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Today, we'll focus on current mirrors and their essential role in differential amplifiers. Can anyone tell me what a current mirror does?
Is it used to maintain a constant current in a circuit?
Exactly! A current mirror ensures that the current in one branch of a circuit is mirrored in another branch. This is crucial for biasing in differential amplifiers.
How do we calculate the DC current through those transistors?
Good question! We can calculate it using the resistor values and the supply voltage. For instance, if we have an 11.4 kΩ resistor with 12V, we can find the current flowing through it.
So, does that mean if the resistor value changes, the current also changes?
Yes, that's correct! The current is inversely proportional to the resistance. Let’s go ahead and replace the resistor value and see how it affects the current.
What happens to the current through the current mirror when the input varies?
Excellent point! The current mirror must adapt to that variation by providing an equal mirrored output, which brings us to small signal analysis and gain calculations.
In summary, current mirrors are key for stabilizing currents in differential amplifiers, particularly important for biasing transistors.
Let’s now move on to calculating the differential mode gain. Who remembers how we go about this?
Is it the output voltage divided by the input voltage?
Exactly! The differential mode gain can be expressed as A_d = g_m * (r_1 || r_2), where g_m is the transconductance. Can anyone tell me what the '||' signifies?
It means we take the parallel resistance of r_1 and r_2.
Yes! Now, how does common mode gain differ?
In common mode, the input signals are the same at both input terminals, right?
Correct! Therefore, common mode gain is much smaller as external variations are effectively canceled out. Let's calculate it using the given resistor values.
So, the common mode gain tells us how well the circuit rejects noise?
Exactly! This rejection is vital for signal integrity. Let’s summarize.
In summary, understanding differential and common mode gains is essential for evaluating amplifier performance.
A vital component for mirroring currents in differential amplifier circuits.
Measures how well a differential amplifier amplifies the difference in signals.
Reveals how well a circuit rejects noises and interferences common to both input signals.
Example of calculating DC current through a transistor using a known resistor value: Given a supply voltage of 12V and a resistor of 11.4 kΩ, the current I = V/R gives approximately 1mA.
Calculating differential mode gain using the transconductance and resistances; if g_m = 0.5 and r1 = r2 = 200kΩ, then A_d = 0.5 * (200k || 200k).
Mirroring current, oh so bright; Keeps our circuits working right!
Imagine a shopkeeper who wants to sell exactly a thousand apples each day. He looks at his original stock and makes sure to always sell the same amount from everyone else. This is how a current mirror functions, keeping everything balanced.
C-D-C: Current Mirror - Differential - Common Mode. It reminds us of the three key concepts in this section.
G-C-R: Gain, Current, Rejection. This highlights the essence of differential amplifiers.
Term
What does a current mirror accomplish?
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Term
Define differential mode gain.
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A configuration used in analog circuits to replicate a current from one branch of the circuit to another, providing stability and biasing.
An amplifier that amplifies the difference between two input signals and rejects common-mode signals.
A measure of how effectively a circuit converts changes in input voltage to output current.
The gain experienced by common signals applied to both inputs of an amplifier, ideally minimized in differential amplifiers.
A linear approximation of a non-linear circuit close to an operating point, used for small perturbations.
This section discusses the functioning and applications of current mirrors in differential amplifiers, emphasizing calculations of DC and small-signal gains.
The section focuses on the role of current mirrors within differential amplifiers, illustrating how they operate to mirror currents and maintain biasing. Included are numerical examples to calculate DC currents, differential mode gain, and common mode gain relevant to these circuits.
In this section, Prof. Pradip Mandal introduces the fundamentals of current mirrors and their application within differential amplifiers, particularly in creating active loads. The key focus is on the operation and analysis of a differential amplifier that employs a current mirror configuration, specifically detailing the interactions between multiple transistors and the significance of mirrored currents. The analysis begins with determining the direct current (DC) levels through various transistors based on circuit parameters such as resistor values and voltage supplies.