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Differential LNA Zinput Problem

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rty94

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Hello I am designing a differential lna stage and my real Zinput is approximately 7 Ohms so it's very low.In which way can I increase it effectively without decrease much my gain ?


It is a problem because I want to connect a balun (single to differential) on the input and I cant design it with an almost 50 Ohm output from the first stage..

Thank you
 

yes that will be a problem. You can not redesign the chip for more like 50 ohms input?

The only place i have seen such a low output impedance was in a CMOS cell phone transmit amp where there had to be numerous cells paralleled up. An LNA should be a snap to get a more normal impedance.
 

As in the previous thread, I recommended you to add series resistances to the emitters of each tail to improve the linearity.It will also increase the input resistance by factor of gm.
 

BigBoss said:
As in the previous thread, I recommended you to add series resistances to the emitters of each tail to improve the linearity.It will also increase the input resistance by factor of gm.
Click to expand...

the problem is that I get a 8 db S21 param and without them the gain is 16 dB.I see a better linearity and Zin thats true ..
Is there any other possible way ?Just to test it.

Thank u again!
 

BigBoss said:
As in the previous thread, I recommended you to add series resistances to the emitters of each tail to improve the linearity.It will also increase the input resistance by factor of gm.
Click to expand...

it has been a while since I screwed around with LNA designs, but I recall a resistance to ground at the emitter really screwing up the noise figure!
 

rty94 said:
the problem is that I get a 8 db S21 param and without them the gain is 16 dB.I see a better linearity and Zin thats true ..
Is there any other possible way ?Just to test it.

Thank u again!
Click to expand...
Linearity of an amplifier depends on few factors that are Bias Point,Source and Load Impedances and of course the Active Device itself.
There are numerous methods to improve the linearity such as applying series and/or parallel feedback,using Tank Circuit at the output to eliminate second and higher order harmonics,Using Emitter Inductor to suppress higher order harmonics ( Warning !, stability issues) or if the circuit is integrated one and it's been used to drive a Mixer ( general case), using a Buffer with Emitter follower will improve the linearity because changings on load impedance ( in Mixer case for instance) will deteriorate the linearity.
There are other "elegant academic techniques" but a technique should be applicable.
 

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