Lecture – 88

Today, we'll be discussing how current mirrors function in common emitter amplifiers. Can anyone explain what a current mirror is?

I think it's a circuit that provides a constant current... but how does it work in amplifiers?

Good question! A current mirror takes a reference current and mirrors it to other parts of the circuit. This helps in maintaining stable biasing. For example, in our amplifier circuit, we can ensure that certain transistors are always biased properly.

So, are the transistor configurations important for this to work?

Absolutely! The characteristics of the transistors, such as their β value and Early voltage, play a crucial role in determining how accurately the current is mirrored. Can anyone tell me how we calculate the collector current given these parameters?

Isn't it based on the biases and resistances connected to the transistors?

Exactly! The collector current can be determined by the bias voltages and resistances in the circuit, allowing us to design efficient amplifiers. Remember, precision is key in achieving the desired performance.

To summarize, current mirrors are vital for biasing in amplifiers, ensuring stable operation, especially in differential amplifiers. Understanding their function leads to improved circuit design.

Now, let's calculate the collector current in our example. Can anyone tell me the relationship between the collector current and the biasing resistances?

I believe there's a formula that involves the base current and the resistances connected.

Correct! Using the β value and assuming negligible base current losses, we can derive the collector current. We set the collector current of specific transistors equal to each other for balance.

The example mentioned 2 mA for the collector current; how did that value come about?

Great observation! The 2 mA was derived through the biasing resistors, which ensured adequate current flow. Can anyone mention the resistor values leading to this current?

Wasn't it 570 kΩ for each biasing resistor?

That's right! Such calculations enable us to achieve a well-defined operating point for our amplifier, ultimately impacting its performance.

In summary, knowing how to calculate the collector current precisely contributes to the effective design and application of amplifiers.

Next, let's address the output resistance and voltage gain of our amplifier circuit. What do you think is the output resistance derived from?

I think it comes from the combination of resistances through the transistors, right?

Exactly! It’s influenced by the small signal model of the transistors in the active region. Can anyone recall what typical value we derived for the output resistance?

Wasn’t it 25 kΩ?

Correct again! A higher output resistance typically leads to better voltage gain, which in this case was approximately 1923. Why do you think such high gain is expected?

It’s because of the active load from the current mirror?

Spot on! The current mirror effectively increases the gain by providing high output impedance. Remember, output characteristics are crucial for designing effective amplifiers.

In summary, the output resistance and voltage gain interplay significantly in amplifier design, impacting overall performance.

Now let’s delve into the effects of Early voltage on our current mirror configuration. How does this affect our calculations?

Doesn’t it impact the collector current accuracy?

Absolutely! A mismatch in the Early voltage between transistors can lead to inaccuracies in the expected collector current. Can someone relate this to the DC output voltage?

I suppose it can cause the output voltage to deviate from expected values?

Exactly! This is critical for high-impedance nodes where even small deviations can cause significant voltage changes.

So we need to minimize mismatches in real applications?

Yes, ensuring matched transistor parameters is crucial. In summary, taking Early voltage into account is essential for precise circuit design, especially in analog applications.