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I am trying to figure out the best values for the tank circuit values (L and C) in order to compare the amplifier's efficiency with the output power I have chosen the tank value L1 and C7 to resonate at 1GHz (see attached), you know we can choose different values for L and C to be resonating at the 1 GHz frequency. What is the most efficient way of selecting the L and C values for such a case?

Class-C PA schematic Current & voltage output without the tank cicruit Current & voltage output with a tank circuit

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2 Answers 2

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You could use a quarterwave stripline, the upper end tied to battery and a fine low ESR RF-rated capacitor, as your resonator. At exactly quarter-wave, the lower end, tied to your collector, will exhibit infinite impedance just like your ideal LC.

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  • \$\begingroup\$ Should the capacitor be connected to stripline as bypass cap? In case of changing the operating freq. from 1GHz to 10GHz, does that will cause any problem? \$\endgroup\$
    – W.Dabbas
    Aug 23, 2018 at 14:04
  • \$\begingroup\$ Yes, the cap is connected at the (upper, VDD, power supply) end as bypass capacitor. At 10GHz, with wavelength of 3cm, quarter wave of 8mm, achieving a precision resonance may be tough. At higher frequencies, ceramic substrates with final-tuning by abrasive air-driven particle etching was used, back in the days. \$\endgroup\$ Aug 23, 2018 at 14:55
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What is the most efficient way of selecting the L and C values for such a case?

On the face of it there are a myriad different combinations but, to narrow this down you need to factor in the load impedance and the target Q factor for the circuit. I do not know what your expected Q is and all I can see is a 252 ohm load resistor that may (or may not) be accurate. The lower that this resistor value is; the lower the Q and the less the gain of your circuit.

Wikipedia explains what Q will be for a parallel RLC circuit: -

$$Q = R\sqrt{\dfrac{C}{L}}$$

So, once you have calculated what your Q is then you can calculate L and C. If you need help understanding what your frequency response will look like when you vary the values, this site has an interactive calculator for a band-pass filter. It's not precisely the same configuration of R, L and C in your circuit but it comes up with the goods for the Q factor: -

enter image description here

For 1 GHz I've chosen an inductor of 5 nH, a capacitor of 5 pF and a load (or drive) resistor of 100 ohms. This produces a Q of 3.16.

I have chosen the tank value L1 and C7 to resonate at 1GHz

Your schematic shows C at 25 uF and L at 100 mH. This has a resonant frequency 100.65 Hz i.e. nowhere near 1 GHz.

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  • \$\begingroup\$ Thanks Andy for your help. In case I managed to choose the tank circuit components correctly, should I expect the output voltage and current to be sinusoide?? \$\endgroup\$
    – W.Dabbas
    Aug 23, 2018 at 13:55
  • \$\begingroup\$ The L and C form a bandpass filter and if the transistor introduces distortion then the sinewave voltage will not be perfect. \$\endgroup\$
    – Andy aka
    Aug 23, 2018 at 15:23

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