how to enable spectre ac simulation with homotopy=dptran

The ac analysis (and the small-signal analyses in general) do not replicate all of the finer control options that dc has. 

So run a dc operating point analysis with homotopy set the way you want first, and then have the ac analysis after that (you can set prevoppoint=yes on the ac options form, although the operating point should be quick as it would start from the previous solution anyway). You can control the order of the analyses using drag and drop.

Regards,

Andrew.

Thanks, It runs much faster now.

******************
DC Analysis `dcOp'
******************

Opening the PSF file ../psf/dcOp.dc ...
Important parameter values:
reltol = 1e-03
abstol(V) = 1 uV
abstol(I) = 1 pA
temp = 40 C
tnom = 27 C
tempeffects = all
gmindc = 1 pS
rabsshort = 1 mOhm
Trying `homotopy = dptran'..

Notice from spectre during DC analysis `dcOp'.
GminDC = 1 pS is large enough to noticeably affect the DC solution.
dV(I12.I4.I41_TOP) = -36.7026 mV
Use the `gmin_check' option to eliminate or expand this report.
Bad pivoting is found during DC analysis. Option dc_pivot_check=yes is recommended for possible improvement of convergence.

Maximum value achieved for any signal of each quantity:
V: V(I12.I55.M14:int_g) = 2.5 V
I: I(V6:p) = 26.02 mA
Convergence achieved in 5544 iterations.
DC simulation time: CPU = 287.623 s, elapsed = 82.4486 s.
Total time required for dc analysis `dcOp': CPU = 287.624 s (4m 47.6s), elapsed = 82.4496 s (1m 22.4s).
Time accumulated: CPU = 297.958 s (4m 58.0s), elapsed = 94.1785 s (1m 34.2s).
Peak resident memory used = 416 Mbytes.

dcOpInfo: writing operating point information to rawfile.

Opening the PSF file ../psf/dcOpInfo.info ...

******************************************
AC Analysis `ac': freq = (1 Hz -> 100 GHz)
******************************************
Operating point computed in DC analysis `dcOp' at T = 40 C.

Opening the PSF file ../psf/ac.ac ...
ac: freq = 1.995 Hz (2.73 %), step = 410.4 mHz (909 m%)
ac: freq = 2.512 Hz (3.64 %), step = 516.6 mHz (909 m%)