# Unsolvable Prob: Microelectronic circuits by sedrasmith 5th Ed.

Discussion in 'Homework Help' started by VINAY_RAO_M, Mar 6, 2011.

1. ### VINAY_RAO_M Thread Starter New Member

Mar 6, 2011
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Pl. Refer to Exercise No.4.31, page no- 306 for which data is referred from the example 4.11.
Here in this problem Ri, R0, Av0, Ais and Gm will be same and it doesnt depend on Rsig and RL.
As per problem Exercise 4.31:
1. When Rsig and RL is changed: In this problem, no information about the vi and vo is given. I can find these vi and v0 only for RL = ∞ condition. For RL = ∞, I got vi = 8.18mv and vo= 81.8mv. When RL is connected then how can I find Rin, Gv and Rout?

2. ### mjhilger Member

Feb 28, 2011
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It would help if you might show the problem and schematic.

3. ### VINAY_RAO_M Thread Starter New Member

Mar 6, 2011
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Hi,
Here i attached the pages of sedra smith. I am having doubt in excercise no 4.31 which is in page no 306 for which example 4.11 is referred.
Thank you.

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4. ### VINAY_RAO_M Thread Starter New Member

Mar 6, 2011
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Sorry, i missed out 1 page. Here i am attaching again. Thank you.

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5. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Apart from the stated changes, I would assume everything else remains the same as stated in the original question - including the unloaded and loaded input and output voltages shown in the table. I'll check if I get the same answers as shown.

6. ### VINAY_RAO_M Thread Starter New Member

Mar 6, 2011
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I thank you for your reply. And about the problem, surely you wont get because when RL changes certainly it will have effect on Rin hence Vin also changes and then similarly Vout also. As i said earlier only Ri, R0, Av0, Ais and Gm remains constant because in these cases load RL either will be short circuited or open circuited.

7. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Your points are valid.

I had a closer look at the exercise 4.31 and the answers provided. It seems that the student is required to take the previously derived amplifier parameter values from example 4.11. Using the same amplifier parameters one then changes the signal resistance and load resistance values as indicated in parts a, b & c of exercise 4.31.

I can confirm the answers provided for part a are correct. I haven't attempted the other parts.

8. ### VINAY_RAO_M Thread Starter New Member

Mar 6, 2011
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Ya. From the example we can take only Ri, R0, Av0, Ais and Gm values. Rest we need to find on our own. About part a: Why the answer is correct? Can you explain it pl?

9. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Why are the supplied part a answers correct?

I assume they are probably correct because I derived the same values using a different method to that provided in the text.

Basically, I derived an equivalent 4-terminal network for the amplifier and did the analysis based on that network. Since my method differs to that shown in the text, it would probably not be of much use to you - although it is informative to undertake the analysis by that approach. The mild criticism I have of the text approach, is that the reader simply plugs values into various formulae without necessarily understanding why.

You might refer to page 303 and plug the derived values for example 4.11 into one of the equivalent circuits A, B or C and then use the equivalent to derive the values for the various conditions noted in exercise 4.31.

Last edited: Mar 12, 2011
10. ### VINAY_RAO_M Thread Starter New Member

Mar 6, 2011
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Hi,
Still my doubts are not cleared yet.

part a) When Rsig changes:
If Rsig changes then surely vi also changes then CURRENTin also changes. Do you feel like, the changes in vi and the current (which is flowing into the two port) is same hence Rin doesn't change or bcz of any other reason?

part b) When RL changes:
When this happens, vo changes hence similarly Av (But Avo is constant) and also vi. As vi changes and also the current (into two port) and hence Rin has to be changed. But the problem here is that, neither i know vo nor vi or the current flowing in to the two port => unknowns are vi,vo,Rin,current (into two port) when load is connected. How can i find Rin with these many unknowns (only known thing is Av which can be calculated from Avo, Ro and RL)?

Last edited: Mar 17, 2011
11. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Clearly the analysis in example 4.11 demonstrates that Rin is not constant but subject to the output loading conditions. Personally, I don't like the text's approach (which you posted), notwithstanding the obvious good reputation of the book. I don't own the book, so I can't look further into any supplementary commentary in the book which may be relevant to explaining the overall analysis. So my negative comment based on this limited knowledge of the book may be unjustified.

I think the equivalent schematics shown are potentially confusing to the unwary student. They (the student) will see a an input resistor Rin shown and might justifiably assume Rin is a constant for a given amplifier. They may overlook the physically possible case in which there is a feedback path between output and input which impacts the Rin value and hence the overall amplifier performance.