# Operational Amplifier + Diode

Discussion in 'Homework Help' started by gianx80, Feb 22, 2011.

1. ### gianx80 Thread Starter New Member

Jan 23, 2011
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Hi, I have to do this exercise in preparation for my first and only exam about electronics. This is the scan of the exercise (the first one):

and this is the scan of the proposed solution:

I have to find the "transfer function" Vo/Vi (is this the correct English name?), Bode plot. For point 2, I can't it into English, sorry.

I'm a total newbie, so I can't understand the proposed solution. Why, for example, Vx/Vi = 1+sRC1? You you explain me the solution step by step?

Dec 20, 2007
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Helo again.

3. ### gianx80 Thread Starter New Member

Jan 23, 2011
6
0
You can find the scan attached to this post.

Exercize 1
In this circuit we have a real diode and a real operation amplifier.
Io = 1 microAmpere, C1 = C2 = 10 picoFarad, R = 10 kilo Ohm.

1) Find the transfer function Vo/Vi and draw the related Bode plot.

SOLUTION

In the small-signal model a the diode acts like a resistor with this value:
rd = Vt/Id = 26mV/1microAmprere = 26 kilo Ohm.
In order do find the tansfer function we use this equivalent circuit:

[DRAWING]

We find that:

Vx/Vi = 1 + R/(1/sC1) = 1+ sRC1

Vo/Vx = (1/sC2)/(rd + (1/sC2)) = 1/(1 + srdC2)

From this we obtain:

Vo/Vi = Vo/Vx*Vx/Vi = (1 + sRC1)/(1 + srdC2)

[The final part of the solution is about point 2].

Now you have all the exercise in English. Could you help me?

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