Help me choose L,C and L1 values for 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.

 

[BigBoss]

RF amplifier design

To design such amplifier you should read a little bit theory and practice from some text books. By asking these questions don't sense anyway...

 

[flatulent]

Re: RF amplifier design

In power amplifiers the output impedance is irrelevant. You set the load impedance by the power supply voltage and power output requirements. Then C is selected for the bandwidth and L is selected for resonance in the parallel circuit. The L and C in the power supply filter are selected for keeping the RF out of the power supply.

 

[xitronics]

Re: RF amplifier design

Hello buddy,


Firstly, there is no need to bias the transistor with R1 and R2. Ii would result into large DC power loss. Since you are using a tank circuit, a class C model is ideal if the load impedance is not too low and the BW requirement is less.

Output impedance of the transistor would be determined by the CE Early Effect impedance Ro, for frequencies near to or at resonance. This also is undesirable as it reduces the amplifier power efficiency ratio i.e Poac/Pidc. To counter it, you need to select a transistor with high Early Voltage Va.

Anyways, you need to have a basic theorotical knowledge of this subject before you implement it in pratice. There are many sites on net that have enough informative content to solve your purpose. If possible, i'll come back with links of some of these pages.

Regards Prasanna.
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[froten]

RF amplifier design

Thank you all for your help.

xitronics I will appreciate that I you suuply me with some links.

I have some knowledge about the RF circuits as I studied that in my faculity and Now I'm trying to use Ihave learned. I made the oscillator and arrived at the power amplifier design, I could make one but I'm not sure about its performance, that is way I'm using this forum.

The load is an antenna with 50 ohms and the problem I have is the valuse for L,C and L1.
The formula f = 1/(2pi sqrt(LC)) gives me the following values 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?

 

[xitronics]

Re: RF amplifier design

Hello ..!

Even I am involved in designing an FM transmitter to have atleast 1 Watt o/p power.

I have designed the RF stages, and simulated them successfully. But the oscillator is misbehaving, giving unexpected o/p. If you can help me, pls read my new post "Colpitts VCO Problem".


Coming back to your question, it'll be good to use L = 1uH and C = 25.4nF. That provides a suffucient Q factor of about 8 for Rload = 50 Ohms.

Selecting a low value of L requires the inductor's internal resistance r to be very very small. Very high L would result into a degradation in Q factor.

Select L1 = 1mH. This would provide very high resistance to AC, thus maintaing a const. current flow through the battery.

Regards Prasanna.
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