98.3.3. Suitable Range of Feedback Resistors

Today, we're focusing on feedback configurations in common emitter amplifiers. Can anyone tell me what a feedback configuration is?

Is it the way we connect feedback to the amplifier to control its gain?

Exactly! There are primarily two types: voltage feedback and current feedback. In our case, we're looking at the voltage-shunt configuration. Can anyone explain why we use this one?

Because it stabilizes the output voltage and ensures consistent gain?

Right! This setup samples the output voltage to control the input current effectively.

So, does that mean we can control the trans-impedance with this configuration?

Good question! Yes, precisely! The trans-impedance Z becomes defined by the feedback network, ensuring stability. In feedback circuits, we often remember: Z = A * R_f, where A is the gain.

To summarize, feedback configurations like voltage-shunt help stabilize our amplifiers by controlling how feedback from the output informs the input.

Now let's discuss how to calculate input and output resistance in our circuit. Can someone tell me about the role of feedback resistors here?

Are they used to determine how much current flows through the circuit?

Absolutely! They help define the input resistance, which can be seen as R_in = r + R_f, where R_f is the feedback resistor. What's crucial is knowing when to ignore those biases.

So, when do we ignore them?

You generally ignore bias resistance in small signal analysis when it is negligible compared to R_f. This results in simplified calculations.

Can anyone tell me the implications on the output resistance?

It gets reduced when using feedback, right?

Correct! The output resistance is also impacted, guiding how we design the feedback network effectively.

Quick recap! Remember, R is used in calculating Z and helps us stabilize trans-impedance, influenced by our feedback network.

Today, let's explore how we choose our feedback resistor values. Why do you think it’s important?

To ensure the circuit performs optimally without distortion?

Exactly! We aim for R_f to be over ten times greater than any load resistance to avert loading effects. How can we represent this mathematically?

R_f should be greater than 10 * R?

Right! And we also need to ensure that the product of beta and Z' exceeds one for stability.

What happens if we don't meet those limits?

If those limits aren't met, we could face instability, fluctuating gain, or increased distortion. Quickly, let's review: always check that our feedback resistors fit within suitable ranges to maintain effective performance.

Finally, let’s apply what we've learned to a numerical example. Can anyone recall our ideal resistor configurations?

We learned R_f has to be much higher than R to avoid loading issues.

Precisely! When we set R to 50kΩ, we satisfy both upper and lower limits of resistance. What result do we get regarding the output resistance?

It becomes lower than the original values, right?

Exactly! So, in summary, choosing the right feedback resistor helps stabilize and optimize gain while managing output effectively. Be sure to apply these principles in real-world circuits!