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Why single-ended to differential LNA?

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bisonlj

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I see some designs use a balun to convert antenna input to differential inputs for LNA, can someone tell me what's the benefit for doing this? Doesn't it add extra cost? If for 2nd order harmonic reduction, isn't it unnecessary at this point? Thanks!
 

BigBoss

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Better Substrate Noise Immunity,doubled diff. Voltage Swing,Less even mode harmonics,compatiblity with other differential structures.
 

bisonlj

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Thanks! But for less even mode harmonics, is it necessary? For example, 2.4GHz RF signal will put 2nd order harmonic at 4.8GHz, which is far beyond operation range, do we still need to care about it?
 

jiripolivka

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I can see mostly an improvement in linearity in balanced LNAs. Concerning the harmonics, a good receiver should include a band-pass filter to prevent interference, then harmonics are rejected.
 

BigBoss

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Thanks! But for less even mode harmonics, is it necessary? For example, 2.4GHz RF signal will put 2nd order harmonic at 4.8GHz, which is far beyond operation range, do we still need to care about it?
This harmonic may interfere anyhow with the main signal so that another unwanted 2.4GHz may appear at the input of the LNA..
You don't want to see it do you??
Almost all RFIC circuits have differential structures with the expense of die size, there should be a strong reason behind..
 

bisonlj

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Thanks! I think another reason is the single-to-diff conversion doubles input signal swing, which is especially important for the faint RF signal input from antenna.
 

biff44

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if u have a chain of components, interconnecting them differentially is best. it offers less leakage.

If u then want to use one of those components as a stand-alone...u need the balun
 

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