# Amplifier Equivalent Circuit problems :(

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#### silverusa

##### New Member
Hey everyone !

I just transferred to another school and Electronics program in new school is a little different.

If anyone can help me get on the right track it would be great.

I need help with :

- Understanding all the questions.
- Picking right equations.
- Meaning of some shortcuts.

etc.

Teacher gave us notes but he seems like a type of person that expect you to know everything.

Once i get on right track i will do rest of the research myself i just need to get started after looong summer.

Thank you !!!

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#### silverusa

##### New Member
Anyone knows this stuff ?

I really need help

Thanks

#### alphacat

##### New Member
Hey,

I believe that the following drawing should help you out solving most (if not all) of the listed questions.

What in the red rectangle is the two-port model of the amplifier.

Good luck.

#### Attachments

• amp.JPG
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#### Grossel

##### Well-Known Member
Well, I beleive an inverting opamp amplifier will match the description of the amplifier. Say amplification -Fu = R2/R1. Then we know that input resistance R1 = 50k.

Turn the formula in respect to R2, we get:
R2 = -Fu * R1 = -(-200) * 50k = 10Mohm.

So the circuit will look like this:

NOw the drawing is finnished. Sorry for the time - but found a little bug in inkscape I got to report

#### Attachments

• rect5034.png
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#### silverusa

##### New Member
You guys are great.

How do you recognize what kind of amp just by looking at values ?

When i get home at 10pm from school today i will try to get same answers as the ones on the bottom of that page.

What equations would i use and why ?

Thank you !

#### alphacat

##### New Member
Have you ever heard about terms like controlled-sources, voltage divider?
Its very simple once you have the two-port model drawn.

#### silverusa

##### New Member
Voltage divider yes. Controlled source i dont think so.

In my old school electronics 1 didnt cover amps at all. I mean we did just overview by the end of the class.

Once i transferred to new school electronics 2 is all about amps and from what i heard electronics 1 also covered bunch of it.

This is why i am having trouble. If i saw how those problems were done it would help me alot.

Thank you

#### alphacat

##### New Member
I'm sorry to hear that, that kind of sucks.
Its weird that you werent required by the new school to take electronics 1 again in their school, since the syllabuses of these two versions of that course arent similar, apparently.

a controlled source is a source that is cotrolled by some other parameter.
In our case, the conrolled source is a voltage-controlled voltage source, it produces voltage of (-200)*vin.

Once you understand that, and understand the voltage divider formula, i think you are capable of solving at least some of the questions here.

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