Mid-band gain

Run the PSpice simulation Select PSpice and then Run from the Capture menu bar. When the simulation is complete, the Probe window will open. Add the trace DB(V(VO) ) to plot the gain in decibels. Use the cursors to label the mid-band gain, and upper and lower -3 dB frequencies. Your cursor values may be slightly different than those shown here. 

We can see that the mid-band gain is 45.7 dB, the upper 3 dB frequency is 6.4 MHz, and the lower 3 dB frequency is 64.4 Hz. You can also use the Upper3dB and Lower3dB goal functions to find upper and lower -3dB frequencies. Select Trace and then Eval Goal Function to evaluate these functions ... 

EXERCISE 5-B For the inverting op-amp below, find the upper 3 dB frequency for mid-band gains of -1, -10, -100, and -1000. Show the Bode magnitude plots for all of the gains on the same graph. 

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 ... 

From (3), we can find out that the amplifier s mid-band gain Am and high-frequency cut-off tuMF are C1/C2 and gm(AMCL) , respectively. 

We would like to find the minimum and maximum gain at mid-band. We will set up an AC Sweep at 1 kHz. Fill in the AC Sweep dialog box as shown ... 

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