Common Base and Common Gate Amplifiers (Contd.): Numerical Examples (Part A)

Let's start with the concept of common base amplifiers. Can anyone explain what makes these amplifiers unique?

Is it because the base is at a fixed voltage, unlike in common emitter configurations?

Exactly! In a common base amplifier, the base is typically treated as AC ground. This helps in achieving low input impedance and high-frequency response.

What about the influence of biasing in this configuration?

Great question! Proper biasing is crucial to ensure that the transistor operates in its active region, which we will explore further in the examples.

Remember: In CB amplifiers, I_E ≈ I_C. The approximate equality helps simplify our calculations.

Now, let’s move to the specific calculations. What are some performance metrics we should consider?

Voltage gain, input impedance, and output impedance?

Correct! Let's calculate the voltage gain using the formula: $A_V = \frac{g_m R_C}{R_{in}}$. Can anyone provide the values for these variables from our example?

We have a g_m value of about 38 mS and R_C is 3 kΩ.

Exactly! Plugging these values in, our voltage gain comes out approximately to 108.85. Excellent work!

To recap, the three crucial metrics we assess are voltage gain, input impedance — which is low in CB amplifiers — and output impedance, which we find to be around 2.83 kΩ.

Let’s talk about how the input source resistance affects our performance metrics. What happens when the source resistance is not negligible?

I remember the attenuation will significantly reduce the overall voltage gain, right?

Exactly! For instance, setting the source resistance to 10 kΩ can drastically lower the overall voltage gain to about 2.758.

Why is that?

Because the input impedance is low, the voltage divider effect comes into play, leading to substantial gain loss. Bear in mind this critical point when designing circuits.

Memory aid: Remember 'Low input impedance leads to Low gain under high source resistance (LILG-HSR)'.

Having gone through the examples, what are some design guidelines we should remember for common base amplifiers?

We should aim for low source impedances to avoid significant gain attenuation.

And consider using larger coupling capacitors to maintain signal integrity.

Perfect! A good design involves analyzing both the input and output resistances along with the capacitance effects, especially in high-frequency applications.

To summarize today, we explored the practical aspects of common base amplifiers, covered our performance metrics, and identified how we can optimize our designs.