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[SOLVED] Gain in AC and Transient Simulation -- different results!?

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Javad1991

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Dear friends,

I have designed a bandpass filter (BPF) using an ideal OpAmp and positive feedback (one can consider it a black box). Based on the AC simulation, the BPF is working correctly and filtering other frequencies around 1GHz and passing the 1GHz signal with 68.9dB amplification.
The problem is, when I do a transient simulation with an input of 1mV at 1GHz, the output is only amplified to 15mV, which means the amplification is near 23dB which is far from the 68.9dB shown in the AC simulation.
Can anyone explain this difference, please?

AC and Transient simulation result:
Capture.PNG


BPF circuit and ideal OpAmp:
Capture3.PNG
Capture2.PNG
 

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Since the Q of the BPF seems quite large, I would increase the number of points in the AC sim to check if its center frequency is exactly at 1GHz. It should also help use conservative accuracy for the transient analysis. You can also play with the strobeperiod to increase the accuracy. At the same time, it might be that some of the internal nodes are not yet settled within the 1000 ns you simulated.
 
Since the Q of the BPF seems quite large, I would increase the number of points in the AC sim to check if its center frequency is exactly at 1GHz. It should also help use conservative accuracy for the transient analysis. You can also play with the strobeperiod to increase the accuracy. At the same time, it might be that some of the internal nodes are not yet settled within the 1000 ns you simulated.
Thank you for your feedback.
Increasing accuracy for transient analysis in combination by playing with strobeperiod worked for me!
 

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