Second Order Filter - Power Dissipation

This is a follow-up to the original problem by Hosam Hajjir. The filter circuit below takes a sinusoidal input voltage at angular frequency ω \omega and produces a sinusoidal output voltage.

At ω = 0 rad/s \omega = 0 \, \text{rad/s} , effectively no active power is dissipated in the circuit. At ω = rad/s \omega = \infty \, \text{rad/s} , the circuit dissipates the maximum possible active power P m a x P_{max} . At what angular frequency does the circuit dissipate active power equal to 1 2 P m a x \frac{1}{2} P_{max} ?

Note: Assume the same input voltage magnitude for all trials


The answer is 550.25.

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2 solutions

Steven Chase
Mar 8, 2019

When the angular frequency is infinite, the equivalent circuit is just the input voltage dropped over one of the resistors. This is the maximum power condition.

Sweep over the angular frequency and store the value of the angular frequency which results in a total power dissipation equal to half of the maximum possible value. The answer turns out to be 550.25 rad/s 550.25 \, \text{rad/s} .

Z C = j ω C Z 1 = R + Z C Z 2 = Z 1 Z C Z 1 + Z C V 1 = V i n Z 2 R + Z 2 V o u t = V 1 R R + Z C V R 1 = V i n V 1 P = V R 1 2 R + V o u t 2 R ZC = - \frac{j}{\omega C} \\ Z_1 = R + Z_C \\ Z_2 = \frac{Z_1 \, Z_C}{Z_1 + Z_C} \\ V_1 = V_{in} \frac{Z_2}{R + Z_2} \\ V_{out} = V_1 \frac{R}{R + Z_C} \\ V_{R1} = V_{in} - V_1 \\ P = \frac{|V_{R1}|^2}{R} + \frac{|V_{out}|^2}{R}

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import math

Vin = complex(100.0,0.0)

R = 1.0 * (10.0**3.0)
C = 1.0 * (10.0**-6.0)

dw = 0.01

w = dw

Pmax = (abs(Vin)**2.0)/R

P_minres = 9999999.0

while w <= 10.0**4.0:

    ZC = complex(0.0,-1.0/(w*C))

    Z1 = R + ZC
    Z2 = Z1*ZC/(Z1+ZC)

    V1 = Vin*Z2/(R+Z2)

    Vout = V1*R/(R+ZC)

    VR1 = Vin - V1

    P = (abs(VR1)**2.0)/R + (abs(Vout)**2.0)/R

    P_res = math.fabs(P - Pmax/2.0)

    if P_res < P_minres:
        P_minres = P_res
        w_store = w  

    w = w + dw

print ""
print ""
print P_minres
print w_store

Karan Chatrath
Mar 8, 2019

Neglecting terms that act as merely scaling factors for the expression, the variation of active power with angular frequency is:

Judging by the monstrosity of the expressions obtained, this approach is a rather tedious one. Feedback would be appreciated

Thanks for the solution. I stayed entirely within the Laplace domain and used a computer program to sweep. The use of a program also allowed for a modular solution format. I have posted my solution.

Steven Chase - 2 years, 3 months ago

Thank you for the problem and for the alternate solution

Karan Chatrath - 2 years, 3 months ago

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