Engineering Board Exam Reviewer · Passive Filters · hard

Two identical RC low-pass sections, each with a cutoff of 1 kHz, are cascaded with a buffer between them so that the sections do not load each other. What is the overall -3 dB frequency?

  1. 0.644 kHz
  2. 1 kHz
  3. 0.5 kHz
  4. 0.707 kHz

Answer

A. 0.644 kHz

Solution

Each section has gain 1sqrt(1+(f/fc)2), so the cascade has 1/1+(f/fc)2=1/ 2 ≤ft(f/fc )2= 2-1. f=fcsqrt( 2-1)=1000(0.6436)=644 Hz.

More Passive Filters questions

  1. A first-order RC high-pass filter delivers 3.5 V rms for a 10 V rms input at 500 Hz. What is its cutoff frequency?
  2. In the loaded low-pass filter shown, R1=1 k , R2=1.5 k (the load), and C=100 nF. Find the cutoff frequency.
  3. The band-stop (notch) filter shown has R=220, , L=25 mH, and C=400 nF. At what frequency is the output voltage (ideally) zero?
  4. A band-pass filter has half-power frequencies of 3 kHz and 12 kHz. Its center frequency, taken as the geometric mean, is
  5. A second-order passive low-pass filter (two reactive sections) rolls off at what rate beyond its cutoff frequency?
  6. A series resonant band-pass filter has a center frequency of 10 kHz and a quality factor of 25. What is its bandwidth?
  7. An RC low-pass filter is driven by a 10 V signal at ten times its cutoff frequency. Find the output voltage.
  8. A filter delivers 5 V at its output when 10 V is applied at its input. What is its voltage gain in decibels?
  9. A filter passes 10 W in its passband. How much power does it pass at the cutoff frequency?
  10. The series RLC band-pass filter shown takes its output across the resistor, with L=10 mH, C=100 nF, and R=100, . Find its bandwidth.

All Passive Filters questions