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.pngRegards
Yousuf