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Non-inverting realization of (s+8)/(s+2)

Short answer

Use a non-inverting opamp with a resistor R1 from inverting input to ground and R2 in parallel with C in feedback. Choose C = 1 µF; then R1 = 166.7 kΩ and R2 = 500 kΩ to realize T(s) = (s+8)/(s+2).

Design an active filter using non-inverting op-amp configuration with following transfer function. T(s) = (s+8)/(s+2)

Approach

A non-inverting op-amp amplifier with impedance Z1Z_1 from the (-) input to ground and Z2Z_2 in feedback has

T(s)=1+Z2Z1T(s) = 1 + \frac{Z_2}{Z_1}
Vin ---------(+)
                \
                 OA >----+---- Vo
                /        |
     +-------(-)         |
     |                   |
     +-------[ Z2 ]------+
     |
   [ Z1 ]
     |
    GND

Matching the given function

T(s)=s+8s+2=1+6s+2  ⇒  Z2Z1=6s+2T(s) = \frac{s+8}{s+2} = 1 + \frac{6}{s+2} \;\Rightarrow\; \frac{Z_2}{Z_1} = \frac{6}{s+2}

Take Z1=R1Z_1 = R_1 and Z2=R2∥CZ_2 = R_2 \parallel C:

Z2=R21+sR2C=1/Cs+1/(R2C),Z2Z1=1/(R1C)s+1/(R2C)Z_2 = \frac{R_2}{1 + sR_2C} = \frac{1/C}{s + 1/(R_2C)}, \qquad \frac{Z_2}{Z_1} = \frac{1/(R_1C)}{s + 1/(R_2C)}

Comparing:

1R2C=2,1R1C=6\frac{1}{R_2C} = 2, \qquad \frac{1}{R_1C} = 6

Normalized values (C=1C = 1 F): R2=0.5 ΩR_2 = 0.5\ \Omega, R1=1/6=0.1667 ΩR_1 = 1/6 = 0.1667\ \Omega.

Check: T(s)=1+6s+2=s+8s+2T(s) = 1 + \dfrac{6}{s+2} = \dfrac{s+8}{s+2}. Zero at s=−8s = -8, pole at s=−2s = -2, dc gain 8/2=48/2 = 4, high-frequency gain 1.

Practical values (impedance scaling)

Choose C=1 μC = 1\ \muF, i.e. km=106k_m = 10^6 (frequencies unchanged):

R2=0.5×106=500 kΩ,R1=0.1667×106=166.7 kΩ,C=1 μFR_2 = 0.5 \times 10^6 = 500\ \text{k}\Omega, \qquad R_1 = 0.1667\times10^6 = 166.7\ \text{k}\Omega, \qquad C = 1\ \mu\text{F}

Answer: non-inverting amplifier with R1=166.7R_1 = 166.7 kΩ\Omega to ground and R2=500R_2 = 500 kΩ\Omega in parallel with C=1 μC = 1\ \muF in feedback (normalized: R1=1/6 ΩR_1 = 1/6\ \Omega, R2=0.5 ΩR_2 = 0.5\ \Omega, C=1C = 1 F). This form works because the zero is farther from the origin than the pole, so ∣T∣≥1|T| \ge 1 at all frequencies.

✓ Answer checked by Claude Sonnet 5.5

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