In order to be able to match the impedance, it is necessary to know the output resistance of the amplifier. In radio frequency amplifier circuits, there is usually a coil in the collector instead of a resistor. Actually, I don't understand why they only use coils. However, if a resistor is used, the voltage gain can be increased. Because Av=-gm.(Rc//RL).

That's not the main problem for now.I don't know how to calculate the output resistance if only the coil is connected to the collector.

enter image description here

Let the amplifier circuit be as in the picture. It is necessary to calculate the output resistance to match the impedance. If there was a resistor in the collector, it would be easy to calculate the output resistance. But I don't know what to do when there is a coil. I couldn't find any information about it on the internet. It just said somewhere that the output resistor can be found with Vce/Ic. I'm not sure if (this is true).In the circuit Vce=6V and Ic=13.4 mA . Vce/Ic=6/0.0134=461ohm . According to this calculation, the output resistance is 461 ohms. But when I connect a test source to the output of the amplifier and test it in the LTSpice program, the output resistance is 65 ohms at a frequency of 10 Mhz. Which of these calculations is correct? Maybe both are wrong. Help me. How do I calculate the output resistor?

  • 2
    \$\begingroup\$ Think about output impedance and the impedance of the inductor rather than resistance. What is the impedance of the inductor at 10MHz? \$\endgroup\$ Commented Apr 8, 2022 at 21:03
  • \$\begingroup\$ Why do you need to match the impedance? \$\endgroup\$ Commented Apr 8, 2022 at 21:10
  • \$\begingroup\$ kevin white,to be able to transfer maximum power when I connect a load to the output. \$\endgroup\$
    – OzGtZ t
    Commented Apr 8, 2022 at 21:49
  • \$\begingroup\$ Do you have the equations for this same output stage with only resistors? \$\endgroup\$
    – Voltage Spike
    Commented Apr 8, 2022 at 21:52
  • \$\begingroup\$ @voltaj spike I dont understand your question."If there is a resistor in the collector, how is the output resistance calculated?" Are you asking this? \$\endgroup\$
    – OzGtZ t
    Commented Apr 8, 2022 at 22:05

1 Answer 1


In order to be able to match the impedance ...

As you have all data about the transistor used,
why don't you "measure" it by the simulation to check it ?

Apply just the definition Zout = v(Vo1)(load=open circuit )/I(R8)(load=short circuit).

NB: note that the theoretical impedance output is not Vce/Ic, but Delta Vce/ Delta Ic which can be a "little" different.

Ok. You have it well done ... Just correct it ... it is not "65 Ohm", but "65*j Ohm" (reactance).
I found j*64 Ohm, very near the impedance of the 1 uH inductor.

For the DC output impedance, I found 349 Ohm.

Now we know "output impedance", we can use this, or this, the most interesting.

enter image description here

  • \$\begingroup\$ Output resistance is virtual .Zo=0+64j. I want to transfer the output to a R=124 ohm load. How do I do the impedance matching. If the output was a pure resistor, I could do impedance matching with an L, T or pi network. But since the output resistor is an imaginary number (Zo=64j ), I don't know a method for it. \$\endgroup\$
    – OzGtZ t
    Commented Apr 10, 2022 at 17:23
  • 1
    \$\begingroup\$ Try this leleivre.com/rf_lcmatch.html ...NB: Some values give NaN ... So Just try Zo=0.1+64j or 1+64j. the circuit may be very different. Choose the "most" "realistic" solution. \$\endgroup\$
    – Antonio51
    Commented Apr 10, 2022 at 20:49
  • \$\begingroup\$ This is also useful will-kelsey.com/smith_chart \$\endgroup\$
    – Antonio51
    Commented Apr 12, 2022 at 7:35

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