RF amplifier design

[froten]

Hi all,

I'm trying to make RF power amplifire but I have some problems like carmonics and elemnt values for LC circuits. In the circuit attached I need a way (critiria or formula) to chose L,C and L1 for an input frequncy of about 1MHz to 2MHz. I have a big problem to choose these elements and to compute the output impedence of the amplifier.

Any help on this will be very appreciated.

 

[Russlk]

Please use .png, .gif, or .jpg format and don't zip it. You need an emitter resistor for stable bias unless you are running class C. The formula for frequency is: F = 1/2/PI/(L*C)^.5. Using smaller L and Larger C will give higher Q and lower impedance. What are you trying to accomplish?

 

[Hero999]

You mean:
f = 1/(2pi sqrt(LC))

This is a tuned amplifier, it has a very narrow bandwidth which isn't much use for your application.

What's the power level of your incomming signal?

What gain do you require?

What's the impedance of your load?

 

[HarveyH42]

Would this work for an RC Monster Truck? I got a cheap one (19.97) at BIG LOTs couple of years ago, attached a small wireless camera on it. Unfortunately, the transmitter is too weak for my to sit in my house. It does okay until about half way across my backyard. I don't know the specs, or even how much of a boost would be legal. Probably just see if I can find a crystal for my helicopter transmitter.

 

[froten]

I know the formula f = 1/(2pi sqrt(LC)) and how to use it, but as per this formula all the follwing valuse are good for 1MHz:
0.1uH 254 nf
1 uH 25.4 nF
10 uH 2.54 nF
100uH 0.254 nF

my quesion is which one is the baset? if it is better to chose valuse with small L , how much we can make it small?

My second question is what is the good value for L1 depending on the frequency, a formula will be very help?

 

[Russlk]

Which is best depends on the load resistance and what you want to accomplish. Large L and small C will give higher impedance and more output voltage, but if the load is low resistance, lower L may be better. You have to analyze the complete circuit.

 

[froten]

this amplifier will be the last stage in an wireless circuit, so the load of the amplifier is the antenna. The antenna resistance is low so small values for L is better, but how much small?

 

[Russlk]

If you are using a resonant 1/4 wave antenna, the resistance will be about 75 ohms. The best way to match to the antenna is to tap down on the coil or use two tuning caps in series to make a voltage divider. Let's say you want 0.1 watts output and VCC is 5 volts, then the max signal at the collector is 10 volts peak to peak = 3.6 volts RMS. The resistance seen at the collector must be: E^2/P = 130 ohms. Since the antenna is about 1/2, tap down half way on the coil, or use two equal values of tuning cap with the antenna connected between them (you have to double the cap value from a single cap). I would stay away from extreme L/C ratios, but I don't think you will see much difference as long as the coil has good Q. Q = Xl/Rs

 

[Hero999]

I repeate you circuit isn't any good for a wide frequency range since it's tuned, also you need an emitter resistor.

How much power do you need?

I found this circuit from Google, the designer claims it'll give 60W at 1.8MHz.

http://users.tpg.com.au/users/ldbutler/MosfetLinear.htm