System Dynamics
System Dynamics
3rd Edition
ISBN: 9780073398068
Author: III William J. Palm
Publisher: MCG
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Chapter 9, Problem 9.28P
To determine

The expressions for the bandwidths of the circuits as shown in figures 1 and 2.

Expert Solution & Answer
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Answer to Problem 9.28P

The expressions for the bandwidths are:

For circuit in figure 1, (0,131R2C21)

For circuit in figure 2,

(0,13(2G1)(1R2C21))

Explanation of Solution

Given:

System Dynamics, Chapter 9, Problem 9.28P , additional homework tip  1 The circuits are as shown below: System Dynamics, Chapter 9, Problem 9.28P , additional homework tip  2

Concept Used:

  1. Kirchhoff’s current and voltage laws are employed for finding the circuit equations.
  2. The bandwidth of a system having transfer function as T(ω)=M(ω)ϕ(ω), is:

(ω1,ω2) such that M(ω1)Mpeak2M(ω2).

Calculation:

For the circuit as shown in figure 1, on applying Kirchhoff’s voltage law:

v1=i3R+v0i3=v1v0R

And the current i3 can be expressed as:

i3=Cdv0dt

Thus,

i3=v1v0RCdv0dt=v1v0R

On taking Laplace transform of this, we get

Cdv0dt=v1v0RRCsV0(s)=V1(s)V0(s)V0(s)V1(s)=1(1+RCs) (1)

Also, for the circuit in figure 1, on applying Kirchhoff’s current law:

i1=i2+i3vsv1R=Cdv1dt+Cdv0dt

On taking the Laplace transform of this, we get

vsv1R=Cdv1dt+Cdv0dtVs(s)V1(s)=sRC(V1(s)+V0(s))Vs(s)sRCV0(s)=(sRC+1)V1(s)V1(s)=(Vs(s)sRCV0(s)(sRC+1)) (2)

Thus, from equation (1) and (2), we have

V0(s)=V1(s)(1+RCs)V0(s)=Vs(s)sRCV0(s)(sRC+1)2V0(s)((sRC+1)2+sRC)=Vs(s)V0(s)Vs(s)=1R2C2s2+3RCs+1

This is the transfer function of the given circuit of figure 1.

Thus, the magnitude for this function is:

M(ω)=1(1R2C2)2+(3RCω)2

And the peak value for this magnitude occurs at ω=0, that is

Mpeak=1(1R2C2)

Thus, lower bound for the bandwidth is ω1=0 and the upper bound ω2 is:

M(ω2)Mpeak2M2(ω2)Mpeak221(1R2C2)2+(3RCω2)212(1R2C2)(1R2C2)2+(3RCω2)22(1R2C2)(3RCω2)2(1R2C2)ω2131R2C21

Therefore, the bandwidth of the circuit is:

(0,131R2C21)

Similarly, for the circuit shown in figure 2, we have

V0(s)Vs(s)=GR2C2s2+2RCs+1

Thus, the magnitude for this function is:

M(ω)=G(1R2C2)2+(2RCω)2

And the peak value for this magnitude occurs at ω=0, that is

Mpeak=G(1R2C2)

Thus, lower bound for the bandwidth is ω1=0 and the upper bound ω2 is:

M(ω2)Mpeak2M2(ω2)Mpeak221(1R2C2)2+(2RCω2)2G2(1R2C2)(1R2C2)2+(2RCω2)22G(1R2C2)(3RCω2)2(2G1)(1R2C2)ω213(2G1)(1R2C21)

Therefore, the bandwidth of this circuit is:

(0,13(2G1)(1R2C21))

Conclusion:

The expressions for the bandwidths are:

For circuit in figure 1, (0,131R2C21)

For circuit in figure 2,

(0,13(2G1)(1R2C21))

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