Numerical Examples and Design Guidelines

Today, we will dive deeper into active loads and their role in amplifiers. Student_1, do you know why we prefer active loads over passive loads?

I think it's because active loads can enhance the voltage gain, right?

Exactly! By using active loads, we can increase the voltage gain significantly. Remember the acronym 'G.A.I.N.' which stands for Gain Amplification In Node circuits. Let's break down how we design these circuits.

What parameters should we look at when designing these active load circuits?

Great question! Key parameters include the current flow through the transistors, their β values, and ensuring that they work within their active regions. Let's illustrate this with a numerical example.

In the example, we have two transistors with different β values, 100 and 200. Now, Student_3, how would we calculate the collector current for the first transistor?

We would use the formula Ic = β * Ib, right? But how do we find Ib?

Correct! To calculate Ib, we can start with the biasing resistors and supply voltage. So for our first transistor, if V_BE(on) is 0.6V, how do we set up the equation?

We would take the supply voltage minus V_BE divided by the bias resistor, right?

Exactly! Great work, Student_4! The relevant calculations will show us the collector current can be balanced across both transistors.

Now that we have our collector current, let's determine the small signal parameters such as transconductance, gm. Student_1, can you recall how we calculate gm?

Is it gm = Ic / Vt, where Vt is the thermal voltage?

Exactly right! Remember, Vt is typically about 26 mV at room temperature. Now, with our Ic value from the previous example, can you calculate gm?

Let’s see, if Ic is 2mA, then gm would be 2/(26e-3), which gives us approximately 76.9mS!

Well done, Student_2! This small signal parameter is critical for further calculations of the circuit's voltage gain. Let's summarize this.