98.4.2. Calculating Feedback Effects

Today, we're going to start exploring how feedback can improve the performance of amplifiers. What is feedback, and why do we use it?

Feedback is when you take a portion of the output and feed it back to the input, right?

Exactly! It allows for control of the amplifier's gain. Specifically, we're looking at negative feedback, which stabilizes the gain. Can anyone guess what Z represents in feedback?

Isn't it the trans-impedance of the amplifier?

Correct! When we apply feedback, Z gets stabilized. Remember, Z here is related to the feedback configuration we use.

Let's discuss how to configure feedback for stability. We often use voltage-shunt or shunt-shunt feedback configurations. What do you think these terms mean?

I think voltage-shunt means we're mixing voltage output with current at the input?

That's a great way to put it! By sampling the output voltage and mixing it with the input in a shunt manner, we can achieve stable Z. Can anyone describe how output voltage affects input current?

If we sample output voltage, it provides a feedback current that adjusts the input current accordingly?

Exactly! This adjustment is critical for stabilizing the amplifier.

Now, let's shift gears and look at the calculations behind input and output resistance. Can anyone tell me what factors affect these resistances?

I think the feedback network and the load connected play a role in determining resistance?

Right! Input resistance can be modeled as [rπ / (1 + βZ')] with feedback applied. And our output resistance changes with feedback, too. Why is this reduction in resistance significant?

It helps reduce distortion and improves signal handling!

Correct! Lower resistances can lead to higher performance.

Let's take a numerical example to solidify our understanding. If we say R is 5 kΩ and look at our feedback configuration, what can we derive?

We can calculate Z' as βR and analyze its effect on other parameters?

Exactly! The key is understanding how these values interact. Let’s calculate Z' now.

Once we have Z', we can draw conclusions about its impact on gain and current!

Well said! This is how practical applications begin.