Common mode to differential mode

[darcyrandall2004]

Hello,
I was examining the following schematic taken from the SA605 datasheet.

Whats the benefit of the circled group of components?

Why is the second input to the mixer not simply AC coupled to ground and the input signal fed to the first input on the mixer?

I simulated the circled components expecting to see a 180 degree phase shift across pins 1 and 2 but instead I found that the two pins are in phase and one is simply of smaller magnitude than the other. Pointless

 

[karesz]

Hi,
What tells AN1994-95-96?
K.

Added after 1 minutes:

Maybe helps some lecture over "Gilbert cell mixer"?

 

[biff44]

A lot of chips have differential inputs or outputs. I am not entirely sure WHY the semiconductor guys do that, because I have NEVER seen them used as proper differential inputs/outputs, and always see some sort of kluge used to form a balun. I really wish they would stop doing that stuff!

In any event, if you simply use one of the differential inputs, and ground the other thru a blocking cap, you lose 3 dB of gain, and you probably have a pretty big impedance mismatch.

If you use some sort of kluge lumped balun, you get the impedance match to 50 ohms, and probably get back 1 or 2 dB of the gain you would have lost. In general, these lumped approximations of a balun do not actually work that well!

 

[darcyrandall2004]

Cheers for the help.

On closer inspection C5(looks like C6) attached to L1 is a rather large 100nF. Pin2 and L1 then could be considered to be AC grounded. The other circuitry then forms a tapped capacitive match to Pin1.

 

[bj_std]

have a look!!!!!!!!!

 

[mbyoussef]

C5 (or C6) is a bypass cap. C1, C2, and L1 form the matching network for the LNA.
The differential LNA has one of its inputs AC grounded.