47.4.4. Output Impedance with Source Resistance

Today we will be discussing output impedance in amplifiers. Can anyone tell me why output impedance matters?

Is it important for how the amplifier interacts with loads?

Exactly! High output impedance can limit the current supplied to a load. Now, let’s define output impedance. It's the impedance seen by the load connected to the amplifier's output.

How does this relate to the source resistance?

Great question! When we include source resistance, we need to consider how it affects our output impedance calculations. This includes looking at small-signal parameters.

Could you give an example of how to calculate output impedance with a source resistance?

Sure! We'll get to the calculations shortly. But first, remember: higher source resistance typically means lower output voltage at the transistor's collector.

Let’s summarize: Output impedance is key for load interaction, and source resistance can significantly affect it.

Now, let’s talk about small-signal parameters that we need to calculate. Who remembers how to find the transconductance?

Isn’t it the collector current divided by the thermal voltage?

Exactly! So if our collector current is 0.5 mA, and the thermal voltage is 26 mV, what's our g_m?

That would be about 19.23 mS.

Correct! And for the output resistance r_o, we have it defined as the early voltage divided by the collector current. Can anyone tell me what this means for our amplifier?

It affects how much the output voltage will change with loading.

Right! Remember these parameters, as they will help us deduce voltage gain and overall output impedance.

To summarize: Transconductance helps in understanding current output in response to voltage, while output resistance conveys the amplifier's response to load changes.

Let’s analyze our circuit for voltage gain. Can anyone tell me what we expect for a common collector amplifier?

We expect the gain to be close to unity.

Absolutely correct! The formula we use takes into account both transconductance and output resistance. Recall that high input impedance is desired, how does it relate to the circuit?

If the input impedance is high, it won’t load the previous stage too much.

Exactly! This is why we strive for input impedance in the mega-ohm range. Now, could someone describe the implications of low output resistance?

Low output resistance means better power transfer to the load.

Great summary! So voltage gain close to one and both high input and low output resistances are design goals. Let’s wrap this up by reviewing.

Finally, let’s look at the upper cutoff frequency. How does the load capacitance interact with the source resistance?

It can lower the bandwidth if the load capacitance is significant, which is also affected by the source resistance.

Exactly right! The higher the load capacitance, the lower the cutoff frequency would be, affecting performance. Now, how about a practical example?

Maybe we could calculate the upper cutoff frequency using the output resistance and capacitance?

Precisely! This is how we can predict how our amplifier will behave at different frequencies.

So, we need to keep an eye on both load and source characteristics when designing circuits.

Great synthesis! In conclusion, we learned how source resistance alters performance and gain, and why that matters.