# RF amplifier for a newbie?

Discussion in 'Wireless & RF Design' started by BlackCow, Apr 15, 2010.

1. ### BlackCow Thread Starter Active Member

May 11, 2009
65
1
So I built a simple AM transmitter but it only broadcasts no more than a foot away. I would like to amplify its output so I could at least hear it around the house. I know I need to make an RF amplifier but I'm not sure exactly how to go about that...

What exactly makes an "RF amplifier" different from any other amplifier?

The only other type of amp I have built is with small transistors for an audio signal, and even then I didn't really understand how to calculate what resistances I need.

Also I have a couple TIP42 power transistors that I was hoping to use to amplify the signal output but I'm not sure exactly how to go about designing the proper circuit.

Any help or a point in the right direction would be greatly appreciated!

2. ### PRS Well-Known Member

Aug 24, 2008
989
35
Have you made a heterodyned transmitter or are you broadcasting at a single frequency. I presume you are in the 550k - 1600k cycle range so that you can hear your broadcast on a commercial AM radio? What are you using for an antenna?

The distance you can transmit is a function of the power of your transmitter. And this assumes that the output resistance of your transmitter is matched to the input resistance of the antenna. If you're using a long wire it's probably 75 ohms. So you have to develop an amplifier that outputs Power = Vpp^2/8*75 and in this case power is, for good measure, 1/4 watt. So Vpp = 12 volts peak to peak into a 75 ohm load.

I don't know what the amplitude of your transmitter is, but it needs to be roughly 12 volts and you need that to feed a 75 ohm load without attenuating by a factor of 2 as occurs when you match source and load resistances. Just a point to keep in mind. As you develop your amplifier use a 75 ohm resistor as a mock load and be sure to put 12 volts peak to peak across it.

Last edited: Apr 17, 2010
3. ### skeptic Active Member

Mar 7, 2010
51
9
It is unlikely that a properly constructed and tuned transmitter would only transmit a foot. Would you post the schematic and we'll see if we can find the problem. In the meantime you should doublecheck your circuit against the schematic to see if there are any errors.

Apr 5, 2008
15,801
2,387
Hello,

Just post your schematic so we can see what we can do about it.

Bertus

5. ### BlackCow Thread Starter Active Member

May 11, 2009
65
1
Ok so this is what I have built at the moment, see attached image.

So I was thinking if I replaced T2 with a power transistor and calculated out the resistors to give the current through the emitter something more like 800 mA it would be a lot more powerful.

That or would I connect the amplifier on the antenna to boost it as like a second stage?

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Apr 5, 2008
15,801
2,387
Hello,

The schematic looks more like a FM transmitter than a AM transmitter.
T1 looks like an audio amplifier and T2 an oscillator.
What is connected at the input?
What voltage is present on R7 (both sides)?

Bertus

7. ### BlackCow Thread Starter Active Member

May 11, 2009
65
1
Perhaps I don't quite understand how AM transmitters work. I was referencing some other schematics out there. I thought the tank circuit did the oscillating and T2 is what modulated the audio input into the AM carrier wave.

R5, the variable resistor, is supposed to control the input of T2 and thus control how much the signal is modulated.

And yes T1 is an audio amplifier, the input is an audio input. (In this case my MP3 player).

Apr 5, 2008
15,801
2,387
9. ### BlackCow Thread Starter Active Member

May 11, 2009
65
1
I'm pretty sure the circuit is producing an AM signal as I can pick it up with an AM radio.

I must say I'm getting very confused as I looked at FM transmitter circuits and it does look similar to this one!

Apr 5, 2008
15,801
2,387
Hello,

A FM signal can be recieved with an AM radio.
When recieving a FM signal on an AM radio detection takes place on the slope of the filter.

Bertus

11. ### PRS Well-Known Member

Aug 24, 2008
989
35
Hey BlackCow, good to see you responding. I think Bertus is right. Your transmitter seems to be for FM. The varacter diode is the clue. I've only seen in in circuits that vary the frequency, hence frequency modulation. I may be wrong about this, but go with simple circuits to start with.

For broadcast AM, as I'm sure you know, you need to amplitude modulate an rf signal with an audio signal. Start simple. Get rid of the variable frequency oscillator for starters and focus on one frequency that, in your area, is not near commerial stations on your Am radio. I've looked over my area's AM band and found that 700 kHz would be a good empty space to play with. And remember not to make your transmitter too powerful. That's illegal. It's also rude. We amateur radio guys need to keep our reputation squeeky clean. Keep your transmitter at or below 1/4 watt and even that may be too much. If it is, attenuate your output so you can just hear it around your house. Otherwise you need a license and the fines for violating this law are high.

So find a very simple AM transmitter circuit, or Bertus and I, and maybe others on this forum, will help you with that if you want. My advice is to leave heterodyning type circuits for later. Go with a single frequency. This means building an oscillator and a mixer, then a tuned amplifier. Are you up to it?

Last edited: Apr 19, 2010
12. ### ELECTRONERD Senior Member

May 26, 2009
1,146
16
Yes, one of the conventional modulation methods uses the varactor diode for FM, not AM.

Austin

13. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
69
What varactor?

I see a *varistor*, which is sometimes used in oscillator circuits to stabilise the amplitude.

It does look like it's supposed to be AM, but it will almost certainly FM to some extent as well.

