sheldon
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Chungming,
1) Using a 1M resistor allows the OTA to setup to the correct input dc common level without requiring the switching. In general, a larger value would be better since it effects the circuit less.
2) The value of the capacitor is not related to the gain. The ratio of the input capacitor and feedback capacitor sets the gain, -Cin/Cf. Having said that, there are some other things you might want to consider when creating testbenches:
1) My philosophy would be that the testbench should not effect the measurement. So I would use an analysis dependent switch
VCM (vcm 0) vsource dc=common_mode_level SCN ( vcm inp) switch position=1 ac_position=0 SCI ( vcm inn) switch position=1 ac_position=0 Now for the dc operating point calculation, the dc level is the forced to the common-mode level, however, for ac analysis the nodes float.
2) As mentioned, the ratio of the input and feedback capacitor ratio determine the gain, not the absolute value of the capacitor. However, slew rate is determined by the maximum value of the output current and the total capacitance at the output node.
slew rate = dV/dT = Iout, max/ Ctotal = Iout, total / ( Cfeedback + Cload)
or
slew rate = Iout, max / ( 6.8pF + 15pF).
So slew rate is proportional to the value of the feedback capacitor.
Additional discussion: If you have access to Ken's book "The Designer's Guide to SPICE and Spectre", you might want to review pages 67 to 106. He discusses various methods for measuring the stability of feedback amplifiers. He also illustrates some "best practices for testbench development". Sorry my version is a little repititive for emphasis.
a) If possible, make the measurement in-situ, that is, as the circuit is intended to be used. The results for the testbenches with the LC network to "open" the loop are not the same as the measurements with the loop closed.
b) Observing the phenomena should not alter the phenomena, that is, making a measurement should not change the result. For example, using the VCVS eliminates the effect of the feedback netwotk loading the circuit and changes the results.
c) Measure the parameter of interest, that is, if you want to understand stability measure the loop gain not the open loop gain of the amplifier. The two port measurement allows the designer to separately measure the forward and reverse characteristics. Using this information, it is easy to see why using the amplifier open loop gain is not appropriate for analyzing loop gain(for the example circuit). At high frequency, the effect of loading on feedback network can not be ignored when calculating the loop characterisitcs.
Best Regards,
Sheldon
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