Performance Analysis — Amplifier Frequency Response

When we design a circuit with tolerance, we may sometimes want to find the worst case upper or lower 3 dB frequency with component tolerances. Unfortunately, calculating a 3 dB frequency requires that we find the mid-band gain and then find the frequency where the gain is 3 dB less than the mid-band. This type of calculation cannot be specified in the Monte Carlo/Worst Case dialog box. However, we can run a Monte Carlo analysis and then determine the 3 dB frequency using the Performance Analysis capabilities available in Probe. In this example, we will illustrate finding the maximum lower 3 dB frequency (FL), minimum upper 3 dB frequency (FH), and maximum and minimum bandwidth (FH - FL) for a common-emitter amplifier. Wire the circuit below  

This circuit contains 5% resistors, capacitors with +80%, -20% tolerance, and a 2N3904 transistor that has been modified to include tolerance in p. The modified model is shown below. See Section 7.C for instructions on how to modify a BJT model. 

All distributions are uniform. We will run an AC Sweep in conjunction with the Monte Carlo analysis. Fill in the AC Sweep and Monte Carlo dialog boxes as shown below  

Use distribution fiandom number seed Sava data from (an 

You may wish to reduce the number of Monte Carlo runs to reduce simulation time. 1000 was chosen to get a better sampling for the distribution. 

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ACIS measurements were performed at frequencies between 1 mHz and 1 kHz using a Solartron Model 1250 Frequency Response Analyzer. Output from the comb specimens was amplified with a Keithley Model 427 Current-to-Voltage Converter before waveform analysis. Reference electrodes were not used owing to the geometry of the encapsulated test specimens. The data reported herein were obtained with a 0.1 V rms amplitude sinusoidal excitation waveform. In one experiment, DC bias was superimposed on this waveform. 

The site specific ground response analysis performed by equivalent linear approach shows amplification for C class site with PGA of 0.53 g, whereas B class site don t amplify the ground motion. The predominant frequency for C class site shifted from 2Hz to 1.7Hz whereas B class site did not alter the frequency content. 

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