The power is miniscule, but I would still expect tens of yards range.

You could be tuning to a harmonic of the oscillator frequency - try different tuning settings on the transmitter and for each tune the full band with the receiver, you may find signals at multiple points.

If you have the tuning capacitance/inductance too high it could be oscillating below the band you are trying to recieve it on, and you are actually recieving on the 3rd or 5th multiple of the osc frequency, which would make the signal very weak.

14. ### PRS Well-Known Member

Aug 24, 2008
989
35
rjenkins is right; it is a varistor. Now that I gave it a good look, the circuit seems to be an AM modulator. There are some features of the circuit that confuse me, but essentially the audio signal at the base of T2 is modulating the rf produced by the tank. And yet the tank's feedback path is the dc voltage supply for the tank. I've never seen that. But just because I haven't seen it, doesn't mean it doesn't work. Evidently the output of this circuit is weak and can't drive the antenna. A fix might be using a Class AB amp such as a complimentary pair between your circuit and the antenna. I can give you the design for a circuit to do this if you want. It includes 2 transistors, 4 resistors and 2 capacitors.

Last edited: Apr 20, 2010
15. ### BlackCow Thread Starter Active Member

May 11, 2009
65
1
That would be awesome. Also the "varistor" was my mistake, it was the closest thing that looked like a "variable resistor" in eagel cad's library.

Anyway, would this class AB amp you are talking about work for any type of radio signal, say if I built an FM transmitter or even a CW transmitter?

I've built audio amps before, what makes an RF amp different? Thanks!

16. ### PRS Well-Known Member

Aug 24, 2008
989
35
I think you have found a very simple circuit that gets a lot of stuff done, BlackCow. I'm interested and I'm thinking of building it myself.

Now that you have clarified the matter of the variable resistor I think we're all on the same page. This resistor was intended to give you control over the percentage of modulation. It attenuates the audio input such that you can find an optimum gain that allows for a good modulation index. This affects the detector circuit in the receiver. You can just turn the pot and listen to the radio to get the best results. By the way where did you get this circuit? I like it.

As for why your circuit is not projecting beyond a foot, I think it's due to this: Consider your output as the voltage source for the antenna. The antenna is a 75 ohm load (presumably). This creates a voltage divider such that

[Rin/(Rs+Rin)]*Vs = Vin to the antenna.

Rin is 75 ohms
Rs is the output resistance of your circuit
and Vs is the output voltage of your circuit.

Look at the equation. If Rs is large compared to Rin then Vin gets stepped on hard. The goal then is to make the resistance seen by the antenna as small as possible. A complimentary pair can make it really small. So tomorrow morning I'm going to to design a CP that gets it done.

But it would sure be nice if you would figure out the output resistance of your circuit. You can do this by finding the resistor which causes the output to fall to half its value when there is no load at all. Given that resistance I can maximize the power to the load. By the way, I'll explain this calculation.

One last thing and this is very important. What are you using for an antenna?

17. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
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If it's medium wave, the antenna will be fractional wavelength so loading will be minimal and an amplifier will not give great improvement (a quarter wave at the top end of MW is over 75m, so you will not need to drive low impedance).

I still think you should verify the operating frequency and tuning before considering adding any amplifier.

Can you give exact details of the coil and tuning capacitor you have used?

18. ### BlackCow Thread Starter Active Member

May 11, 2009
65
1
The variable capacitor is rated at 265 pF. For my calculations I said between 30 - 290 pF to take in to account any stray capacitance. The coil is made up of an empty toilet paper roll with 114 turns of copper enameled wire with a few taps in the middle. It is supposed to be a 310 uH coil. It worked fine tuning in the AM band when I built a crystal set and it works fine with this transmitter (just not very far).

My antenna I just used a a piece of wire about 12 inches long. I put a multimeter along both sides and measured the resistance at 78 Ohms. I'm not sure what else you are asking about the output resistance, I used all the resistor values in the schematic if that helps.

Last edited: Apr 21, 2010
19. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
69
Your antenna will be doing next to nothing.

Assuming you have the transmitter tuned somewhere in the middle of the band, about 1MHz, the wavelength is 300 Metres.

That makes a straight wire quarter wave 75 Metres or about 240Ft.

You would also need a good RF ground or a counterpoise (another long wire).

This is one reason walkie talkie radios & the like generally work on VHF or UHF, you can have a properly matched and resonant antenna is a reasonably small size.

You would get better low-impedance antenna matching by using a coupling coil wound over the main coil - something like 10 to 12 turns, with one end to the antenna and the other end grounded. (And forget the existing antenna connection).

You still need a longer antenna wire, some yards and with it suspended, out of contact with anything conductive or damp.

I'd be very wary of boosting the power in any big way, as transmitting in a broadcast band at anything other than microscopic power is illegal in most parts of the world.

20. ### BlackCow Thread Starter Active Member

May 11, 2009
65
1
Holy crap that's a huge antenna!

If I were to build my own receiver is there any band I'm allowed to play with? Perhaps I should get an amateur radio license

But I would like to see the AM transmitter brodcast with some reasonable range, even if just temporarily to see it work. I don't think around my house is unreasonable.

I'm mainly interested in how an RF amplifier would work, and what makes it diffrent from a regular audio amp. I am having trouble finding info on them with google.

Last edited: Apr 21, 2010