how to design bypass mode for wcdma LNA

[lijulia]

w-cdma lna

I design cascode LNA for wcdma, and added the bypass switch between the input and output , but the s11 & s22 change a lot from high-gain mode. how to solve this problem?

Thanks!

 

[dhlee]

lna bypass mode

In my opinion, at high-gain mode lowering the reflection coeff. of LNA may be meaningless. Because the input signal is large enough at the bypass mode, the power loss induced by the poor VSWR of LNA does not affect system performance.

 

[djalli]

bypass mode loss

Can you post the schematic. I am interested!

thank you
djalli

 

[lijulia]

wcdma & pdf

sure, the input power is high for bypass mode is high. But the performance of the duplexer won't be good if LNA input is not matched to 50ohm, right?

 

[vfone]

lna bypass design

This document will help you:
**broken link removed**
rgds

 

[dsjomo]

wcdma lna

I guess, you tide your switch to an high-Q output node, and input impedance varies because output suffers process variation or inaccurate modelling or parasitics.

For a transistor operating under high frequency, Gamma-in is not only a function of S11 but also a function of S22. And so does Gamma-out, this is because of the Cbc capacitance. High-Q node is easy to be affected by inacurrate modelling or parasitics. So both Gamma-in and Gamma-out present totally different value compared to the designed value.

The followings are my suggestions......

1) Lower down the node Q which you tide your switch to. For example, shunting a resistor.
2) Current steering method. But whatch out the low gain linearity. For WCDMA, this mighy not be a applicable way.
3) um...... this might be the most effective way, but here i can only give you some hints.
Hint #1 : For a cascode LNA the upper stage is common-base stage AMP.
Hint #2: The mag. of S12 of common-base AMP is vary small.
Hint #3: Common-base AMP could be treated as isolated 2-port network.
Hint #4: For an isolated 2-port, Gamma-in is no more a funcion of S22.

Wish these could help you.

 

[dhlee]

inductorless lna wcdma

lijulia said:
sure, the input power is high for bypass mode is high. But the performance of the duplexer won't be good if LNA input is not matched to 50ohm, right?

==> What performance of duplexer did you refer to?
the insertion loss?? filtering characteristics??

I think neither maybe affect the system performance in high input power.

BR.