Output Resistance of the CC Stage

Welcome, everyone! Today, we will be discussing the common collector stage or emitter follower. Can anyone explain what the primary function of a CC stage is?

Is it to provide some sort of impedance matching?

That's correct! The CC stage allows for high input impedance and low output impedance, making it suitable for connecting different circuit stages.

So, how does that affect the output resistance?

Great question! The output resistance in a CC stage is primarily determined by the load connected to it. Now, let's remember the mnemonic 'CC is Cool for Coupling' to emphasize its role in interfacing stages.

So, does that mean the CC stage can also enhance bandwidth?

Exactly! By using a CC stage in conjunction with other amplifiers, such as the common emitter stage, we can extend bandwidth while maintaining the same gain.

Now, let's look at how we can calculate the output resistance in a CC stage. Can anyone recall what factors come into play here?

I think it depends on the output resistor and the transistor parameters.

Exactly! The output resistance, primarily influenced by the load 'R', dictates how efficiently the stage drives connected loads. Let's go through an example where we calculate this using given values.

Could you remind us what values we need for the calculations?

You will need the load capacitance and resistor values! Let's recall the formula: R_out = R + r_e, where r_e is the intrinsic emitter resistance.

So, what's the output resistance if we have R = 3kΩ and r_e = 375Ω, as mentioned earlier?

Adding those gives us an output resistance of about 3.375kΩ. Good job!

Next, let’s dive into the voltage gain of the CC stage. Can someone explain how output resistance affects voltage gain?

Higher output resistance typically leads to a lower voltage gain, right?

Correct! In our previous example, with a gain of approximately 1, we see that the overall gain remains about 6 with the cascading stages.

And how does this relate to bandwidth?

Bandwidth is indeed extended using the CC stage. As you increase gain with low output resistance, the upper cut-off frequency also increases. Remember the formula: f_U = 1 / (2πRC).

So, using capacitors in this formula, we can also extend the bandwidth based on our design values?

Absolutely! With good design, you may see improvements up to several MHz, harnessing the effectiveness of the CC stage.