# microstrip BP filter from an elliptic prototype - RF Cafe Forums

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Post subject: Matching networks for differential design Posted: Tue Dec 27, 2005 9:12 pm

i need to design an amplifier that has differential input and output and match it for 50 ohms. however, since i've differential input/output how do i match for 50 ohms...some one told me you match it for 100 ohms but that didn't make sense

also, what's the best way to test for matching in the case of differential, meaning how should the match be tested??

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Post subject: Differential ImpedancePosted: Thu Dec 29, 2005 8:29 am

In the amplifier you're talking about, there will be 3 input impedances: Input 1 to ground, input 2 to ground, and input 1 to input 2. You can't assume Zdiff = Zin1 + Zin2 at all. Consider a 1:1 transformer terminated in 50 Ohms - you could ground either input and not affect the impedance between the two inputs at all. Or you could place, say, a 1k resistor from one input to ground, and a second 1k resistor from the other input to ground. Zin1 = 500 Ohms, Zin2 = 500 Ohms, Zdiff = 48.78 Ohms (50 || 2k).

The easiest way to measure differential input impedance is with a transformer of known behavior. The best way is with a differential vector network analyzer - and that's \$\$\$\$.

Good Luck!

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