Zeros in a circuit transfer function

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Discussion Overview

The discussion revolves around constructing a circuit that achieves a specific transfer function, H(s), which includes a simple zero and a double zero without introducing additional poles. Participants explore various circuit configurations and the practical implications of their designs.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes splitting the circuit into four stages to derive the transfer function, detailing the poles for three stages and seeking guidance on constructing a fourth stage, H2(s), that introduces the required zeros without adding poles.
  • Another participant suggests using an operational amplifier stage with a capacitor input and resistor feedback to create a zero at the origin, along with a parallel inductor-capacitor stage for the complex-conjugate zero pair, noting the impracticality of such designs in real life.
  • A later reply emphasizes that the proposed transfer function is not realizable practically due to the requirement for zero gain at a specific frequency, suggesting that the problem is more theoretical and acceptable for textbook scenarios.
  • Further contributions mention that while synthesizing complex-conjugate zeros or poles is possible, achieving them exactly on the imaginary axis is impractical, as it would imply an infinite-Q circuit.
  • Participants discuss alternative methods, such as Twin-T and bridged-T networks, for placing complex zeros near the jw axis, indicating that while challenging, it is not impossible to achieve complex zeros close to the desired location.

Areas of Agreement / Disagreement

Participants generally agree on the impracticality of the proposed circuit designs in real-world applications, but multiple competing views remain regarding the methods for achieving the desired transfer function and the implications of theoretical versus practical circuit design.

Contextual Notes

Limitations include the dependence on idealized circuit components and the unresolved nature of how to effectively implement the required zeros without introducing poles, as well as the implications of infinite-Q circuits.

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Homework Statement


Complete the circuit in the figure in order to get a transfer function as: H(s) = k*[s(s^2+10^10)]/[(s+10^3)(s+10^5)(s^2+1.41*10^5s+10^10)].


The Attempt at a Solution


If I split the circuit in four stages: H1(s), H2(s), H3(s) and H4(s), I obtain:

H1(s) = -(1/(R1*C1))/(s+1/(R2*C1)). One single pole.
H3(s) = -(1/(R3*C2))/(s+1/(R4*C2)). One single pole.
H4(s) = (1/(LC3))/(s^2+(1/(R5*C3))s + 1/(LC3)). Complex pole.

How can I build an H2(s) stage to include a simple zero and a double zero? How can I build a circuit like that? It cannot introduce poles!

Thank you.
 

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Highly impractical, but you can add 1 op amp stage with a C input & R feedback (giving you the zero at the origin), and a second stage with a parallel L-C input and L feedback (giving you the complex-conjugate zero pair on the imaginary axis).

In real life you would not have two single-pole stages like you have to deal with ...
 
rude man said:
Highly impractical, but you can add 1 op amp stage with a C input & R feedback (giving you the zero at the origin), and a second stage with a parallel L-C input and L feedback (giving you the complex-conjugate zero pair on the imaginary axis).

In real life you would not have two single-pole stages like you have to deal with ...

Thank you.

Curiously, what happens in real life if you build a circuit like this?
 
First, realize that your transfer function is not realizable in any practical way. That's because it calls for zero gain at w = sqrt(10^10) = 1e5 rad/s or 1e5/2pi Hz. That is impossible to get and still have finite gain at other frequencies. So this problem is a "textbook" problem & so it's OK to use impractical circuits like what I gave you.

The s in the numerator would be made part of a Ts/(Ts + 1) circuit so you'd get you zero at the origin plus your pole at s = - 1/T. That's just a series R-C in the input and R in the feedback:
Vout/Vin = -Zf/Zi = -Rf/(Ri + 1/sC) = -(Rf/Ri)sRiC/(sRiC + 1).

There are fancy networks (like 3 R's and 2C's in the feedback etc.) for synthesizing complex-conjugate zeros (or poles) but never if the poles or zeros have to be right on the imaginary axis, since that implies an infinite-Q circuit.
 
rude man said:
There are fancy networks (like 3 R's and 2C's in the feedback etc.) for synthesizing complex-conjugate zeros (or poles) but never if the poles or zeros have to be right on the imaginary axis, since that implies an infinite-Q circuit.

It's not difficult to place a complex zero right on the jw axis (or close to the jw axis on either side). Twin-T and bridged-T networks are one way to do it.
 

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