer your questions but I would say that having too much gain is unlikely to be a problem. Finding the gain of amplifiers near 100 MHz is complicated and becomes more complicated as the frequency increases, partly because the antenna's characteristics needs to be taken into account as well as the output characteristics of signal source.

Even though I cannot answer your question, I offer a few comments which are aimed at improving your chances of obtaining satisfactory results.

Not seeing the circuit of the transmitter I can only guess that the parallel values of R2, R4 and the 25 pf input capacitance of the transistor (varies by manufacturer and power supply voltage) will greatly load down the signal source, particularly if the signal source is the typical Colpitts oscillator used in wireless microphone circuits. If that is the case, an emitter follower can be inserted between the oscillator and the input of the 2N2222 amplifier. In an optimized circuit inductors and capacitors would be used between stages of the transmitter to assure that efficient coupling of power from one state to the next. Considered that a slightly more advanced art.

The transistor in the schematic is marked "2N222"; it is missing a "2".

C1 should be a low value, low loss ceramic capacitor, such as a 100 pf - 470 pf COG or NPO type, NOT an electrolytic.

When R1 and R3 have equal values the gain will be 1x at low frequencies, and much less at higher frequencies; put a capacitors like the one suggested for C1 across R3 to boost gain at radio frequencies.

C2 should be very much like that recommended for C1.

If you can use an RF choke or a parallel resonant circuit in place of R1, you will probably get much better results than when using a resistor. A matching network such as a Pi network would do wonders for output power, but consider a Pi network for a future project. The term "Pi network" has nothing to do with raspberries

The only 2N2222 that is intended for RF use that I have come across is the 2N222A from Continental Devices India Limited
http://www.es.co.th/Schemetic/PDF/2N2221A.PDF Note the 25 pf input capacitance -I would go for the 470 pf value for C1.

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