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We can use transformers to carry out impedance matching. However, using various configurations of BJT circuit can achieve the same effect. Here I am talking about common collector, common base and common emmiter configurations. In different configurations we get different input output impedances that can be controlled to much extent.

When do we use a transformer to match impedances and when do we use other circuitry. I understand that if we have very high power applications than transofmer can provide isolation, but what about other not so high voltage applications?

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I'd use a transformer when I want to isolate one signal from another. The transformer provides galvanic separation between two circuits and this is needed in several applications. One that springs to mind is a data coupler between a rotating machine and a static data receiver - you can't use wires but, you can use a rotating transformer to couple the data.

I've used near-field data transfer of modulated data as well and, in effect, antennas have become a transformer.

You can find RS485 chips that now embody isolation transformers and, ADI have a series of data coupling chips that incorporate tiny transformers to provide galvanic isolation.

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  • \$\begingroup\$ OK, so when do we use the BJT to do this? What I understand is that now we try to use transformer at most places. Is this true? What about other ways to match impedance? When are they used? \$\endgroup\$
    – quantum231
    Commented Apr 28, 2014 at 11:21
  • \$\begingroup\$ @quantum231 Impedance matching is done using a resistor or network of R, L and C - the BJT or transformer is an "aide" and sometimes a hindrance to matching impedances but it's the resistor/network that does the Z matching. It is done to avoid data reflections corrupting data or at an RF level to maximize power throughput. \$\endgroup\$
    – Andy aka
    Commented Apr 28, 2014 at 11:30
  • \$\begingroup\$ I have heard/read about using transformers and BJTs being used for impedance matching but not passive components like R,L,C. I am not sure what you are referring to here. You might be referring to high speed digital or RF where we use termination resistors. I know about that though this question is not related to that. \$\endgroup\$
    – quantum231
    Commented Apr 28, 2014 at 14:16
  • \$\begingroup\$ Maybe if you can provide a link to what you have heard/read? \$\endgroup\$
    – Andy aka
    Commented Apr 28, 2014 at 14:40

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