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Silly question:

Say I have an amplifier with a 50\$\Omega\$ output and I want to connect it to an SMA connector. As far as I understand I need to match the 50 ohms to maximize the power transmitted. Now the SMA connector itself already has 50\$\Omega\$. So what do I do about the trace on my PCB connecting the amplifier and the connector? How do I match this? Wouldn't adding a trace always throw off my impedance matching here?

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  • \$\begingroup\$ How long is the trace? If it's short enough, the effect on impedance matching should be negligible. \$\endgroup\$
    – uriyabsc
    Commented May 3, 2023 at 12:54
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    \$\begingroup\$ What signals or frequencies will travel through the system? Will it be kHz, MHz, GHz? \$\endgroup\$
    – Justme
    Commented May 3, 2023 at 13:02
  • \$\begingroup\$ Also it depends on the characteristics of the PA. \$\endgroup\$
    – Lundin
    Commented May 3, 2023 at 13:19
  • \$\begingroup\$ A trace on FR4 of dielectric thickness t above a ground plane will present a suitable 50 ohm impedance if it has a width of 2t. Connect the amplifier output to the SMA with such a trace. Other substrate materials may have a different ratio. \$\endgroup\$
    – Neil_UK
    Commented May 3, 2023 at 13:28
  • \$\begingroup\$ It is important to note that we are talking about characteristic impedance here. This is not the same as a resistor. You can design a 50-ohm PCB trace using one of the many online calculators. For RF, this is often done using a microstrip configuration. Each element in your RF chain will ideally have a 50 ohm characteristic impedance. Amplifier, microstrip feed to SMA, SMA, cable. Frequency plays an important role here. \$\endgroup\$
    – Troutdog
    Commented May 3, 2023 at 17:37

2 Answers 2

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Now the sma connector itself already has 50Ω

Not really; it behaves as a fairly lossless component that is designed dimensionally to be compatible with a 50 Ω transmission-line. It doesn't present a continuous 50 Ω impedance at all. It's "said to be" 50 Ω just so you know you can use it virtually losslessly on a 50 Ω transmission line.

So what do I do about the trace on my pcb connecting the amplifier and the connector? How do I match this?

You design the PCB trace (and PCB ground plane) with such dimensions that make it compatible with a 50 Ω transmission-line. It's then "said to be" 50 Ω compatible because it will convey signals fairly losslessly on a 50 Ω transmission line.

Wouldn't adding a trace always throw off my impedance matching here?

If incorrectly dimensioned yes. A 50 Ω example of a PCB trace from here: -

enter image description here

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  • \$\begingroup\$ so I need to design my trace in a way that it has an impedance of 50 Ohms towards ground? Sorry if im asking stupid things here, I'm just really lost. \$\endgroup\$
    – Axodarap
    Commented May 3, 2023 at 12:57
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    \$\begingroup\$ Yes you do so, you need to understand why this is necessary by understanding transmission lines and why reflections will occur if there are impedance mismatches. Of course, if you are talking about an audio amplifier then it doesn't matter. However, you tagged "impedance matching" so I expect it's a high frequency RF amplifier you refer to. \$\endgroup\$
    – Andy aka
    Commented May 3, 2023 at 13:00
  • \$\begingroup\$ yes I'm talking about an RF amplifier here. Now say I attach a 50 ohm antenna to my sma connector but i also have a 50 ohm transmission line and a 50 ohm output, isnt the output then missmatched to the antenna? \$\endgroup\$
    – Axodarap
    Commented May 3, 2023 at 13:03
  • \$\begingroup\$ No, 50 ohm source, transmission line and load is a correctly balanced system, you should see minimal reflections as the signal moves from each part of the circuit to the next. The impedances of the line and load don’t sum (or parallel). \$\endgroup\$
    – Frog
    Commented May 3, 2023 at 13:05
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    \$\begingroup\$ @Axodarap - Here's a qualitative way of looking at it. If the interconnect is truly a transmission line, then the source (amplifier) really doesn't "see" the load at the end, but only "sees" the input to the transmission line. The end of the transmission line "sees" the load. \$\endgroup\$
    – SteveSh
    Commented May 3, 2023 at 13:46
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There are PCB calculators all over the net. Nearly all PCB houses have them. See address at the top of the picture.
I was going to list some here but nearly all want me to log in and I don't want to. you will need to know information about your PCB like thickness and stackup.
http://saturnpcb.com/

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    \$\begingroup\$ Can you expand this answer and answer the question? \$\endgroup\$
    – Voltage Spike
    Commented May 3, 2023 at 14:13

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