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    Biasing Circuit for LNA with Chebyshev Bandpass Filter

    Hi,

    As my understanding on biasing circuit, it is used to provide stable and desired voltage level to the amplifier, so that the amplifier can be operated in desired region (eg. saturated region).
    My question is that if I have an LNA with schematic as shown in the picture below, how is the biasing circuit for the inductive degeneration configuration NMOS constructed for Vbias1?
    Click image for larger version. 

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    If a current mirror configuration biasing circuit (red square) is used, the sp (eg, S11, S21) will be affected.
    Click image for larger version. 

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    In my opinion, they are affected because they are interpreted as part of input matching. Please correct me if I'm wrong. Is there any biasing circuit for LNA with Chebyshev bandpass filter instead of connecting directly to a power source?

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    Re: Biasing Circuit for LNA with Chebyshev Bandpass Filter

    For first Figure, there is nothing wrong since C2 and L2 are part of matching.
    In second Figure, the Current Mirror is not part of matching so the effect of the Current Mirror is negligible.I couldn't understand why you would connect the LNA through Chebyshev BPF..



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    Re: Biasing Circuit for LNA with Chebyshev Bandpass Filter

    Sorry for my bad description. Let me reuse it here. I would like to use the current mirror in second figure as biasing network for first figure. But when I do so, the s parameter is affected.
    To answer your question, as of my understanding, inductive source degeneration LNA is for narrow band application. By connecting it with Chebyshev bandpass filter, wideband design can be achieved.



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    Re: Biasing Circuit for LNA with Chebyshev Bandpass Filter

    The Current Mirror shouldn't affect the s-parameters of the LNA.Are you sure ??
    How/Where you would use Chebyshev BPF to expand the Bandwidth of the LNA and how NF will be affected in this case ??



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    Re: Biasing Circuit for LNA with Chebyshev Bandpass Filter

    Quote Originally Posted by BigBoss View Post
    The Current Mirror shouldn't affect the s-parameters of the LNA.Are you sure ??
    How/Where you would use Chebyshev BPF to expand the Bandwidth of the LNA and how NF will be affected in this case ??
    Look at the picture below, it's what I wanted to do. The red circle is the Chebyshev bandpass filter + Inductive Source Degeneration and the blue one covered the current mirror biasing. Take note that there is another resistor R16 and R15 in the biasing circuit. Thus, the input impedance is different from connecting to the power supply instead. Furthermore, R16 is necessary and big in value to have a good isolation. This further increase the input impedance in input matching network.
    Click image for larger version. 

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    Re: Biasing Circuit for LNA with Chebyshev Bandpass Filter

    The circuit isn't the equivalent of post #1 which connects L2/C2 to a low impedance node, so not surprising to get different S11 and kill the original Chebyshev characteristic. You need to place a bypass capacitor at the L2 foot point.

    Or design a different matching network from the scratch.



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    Re: Biasing Circuit for LNA with Chebyshev Bandpass Filter

    Quote Originally Posted by FvM View Post
    The circuit isn't the equivalent of post #1 which connects L2/C2 to a low impedance node, so not surprising to get different S11 and kill the original Chebyshev characteristic. You need to place a bypass capacitor at the L2 foot point.

    Or design a different matching network from the scratch.
    Is it like this?
    Click image for larger version. 

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    Still s-parameter got affected. But the transistor is able to turn on with this bypass capacitor added.
    I've came up with another type of biasing circuit as shown below,
    Click image for larger version. 

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    I move the R1 into biasing circuit from original location(above L2). And the voltage supply to the biasing circuit is no more 1.2V, but 600mV, which is the voltage supplied to L2 before I trying to implement a biasing circuit.
    I not sure if I design like this, is the biasing circuit still maintain its function, to provide a stable voltage level indirectly to the transistor Q1?



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