# Design a tank circuit for class C power amplifier using ADS

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?

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.

• 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? Aug 23, 2018 at 14:04
• 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. Aug 23, 2018 at 14:55

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: -

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.

• 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?? Aug 23, 2018 at 13:55
• The L and C form a bandpass filter and if the transistor introduces distortion then the sinewave voltage will not be perfect. Aug 23, 2018 at 15:23