Numerical Example

Today, we'll explore how feedback affects common emitter amplifiers. Can anyone tell me what feedback typically does in electronic circuits?

I think it helps stabilize the gain.

Exactly! Feedback stabilizes gain and improves linearity. Now, we also have different configurations of feedback — can anyone name one?

There's voltage feedback and current feedback.

Great! In our example, we examine voltage-shunt feedback. Remember, voltage feedback modifies the amplifier behavior, affecting input and output resistances.

But how do we calculate those resistances?

We'll go through that later. For now, keep in mind the role of feedback configurations.

Summarizing today's key points: feedback stabilizes gain, and we have various configurations such as voltage and current feedback.

Let's look at a numerical example now. The supply voltage in our example is 10 V, and we have a base bias resistor of 940 kΩ and a feedback resistor of 5 kΩ. What do you think will happen with these numbers?

We can start calculating the current through the amplifier.

Correct! The next step is to find V_BE, which is approximately 0.6 V. What follows this?

We should find the collector current!

Excellent! With a current gain (β) of 100, how do we calculate the collector current?

By multiplying the base current by 100.

Exactly! The collector current becomes 1 mA. Let's sum up: we've utilized the given values to understand how parameters feed into our equations.

Now, we will calculate the input resistance r_π, which is vital. Who can recall its formula?

Isn't it derived using the transistor parameters and base current?

Correct! In our example, this results in approximately 2.6 kΩ. Next, how do we find the output resistance?

We consider the load effects and the transistor parameters.

Exactly right! Remember, we also need to consider R and R' to understand their impact on the circuit. What trends do we see?

Both resistances seem to lower, reducing overall circuit impedance.

Good observation! Summarizing: we used the device parameters to calculate both input and output resistances, highlighting the importance of those calculations.

Now, let’s discuss the suitable range for our feedback resistor R. What considerations do we need to make regarding R's relationship to r and R'?

R should be much higher than the output resistance R' to avoid loading effects.

Exactly! Also, what should R be in relation to input resistance r?

R should also be much higher than r.

Well done! When we combine these conditions, we find a meaningful range for R. What values do we derive?

Lower limit is around 5 kΩ and upper limit can reach 500 kΩ.

Excellent work! Just to recap: we validated the feedback resistor range to ensure proper circuit function.