I am designing a current-mode passive mixer which consists of gm,mixer core and tia stages. The mixer core is ac coupled to the first stage. I wonder about the input offset voltage of tia, is it feedback by the resistor or other ways?
   I tried to use the feedback resister. The input and output common mode voltage is vcc/2 at most times. By as I tried to enlarge the ratio of input and load mosfets to mitigate flicker noise. The cm voltage becames vcc. The input mosfets are pmos. Does it means that a start-up circuit is needed?
   Thank you for your help

   Thank you, Mayank~
   The PM of both DM(open loop) and CM loop are 90° and 60°, I think it`s enough. But the problem still exists. I don`t know how does spectre do dc analysis, but I don`t think the problem relates to ac response.
   Since there is no dc voltage or current after mixer core. If the input node of tia begins with a random voltage like vcc. Then it seems that the whole circuit can`t work. Why not just bias the input node (vcc/2) of tia with a large resistante?

Hi,


I think you were right in that your circuit is not starting up. If you show us your schematic it would help but I can understand if you don't want to. You can ensure start-up by either adding some start-up transistors, or by making sure your common-mode feedback loop allows the circuit to start-up.

Out of curiousity, why did you AC couple the mixer core to the op-amp? Are you trying to get a high-pass function? If so, the corner frequency of your HPF depends on the average resistance of the mixer core (for a double balanced mixer core). The smaller the on-resistance, the larger the capacitors you need.

Also, if your mixer core is a double-balanced type (as is normal), you should look out for your TIA loop gain which will depend on the average resistance of the mixer core. Your loop gain will affect the TIA linearity.


cheers,
Aaron

Quote:


AC Couple the Mixer's Output to the input of TIA through a decoupling cap.
DC Bias the input at TIA by dropping mixer o/p current into a resistor to VDD/GND.
This is what is done generally. Rest of your circuit looks fine to me.

Happy to see you Mayank~ The receiver is designed to directly down-convert the rf signal. As the IF frequency is zero, the mixer core cannot ac couple to tia or the cap may be too large.
Thank you rfcooltools.com, the suggestions are good. However, when I use the tool initial condition in spectre simulation to do transient simulation, I find that the ota with resistance feedback can start if the initial voltage at input node is 0 to near vcc. Though the dc simulation remains wrong...

Thank you RFICDUDE!
The CM of input of TIA should be feedback by the resistance so it is the same as output CM voltage.I use a large resistance to bias the input of TIA just to avoid start-up problem.
kibby

Hi Kibby,


looks like you already have plenty of good suggestions. Anyway from your schematic there is clearly a start-up issue. I think a good option would simply be to add a couple of transistors in the right places to start-up the op-amp. They can safely turn off when the op-amp is on and may not affect the operation much. Alternatively, you could change your CMFB arrangement so that the op-amp is able to start up.

If you add biasing resistors, you may run into a couple of problems. The biasing resistors would have to be small enough to ensure the op-amp is able to start, and large enough not to add noise to the circuit. If you can live with the small capacitance they add, start-up transistors are a better idea IMO.

BTW that is an interesting arrangement you have. I guess your loop gain is now affected by the transconductor's output impedance. Looks like the loop gain will be difficult to calculate...

cheers,
Aaron