RF Amps

I am looking for an easy way to build a small RF amp. it would not need to pass a frequencies of higher than 50Mhz and lower than 20Mhz, however it is not a problem if it does pass a wider range. Also it needs to be able to run off of 12v or less (so i can use a battery).
Basically I'm trying to modify a remote control for an RC car to give it a bit farther range. I have tried looking for a simple transistor amplifier circuit online, but i either find one with no values for the resistors, or they look very complicated. I have some electronics background, but compared to what I hear people talking about in forums usually, I'm defiantly an amateur at best .
Anyway, thanks for any help you can provide,
Warren

zane9000 said:

I am looking for an easy way to build a small RF amp. it would not need to pass a frequencies of higher than 50Mhz and lower than 20Mhz, however it is not a problem if it does pass a wider range. Also it needs to be able to run off of 12v or less (so i can use a battery).
Basically I'm trying to modify a remote control for an RC car to give it a bit farther range. I have tried looking for a simple transistor amplifier circuit online, but i either find one with no values for the resistors, or they look very complicated. I have some electronics background, but compared to what I hear people talking about in forums usually, I'm defiantly an amateur at best .
Anyway, thanks for any help you can provide,
Warren

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RF power amplifiers are usually tuned to a specific frequency, not wideband, the input and output impedances are usually matched as well, mostly to either 50 or 75 ohms.

As you say, they are quite complicated, and require accurate setting up, not really a project for the inexperienced.

zane, there's usually a good reason as to why the output power of RC controls is limited - it prevents interferance with other electronic things.

How far a range did you have in mind? surely not beyond line of sight?

A much simpler way to extend the range is a better antenna at the transmitter (control box). You obviously can do little about the RC car's antenna.
The simple rod antenna's transmit a signal equally strong in all directions. If you replace it with a directional antenna you can get a much greater range at the inconvenience of having to point it in the general direction of the car. That should not be too difficult as you are looking at it anyway.

Find out the exact frequency of your control unit and then go to the library and read up on directional antenna's for that frequency.
They are much easier to build than a linear amplifier, the hardest part would be to find a way to connect it to the unit.

Klaus

wow, thanks for the great idea. i dont know why i didnt think of a directional before.
sorry for my ignorance, but why wouldnt a plan old transistor amp do the trick? i know ive seen them before for amplifying signals picked up by phototransistors... i know its different, i just dont understand why.
Thanks again,
Warren

zane9000 said:

sorry for my ignorance, but why wouldnt a plan old transistor amp do the trick? i know ive seen them before for amplifying signals picked up by phototransistors... i know its different, i just dont understand why.

Click to expand...

An RF amplifier is a very critical device, it can easily be destroyed by slight mistuning, you also have the huge problem of instability, a wideband RF amplifier is very likely to self-oscillate. The input and output must be carefully matched to their connections, you've perhaps heard of SWR meters which you use for adjusting aerials? - these measure the power going to the aerial, and the power bouncing back!. With a perfect aerial all the power will be radiated, and none bounce back, however this is unobtainable - any power bouncing back heats the transistor up even more, resulting in short life (sometimes VERY short!).

So, essentially, you have a tuned circuit feeding into the transistor, matching the transistor to the source impedance, then a tuned circuit on the output, matching the output to the aerial impedance. A tuned circuit, by definition, is tuned to a specific frequency - so this limits the range the amplifier can work over.