Question

When Agarwal says, in Lecture 7, to "shrink" the signal in the Small Signal Model, how do you actually do that? I can understand the boost part (add a constant DC voltage source). But to shrink a signal, are you adding a resistor, or something else?

Answer 1

@gbathree shrink a signal implies that we have used a signal with a small amplitude.
that's why the signal is also called small signal

Answer 2

right... but if the signal starts out big, how do we get it small? His example of the LED sending a signal to send music is a good example - how exactly did he "shrink" his original signal? I'm assuming it's a resistor, but just want to make sure.

Answer 3

he just decrease the amplitude of the signal and it shrinks

Answer 4

Do you get my point?

Answer 5

I understand what you need to do, I just don't understand how you do it in the real world. That is, what element do you use (resistor, diode, etc etc) do you use to decrease the amplitude.

Answer 6

Ok, got to Lecture 8 - I suppose he used a Dependent Source (like an amplifier, but in reverse).

Answer 7

@gbathree
in the example with the diode there is a typical approach used to make a distinction between bias point and signal. With the bias (dc voltage) we just decide the working point on the device characteristic (for the LED it's an exponential curve, so it' non linear) then we superimpose the signal (the part that carries information) with a small amplitude, so that the variation on the curve around the working point is very small. In that way the little part of the curve used tends to be a linear approximation and there is no more distortion. This is the meaning of signal "shrinking".

Answer 8

circuity and graphical illustration of ur question @gbathree

Answer 9

@nick67 and @ali110 - thanks for the answers, but unfortunately my question is what device causes the peaks and valleys of the signal to become smaller? So if the signal initially oscillates between -5V and 5V, we want to "shrink" the signal so it goes between -0.1V and 0.1V (for example), then shift the signal using a constant DC bump (let's say +2V) so now we go from 1.9 - 2.1V (that may be useful for a red LED). Got all that.

Question is: How do actually, physically make the signal go from oscillating between -5V and 5V to oscillating between -0.1V and 0.1V? I assume it's an element which is the reverse of the amplifier or something, but the element itself was not mentioned in the lecture, thus my question. So my question is an equipment question, not a "I don't get the idea" question.

Unless I'm way off here (always possible).

Answer 10

@gbathree
hi, you can use a simple voltage divider or, for a more sophisticated solution, an inverting op-amp with a gain lower than 1; this way you can reduce the signal amplitude as you want

Answer 11

Ok, got it - thank you nick! Sorry it took so long to get the idea across :)

Answer 12

you're welcome