# Transfer Function, numerator order>denominator

Discussion in 'Homework Help' started by BlackSuede, May 24, 2014.

1. ### BlackSuede Thread Starter New Member

May 8, 2013
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0
I have an assignment problem in my circuits class as follows;

Let us assume we have a DC voltage measured in a circuit, called Vin. We intend to build a circuit such that it has a transfer function defined as

H(s) = Vout(s) / Vin(s)
= (s^2 + s + 1) / s

Draw this electric circuit. The circuit can be composed of resistors, inductors, capacitors and opamps only. Then verify your circuit for any arbitrary DC voltage used instead of Vin by a simulator.

My workings so far;

H(s) could be simplified to = (s^2)/s + s/s + 1/s
= s + 1 + 1/s
= 1/(1/s) + (1/s)/(1/s) + (1/s^2)/(1/s)

I have assumed that Vin will be Vin(s) = 1/s which represents a u(t) step function in time domain - i.e. a DC voltage.

The Vout would therefore be Vout(s) = 1 + 1/s + 1/s^2
The inverse laplace of this would be the following in time domain;
delta(t) + t + 1

I would appreciate any guidance if I am approaching this the right way - and how would I possibly go about finding the circuit for such a function?

2. ### Papabravo Expert

Feb 24, 2006
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A circuit that diverges to ∞ for large values of t will be difficult to realize. What makes you think such a circuit is possible?

3. ### BlackSuede Thread Starter New Member

May 8, 2013
9
0
I don't think this is a practical circuit - just more to test theoretical knowledge. The simulation I would think should be over a small time period.

I have progressed through the problem further now. From my previous working,

Vout in time domain = t + 1 + delta(t)
Vin in time domain = u(t)

• t can be obtained using a integrator op-amp circuit with u(t) as the input
• delta(t) I think can be obtained with a differentiator op-amp circuit with a u(t) input
• the 1 output is simply the same as u(t)

So using a summing amplifier could I combine the output of these three circuits all using the same u(t) input in parallel?

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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The transfer function produces an output which is related to proportions of the actual input, the derivative of the input and integral of the input.

A typical application (say) in closed loop feedback control would the classic PID compensator. So there is indeed a practical use.

5. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
The integrator must be subjected to inputs of opposite sign or its output will diverge.

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
In the case of a PID compensator incorporated in a feedback control system the system error signal applied to the PID input could be of either positive or negative excursion. The integration term is used to null the system steady state error to zero.

Last edited: May 26, 2014
7. ### mat121288 New Member

May 26, 2014
2
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Hi,

im also trying to solve the same problem for an assignment. Are you planning to use 3 op-amp circuits connected end to end?