Output impedance matching question

[raebrm]

Output impedance

Hi

I came across this voltage source (attached below) but i am unsure what its output impedance is. Is it 50 Ohm and hence must be matched with 50 ohm transmission line and load? How do we proof its output impedance mathematically?

thanks in advance!

 

[JoannesPaulus]

Output impedance

The impedance of this source is 0. If you want a 50ohm source you need to have the resistor in series with the source.

 

[raebrm]

Re: Output impedance

Hi, silly me...i think my simplified circuit is wrong.
The picture below is the actual circuit that I wanted to ask about. In this case is the output impedance 50 Ohm? and why?

 

[cdh7]

Re: Output impedance

I've seen similar circuits used to drive twisted pair cables with a 100 ohm characteristic impedance in Ethernet networks. I believe the termination resistors are there so that any reflections coming from the other end of the cable due to impedance mismatches are stopped rather than reflecting back and forth multiple times.

Regards,
Chris

 

[amraldo]

Re: Output impedance

For the first circuit:
To get the output impedence:
1. All voltage source are short circuit
2. Get the equivalent resistance between the output terminals.
In your case, the VCC and Vs will be short circuit. You will have the 50 ohm in parallel with a short circuit. So the overall impedance is zero.
--
Amr Ali
www.embedded-tips.blogspot.com

Added after 3 minutes:

For the second circuit its 100 ohm in parallel with the amplifier output resistance.
--
Amr Ali
www.embedded-tips.blogspot

 

[ahelmi]

Thanks for the replies, they helped a lot.

Another question, if I have a current source instead of a voltage source, do I consider the current source as a short or open circuit when calculating the output impedance? An example of such a configuration is attached

thanks!

 

[amraldo]

Open Circuit.
The idea is to make the effect of the input zero and no effect of the source.
In case of voltage source you make it short circuit to get zero input voltage.
In case of current source you make it open circuit to get zero input current.
--
Amr Ali