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Today, we’re going to discuss how current mirrors are used in amplifiers. Can anyone tell me what a current mirror does?
Isn't it a circuit that copies current from one branch to another?
Exactly! It helps in setting consistent bias currents across different transistors. Now, how do we ensure the collector currents in our circuit remain the same?
We have to make sure the currents through the biasing resistors are properly calculated, right?
That’s correct! We use Ohm’s law to determine the necessary resistor values. Let's remember the formula for current through a resistor: I = V/R.
So, if we want 2 mA through the collector, we need the BASE currents and resistors to ensure that?
Yes! The base current must correspond with the required collector current given the β value of the transistors involved.
In summary, we need precise biasing to ensure equal currents that are vital for our amplifier's performance.
Let’s explore how we calculate the collector current, starting with transistor-1. Given it has a β of 100, how can we find I_B?
If I_C is 2 mA, then I_B will be I_C/β, right?
Precisely! So, 2 mA divided by 100 gives us 20 µA for I_B. Now, what implications does this have for our resistors R1 and R2?
We should have both resistors set to produce this base current consistently?
That’s correct! The calculations lead us to set both R1 and R2 to approximately 570 kΩ.
And this means we can mirror that current into transistor-4 accurately?
Exactly! The alignment of currents allows us to maintain stability within the amplifier. Always remember the connection between β and I_B for accurate biasing.
Having calculated the collector currents, we need to discuss the output resistance. Can anyone remember the formula for output resistance in this context?
Is it the parallel combination of r_o1 and r_o4 for the output impedance?
Correct! And if both are about 50 kΩ, what would that give us?
That should result in an output resistance of around 25 kΩ.
Well done! Now, let’s consider the voltage gain — what would we calculate based on our output resistance?
Using the gain equation, with our calculated values, we can expect a gain near 2000?
Exactly! This high gain is a hallmark of amplifiers using active loads. However, remember the role of Early voltage in determining the actual output voltage as well!
Real-world scenarios often introduce variables like base current loss. How does this affect our calculations?
It might lead to discrepancies in our intended collector currents!
Exactly. If we have higher β values in transistors. Reflecting on the impact on output voltage, what happens if the required current exceeds what our circuit provides?
Then the output voltage will drop below what we calculated, right?
Right! This highlights the significance of precise calculations in amplifier design, as small differences can lead to larger impacts in voltage output.
So, we need to maintain close tolerances when designing such circuits?
Absolutely! Balancing all these variables ensures our amplifier functions effectively and reliably. Great discussions today!
A circuit configuration designed to copy current from one branch of the circuit to another.
The output current taken from a transistor's collector, essential for amplifier functionality.
Resistors used to set the base current in transistors, influencing stability and performance.
Reflects the amplifier's ability to increase the amplitude of a signal, critical for audio and signal processing.
Impacts the output characteristics of transistors and needs to be considered in practical scenarios.
In circuits of mirrors with currents so bright, Biasing is key to make transistors right.
Imagine a group of identical twins, each in charge of a current stream. They need to work together to keep the flows equal, just like how transistors use current mirrors to maintain balanced outputs in amplifiers.
C.B.A. - Remember: Collector, Base, Amplifier functions in amplifying circuits for clarity!
MIRROR: Maintaining Identical Resistor Ratios for Output Resilience.
Term
Collector Current (I_C)
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Term
Bias Resistor
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Term
Voltage Gain
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Term
Early Voltage
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The current flowing from the collector of a transistor, which is controlled by the base current.
A steady current applied to the base of a transistor to keep it in the active region.
A parameter that quantifies the output conductance in a transistor, impacting its performance by affecting the collector voltage.
The ratio of output voltage to input voltage in an amplifier, representing its amplification capability.
The operating region of a transistor where it acts as an amplifier rather than a switch.
This section focuses on calculating the collector current in a common emitter amplifier using current mirrors and analyzing its impacts on the amplifier's performance.
The section discusses the principles of a common emitter amplifier using current mirrors, emphasizing the calculations of collector currents in various transistors, the setup of resistors to achieve the desired currents, and the implications for output resistance and voltage gain.
In this section, we delve into the workings of a common emitter amplifier that employs current mirrors for biasing. The discussion begins with the assumption that the transistors in the circuit are identical, enabling straightforward calculations of collector currents. The section illustrates how to calculate the collector current of transistor-1 (I_C1) and the collector current of transistor-4 (I_C4), ensuring that they match in value (2 mA). Given the common beta value (β=100) of the transistors, the required biasing resistors (R1 and R2) to establish a normalized base current are also calculated, resulting in a value of approximately 570 kΩ for both resistors.
Furthermore, the small signal output resistance and voltage gain of the amplifier are evaluated under the assumption that all devices are operating in the active region. The calculations produced an output resistance of 25 kΩ and a remarkably high voltage gain close to 2000, confirming the performance expected from an active load configuration.
The section also discusses the significance of Early voltage in determining DC output voltage and the dependency of the currents between the various transistors while taking into account possible mismatches and base current losses. A subsequent example highlights how increased β values in transistors affect the output voltage and current flows. Overall, this section reinforces essential concepts in amplifier design and introduces practical calculation techniques for performance parameters.