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    Design class A RF power amplifier

    Hi everyone,

    I am a student at Sheffield University and I don't have a solid experience in the RF design.
    I am working on a project trying to design a class A amplifier using BPF740F BJT transistor at 1GHz to show how the output voltage swings between 1V and twice the V_DC and current also swings between 0mA and twice I_Cmax for the selected transistor within the linear region, then I am going to drive the base of the transistor a bit harder to switch from class A to class C (no longer work in linear mode).
    What I have done are: I have calculated the R_opt (112 Ohms) depending on load line method and figured out the V_DC(5.5 V), then I did a matching for the input, but still getting wrong output.

    Once I am increasing the biasing on the base, the Ic exceeds twice the maximum collector current (which is 40 mA), and the collector-emitter voltage decreases to be less than twice the DC (11 V).

    I am struggling to figure out how to solve this problem, or what should modify of consider in order to solve this issue?

    Please find attached a screenshot of my schematic I have done using ADS
    If you require further information please let me know.

    Your help is much appreciated!Click image for larger version. 

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  2. #2
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    Re: Design class A RF power amplifier

    Hi

    Are you increasing the bias when you apply the signal?
    I am a little unsure of the procedure.

    You would normally set up the bias and then you can increase the RF signal. For linear class A the voltage swing is between Vce_min and twice the Vc (11V). The current is another matter depending in the device. In your case the current should be between 0 and Imax of your device.

    Kind regards



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  3. #3
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    Re: Design class A RF power amplifier

    Quote Originally Posted by tyassin View Post
    Hi

    Are you increasing the bias when you apply the signal?
    I am a little unsure of the procedure.

    You would normally set up the bias and then you can increase the RF signal. For linear class A the voltage swing is between Vce_min and twice the Vc (11V). The current is another matter depending in the device. In your case the current should be between 0 and Imax of your device.

    Kind regards
    Hi Tyassin,
    Thank you very much for your reply.
    I have tried to change in the biasing and the amplitude of the signal but It didn't work, therefore I have applied a direct biasing to the base, so I got the collector current swings between 0 and 41 mA, but the output voltage it doesn't seem correct (Please check the attached screenshots)

    What I am actually looking for is to let the output voltage swings between Vce_min and twice the Vc (11v), and the collector current should swing between 0 and Imax of my device which is 45mA

    I am really very confused and I don't know if I am doing right or wrong!!! Could you please explain the steps of setting up the bias first.

    Many thanksClick image for larger version. 

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  4. #4
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    Re: Design class A RF power amplifier

    If the corner current ( Imax) is 20mA, the Idc should be 10mA and Ropt=5.5/10mA=550 Ohm.



  5. #5
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    Re: Design class A RF power amplifier

    Quote Originally Posted by W.Dabbas View Post
    output voltage swings between 1V and twice the V_DC
    Does this mean you wish the output voltage to be higher than the supply V?
    It is possible to use a series LC, to boost AC amplitude. Resonant frequency needs to match incoming freq.
    The L:C ratio is important. Usually the inductor's Henry value needs to be 1000 or 10000 or 100000 times the capacitor value.
    Values should be selected in consideration of the available current to drive them.
    Small A, small C, large L.
    Large A, large C, small L.



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  6. #6
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    Re: Design class A RF power amplifier

    Hello there, i'm not sure if this might be the case solver but, have you checked if your input and output impedance are optimized (equal) to gain max power. If they are not equal you'r signal might be partially reflected thus you lose gain specially with such high frequencies. I hope this might become helpful.

    Best regards,

    Borna



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