[SOLVED] Impedance matching for High input amps

Hello,



Had a question about whether or not I need concern myself with impedance matching. The circuit above is pretty standard: filter a down converted IF and amplify for further processing. The IF is ~30 MHz, the mixer (SYM-2500+) i can only assume is matched to 50 Ohms. The LPF are series LC circuits designed for 50 Ohm terminations. Since its a low enough frequency, I was going to use a low noise op-amp in a non-inverting topology, feeding directly to the positive input of the amp.
I realize i just said it was low enough frequency, but should I be concerned about filter and mixer not seeing a 50 ohm termination at the input of the amp? At 30 MHz, is the infinite impedance of the amp even going to matter?

Im going to see what LT-spice says, but just wanted a second opinion or three.
Thanks in advance,
Sami

Impedance mismatch=reflections. Why don’t you just put a 50 ohm termination at the input to the opamp?

Wait, I just noticed you DO have a termination. Just put it close to the opamp input.

Hawaslsh said:

wanted a second opinion or three

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You need 3 opinions for input impedance matching of an opam? Do you want to send it in space?

LC filters are designed with specific termination.The frequency characteristic is definitely affected if you change the source or load impedance.

barry said:

Just put it close to the opamp input.

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I figured that might be the answer, but I had no personal experience doing it. Follow up for my edification. What if the situation were reversed and the amp was driving the mixer. Would a series 50 ohm resistor be enough to match the low output impedance of the amp?

Thanks again for your help,
Sami

Hawaslsh said:

I figured that might be the answer, but I had no personal experience doing it. Follow up for my edification. What if the situation were reversed and the amp was driving the mixer. Would a series 50 ohm resistor be enough to match the low output impedance of the amp?

Thanks again for your help,
Sami

Click to expand...

Yes. That's done all the time.