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Problem with DC convergence in Bias Circuit

SC202503236954

Hi, I have some problems while simulating Cascode Bias Circuit.

I draw the schematic as below, and I tried to simulate the circuit, but Spectre said there are some problems related to DC convergence.

I don't know the thing that I did wrong. Please tell me about it if you know about this error.

<Content of the Error>

ERROR (SPECTRE-16385): There were 13 attempts to find the DC solution. In some of those attempts, a signal exceeded the blowup limit of its quantity. The last signal that failed is I(V0:p) = 3.01741 GA, for which the quantity is `I' and the blowup limit is (1 GA). It is possible that the circuit has no DC solution. If you really want signals this large, set the `blowup' parameter of this quantity to a larger value.

ERROR (SPECTRE-16080): Cannot print DC solution because DC did not converge. Resolve the convergence issue and rerun the simulation.

 

  • SC202503236954

    Hi everyone.

    I'm working on a Cascode Bias circuit using gpdk090 PDK in Virtuoso and running DC analysis with Spectre. However, the simulation keeps failing with DC convergence errors.

    The error log mentions things like:

    No DC path from node `Bias1' to ground, Gmin installed to provide path.
    No DC path from node `Bias3' to ground, Gmin installed to provide path.

    ERROR (SPECTRE-16385): There were 13 attempts to find the DC solution. In some of those attempts, a signal exceeded the blowup limit of its quantity. The last signal that failed is I(V0:p) = 3.01741 GA, for which the quantity is `I' and the blowup limit is (1 GA). It is possible that the circuit has no DC solution. If you really want signals this large, set the `blowup' parameter of this quantity to a larger value.

    The Bias1–Bias4 nodes are the gates of transistors in the biasing structure. These nodes are not directly connected to any obvious DC current path.

    My question is
    What is the fundamental reason for this kind of DC convergence issue in a cascode bias configuration?
    And what are the **recommended or standard methods** to resolve this kind of problem in Spectre simulations?

  • sgcad
    in reply to SC202503236954

    As far as I can tell, this is probably an issue with the circuit, not the simulator. There are at least two nets (Bias1 and Bias3) that are connected only to gates and thus have no well-defined DC operating point. The circuit simply doesn't seem to work as shown. I'm not completely sure what the intended functionality of this circuit is, but perhaps these nets should be connected to drains (NM0 and PM10) to form diode-connected FETs?

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