Hello,
I am designing a regular fully differential amplifier at 2.5GHz, nmos
input stage and pmos loads. I am having difficulties with matching the
output to 50 ohms.

When I try to connect a port to the output node, I measure a negative
real resistance. I do not know what is happening.

The same thing happened when I tried to measure the input impedance but I just used one port with the +end of the port connected to the +input amplifier node and the -end to the port connected to the -input of the amplifier, and it worked for the input. Sadly I cannot get it work for the output.

I have also tried to put a Balun but nothing seems to work.

Please find the pictures of the schematic and real impedance measured at the output. I would greatly appreciate if some on can help me figure out the correct setup to measure the output impedance, without getting this -ve real impedance.

In the Amplifier all transistors are operating in the saturation region, both nmos current and pmos currents are balanced (Although I will need a CMFB to ensure that, which I will Put together later, my real concern at the moment is the design of the impedance matching network and correct measurement of the output resistance)

Circuit Schematic

http://i51.tinypic.com/3y3it.png

Real Part of the output impedance

http://i52.tinypic.com/345ou53.png

Regards
Yousuf

Hi vp1953,

Here are the results that I got systematically.

First without any impedance matching networks. I am getting real impedance at both the input and the output. So I believe the amplifier is working fine

circuit diagram
http://i54.tinypic.com/2lnfehg.png

real impedance at the input and output
http://i56.tinypic.com/ehzqc3.png

Then I make the input impedance matching network and things get weird. Below is the S11 and the output impedance ( comes out negative )

S11 input
http://i56.tinypic.com/33mwf8p.png

Output impedance with input matching in place ( coming out negative)
http://i55.tinypic.com/fay1r7.png

circuit diagram
http://i56.tinypic.com/2po5b9d.png

Then I decide to make the output impedance matching first. Again comes out weird. This time the input impedance is negative.

circuit diagram
http://i52.tinypic.com/vpewl2.png

negative impedance at input
http://i55.tinypic.com/x55x5i.png

S22 at output
http://i55.tinypic.com/x55x5i.jpg

I think as soon as I put the matching network things stray out. I believe the simulator cannot recognize that it is a fully differential network.

Once again I really appreciate any help or suggestions

Regards

Yousuf