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    Problem about amplifier biasing circuit

    Hi,

    For CMOS integrated LNA in this picture, is LD a part of the matching network?

    Usually if LD and C1 is a bias Tee, LD should be a RF choke and C1 should be a DC blocking cap. But such large L and C can't be built on chip. So I assume they are just matching network. But why do we sometimes need a bias Tee? If LD is not large, will the amp be affected if VDD is not an ideal voltage source?Click image for larger version. 

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    Re: Problem about amplifier biasing circuit

    Ld can be used either Matching Purpose or RFC.Depends on the Designer..
    But if this LNA is a RFIC type Matching is not very necessary due to very short distances in RFIC, Ld in this case might be RF Choke Coil.It does not have to be very large Inductor.



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    Re: Problem about amplifier biasing circuit

    But that's where I'm confused. If Ld doesn't need to be large, why do we sometimes need a RF choke?



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    Re: Problem about amplifier biasing circuit

    Sometime Ld is replaced by a resistor at the cost of losing efficiency of the circuit.
    Also there are situations (for narrow band circuits) when Ld is not a choke, but is an inductor which together with the output reactive components (in this case, C1 and C2) is used to make the output impedance matching network.



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    Re: Problem about amplifier biasing circuit

    Quote Originally Posted by sqx View Post
    But that's where I'm confused. If Ld doesn't need to be large, why do we sometimes need a RF choke?
    In order to expand The Voltage Headroom.If we use a Resistor, we'll loose Voltage Headroom..
    RFC can sometimes be a part of Matching Circuit moreover even Bonding Wires are also be used for Matching Circuit or RFC.. There are many options..



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