PROVIDING DC PATHS TO A PASSIVE FILTER A passive filter is shown in Figure 15.14. The output is taken from node 9. Plot the magnitude and phase of the output voltage separately against the frequency. The frequency should be varied from 100 Hz to 10 kHz in steps of one decade and 101 points per

Question

PROVIDING DC PATHS TO A PASSIVE FILTER

A passive filter is shown in Figure 15.14. The output is taken from node 9. Plot the magnitude and phase of the output voltage separately against the frequency. The frequency should be varied from 100 Hz to 10 kHz in steps of one decade and 101 points per decade.

Accepted Answer

The nodes between [latex]C_{1}[/latex] and [latex]C_{3}[/latex], [latex]C_{3}[/latex] and [latex]C_{5}[/latex], and [latex]C_{5}[/latex] and [latex]C_{7}[/latex] do not have DC paths to the ground. Therefore, the circuit cannot be analyzed without connecting resistors [latex]R_{3}[/latex] , [latex]R_{4}[/latex] , and [latex]R_{5}[/latex] as shown in Figure 15.14 by dashed lines. If the values of these resistances are very high, say 200 MΩ, their influence on the AC analysis would be negligible.

The PSpice schematic of a passive filter with DC paths is shown in Figure 15.15.

The listing of the circuit file is as follows:

Example 15.4 Passive filter

SOURCE VIN 1 0 AC 1V ; Input voltage is 1 V peak
CIRCUIT R1 1 2 10K

R2 9 0 10K
* Resistances R3, R4, and R5 are connected to provide
DC paths:
R3 3 0 200MEG
R4 5 0 200MEG
R5 7 0 200MEG
C1 2 3 7NF
C2 3 4 70NF
C3 3 5 6NF
C4 5 6 22NF
C5 5 7 7.5NF
C6 7 8 12NF
C7 7 9 10.5NF
L1 4 0 1.5MH
L2 6 0 1.75MH
L3 8 0 2.5MH

ANALYSIS * AC analysis for 100 Hz to 10 kHz with a decade

* increment and 101 points per decade:
.AC DEC 101 100 10KHZ

* Plot the results of AC analysis for the magnitude
* of voltage at node 9.
.PLOT AC VM(9) VP(9) ; Plots on output file
.PLOT AG VP(9) file ; Plots on output
.PROBE ; Graphics post-processor

.END

The frequency response for Example 15.4 is shown in Figure 15.16.

Question 15.4: PROVIDING DC PATHS TO A PASSIVE FILTER A passive filter is s...

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