99. The characteristic equation for the output voltage of an All-pass filter is given by:
Correct Answer: 1
Solution
Based on the question, the correct characteristic equation is Option 1:
Vo = Vin [ 1 - 2 / (jωRC + 1) ]
Comparison: How others look
To identify filters quickly, look at the numerator (the top part of the fraction):
1. Low-Pass Filter (Blocks Highs)
Notice: It has only a "1" on top. It is the simplest one.
Vo = Vin [ 1 / (jωRC + 1) ]
2. High-Pass Filter (Blocks Lows)
Notice: It has the frequency symbol (jωRC) on top.
Vo = Vin [ jωRC / (jωRC + 1) ]
3. All-Pass Filter (Phase Shifter)
Notice: It uses a "1 minus 2/..." structure to keep the output volume exactly the same as the input.
Vo = Vin [ 1 - 2 / (jωRC + 1) ]