Analog Electronic Circuits

Today, we'll discuss current mirrors. Can anyone tell me what a current mirror does?

Is it something that replicates a certain amount of current?

Exactly! Current mirrors are crucial for replicating reference currents in circuits. Remember the term ‘mirroring’—it’s about copying a reference current accurately.

What challenges do we face with current mirrors?

Great question! We often encounter issues like base current loss and early voltage. We can refer to these as non-ideality factors.

What about Beta-helper?

Beta-helper is a technique used to minimize these losses by modifying the transistor configuration. We'll cover more on that soon.

To summarize, current mirrors copy currents but can suffer from non-ideality. Next, we'll delve into how cascode transistors can help.

Now that we understand the basics of current mirrors, let’s talk about cascode transistors. Why do you think we would add another transistor to a current mirror?

Would it help to improve the output resistance?

Exactly! The addition of a cascode transistor increases output resistance, making the current mirror more effective at maintaining the desired output current.

How do we analyze the circuit performance with this modification?

We can calculate the values of different voltages and currents in the circuit. For example, the gate voltage related to the bias current is crucial here.

What parameters do we need to consider?

Good point! We consider the aspect ratios of the transistors, the bias voltage, and the thresholds. Let’s go through those calculations together.

In summary, using cascode transistors significantly enhances mirror performance, especially in terms of output resistance.

Next, let’s delve into the numerical examples involving MOSFET configurations in current mirrors.

What specific equations do we use when calculating the bias currents?

We utilize the equation I_DS = K(V_GS - V_th)^2 to determine the drain-to-source current. Remember, K is the transconductance factor!

What’s V_GS again?

V_GS is the gate-to-source voltage, critical for setting up the operating point of the MOSFET.

And how do we find the gate voltages?

Great question! We derive the gate voltage from the bias equations, setting the appropriate thresholds. Let’s solve one together.

To summarize, the relationships between V_GS and I_DS are pivotal in MOSFET current mirror designs.

Now, let’s switch gears and look at BJT current mirrors. How does this differ from the MOSFET version?

I think BJTs might have different current-voltage characteristics?

Exactly! The BJT operates with base-emitter junctions, which is essential to keep in mind. Also, we generally assume V_BE to be around 0.6 volts for silicon BJTs.

How do we handle the biasing for BJTs?

For BJTs, the bias current can be set by resistors biased from a supply voltage. We can analyze this using similar currents to MOSFETs, called collector currents.

What about small signal output resistance for BJTs?

Excellent point! For BJTs, we compute resistance using g_m and r_o, just like in MOSFETs. Calculating the output resistance will reveal the circuit's effectiveness.

In summary, BJT current mirrors parallel MOSFET design principles but need adjustments for their unique characteristics.

Finally, let’s cover how current mirrors can be used in common source and common emitter amplifiers.

How do current mirrors benefit amplifiers?

Current mirrors act as active loads in amplifiers, which helps increase gain. They ensure a stable operating point for transistors.

What about calculating the voltage gains?

To find gains, we multiply transconductance with load resistance, similar to how we computed in earlier examples.

Can you summarize the steps for me?

Sure! First, calculate the bias currents, then determine transconductance and finally compute voltage gain using regular Ohm’s law.

So, to summarize our session today—current mirrors, their applications in amplifiers, and how we compute their gains are vital aspects of analog circuits.