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Today, we're going to explore current mirror circuits. Who can tell me what a current mirror is?
Is it a circuit that keeps the current constant?
Exactly! A current mirror is designed to produce a constant current. In differential amplifiers, it helps set the tail current, enhancing performance. Remember the acronym C.A.R.: Constant, Accurate, Reliable.
Why do we need a constant current in a differential amplifier?
A constant current enhances stability and linearity in the amplifier's operation. Now, can anyone explain how we derive the reference current?
I think it comes from a common voltage in the circuit.
Yes, very good! The reference current comes from a certain voltage, such as a supply voltage, ensuring appropriate biasing of the BJTs in the differential amplifier.
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Now that we understand current mirrors, let's move on to small signal models. How do these models help in analysis?
They help simplify complex circuits for easier calculations, right?
Exactly, they allow us to analyze the circuit behavior for small AC signals. What components do you think are involved in these small signal models?
We have resistances and parameters like transconductance.
Great! In a current mirror circuit, you will also take into account parameters like 'rΟ' for BJTs, which play a crucial role in determining the gain. Letβs break down how we calculate the common mode gain from our small signal model.
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Can anyone explain the difference between common mode gain and differential mode gain?
I think common mode gain refers to the gain when the same signal is applied to both inputs.
That's right! And what about differential mode gain?
Itβs when we apply opposite signals to the inputs.
Exactly! In applications, we often care about maximizing the differential mode gain while minimizing common mode gain. This ratio is key to effective differential amplifier design. Who remembers how current mirrors help in this regard?
They create active loads which enhance the differential mode gain.
Exactly! By using active current mirrors for the load, we improve efficiency and performance of the amplifier significantly.
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Today, we'll discuss active loads. Who can summarize what we mean by active load in a circuit?
Itβs when we use active components instead of passive components to manage current.
Correct! And how does this compare to using a passive load?
Passive loads can limit the signal they carry, but active loads improve the gain!
Very good! The replacement of passive loads with active current sources means our circuits can gain significantly better performance metrics. What examples can you think of that implement this design approach?
Many op-amp circuits use this configuration to enhance gain and bandwidth.
Exactly! Active load configurations are found widely in modern analog design.
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The section explains how current mirror circuits are employed in differential amplifiers, particularly those constructed using BJTs. It covers the design principles and functionality of differential amplifiers using current mirrors to achieve improved gain and performance.
This section delves into current mirror circuits within differential amplifiers, specifically using bipolar junction transistors (BJTs). It emphasizes the role of the current mirror in establishing the tail current required for differential amplifier operation, differentiating between loading with passive and active elements. The concept of replacing resistance with a current mirror to enhance performance is examined in detail.
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So, here we do have the differential amplifier which is having tail resistor as a passive element. Now, here instead of R, T we are using a transistor-3 which is getting a bias from transistor-4 and the bias circuit. In fact, similar to the previous case, you can see that this is the current mirror circuit which is helping us to set the tail current here.
In a differential amplifier, the tail resistor is often used to set the operating point. In this case, a transistor (labeled as transistor-3) is used instead of a resistor. This transistor is biased by another transistor (transistor-4), and together they form a current mirror circuit. This current mirror is crucial as it helps define the tail current, which influences the performance of the amplifier. Essentially, the current mirror allows the circuit to regulate current more effectively compared to a simple resistor.
Think of the current mirror as a water pipe system where you want to maintain a certain water flow rate (current). Instead of using a faucet (resistor) that is limited in adjustment, you use a pump (transistor) that can adapt and maintain the flow even when other factors change. This makes your system much more efficient.
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Now in this case the reference current I_REF, it is coming from V_CC. This reference current based on the reverse saturation current ratio of transistor-3 and transistor-4, we do get current here which is I_C3 = Ξ» Γ I_REF multiplied by two nonideality factors; one is due to early voltage and another one is due to Ξ²-loss or base-bias loss.
The current through the mirror (I_C3) is linked to a reference current (I_REF) that is derived from a voltage source (V_CC). This relationship is mediated by the characteristics of the transistors used, including their saturation current and how they respond to variations in voltage (known as Early effect). Non-idealities such as Early voltage and beta losses must be accounted for as they can affect the accuracy of the current mirror operation, and these are adjusted by multiplying the reference current. Thus, the real output current may differ slightly from what we expect due to these factors.
Consider a chef who needs to keep a steady flow of pasta in water (reference current). If the kitchen's stove is too high or low (representing Early voltage or Ξ²-loss), the water can boil over or go cold, affecting how the pasta cooks. Just like adjusting the heat helps control the cooking process, adjusting for these non-ideal factors helps achieve the desired current.
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Once you get the reference current here or the tail current here set by the reference current, then we can analyse this circuit by considering its small signal model.
After establishing the tail current, we can analyze the differential amplifier's behavior using small signal models. This involves linearizing the circuit around its operating point to find how small variations (signals) affect its output. This kind of analysis is vital for understanding the amplifier's response and designing it for optimal performance.
Imagine tuning a musical instrument. After setting it to the correct pitch (establishing the tail current), you then play small notes (small signals) to see how well it resonates. By making minor adjustments, you ensure that the instrument produces the best sound (output) possible.
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Now similar to the previous case, this part can also be replaced by an active load, and that load current of course should be consistent with whatever the current we do have flowing through transistor-3.
In the design of differential amplifiers, replacing a passive load with an active load can enhance performance. An active load allows for better control over the current, similar to how the current mirror controls the tail current. It ensures that the load current remains consistent with what flows through the main transistors, improving overall gain and efficiency.
This can be compared to using adjustable weights in a workout session versus fixed weights. Adjustable weights (active loads) allow for fine-tuning your training so you can maintain optimal resistance as you progress, while fixed weights (passive loads) donβt offer that flexibility, which might limit your overall performance.
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Here we can say that the common mode gain and differential mode gain can be derived using small signal equivalent circuits.
Gains in a differential amplifier indicate how effectively it responds to signals. Common mode gain refers to how well the circuit handles identical signals applied to both inputs, while differential mode gain deals with the circuit's response to signals that differ between the two inputs. By analyzing the small signal equivalent circuit, we can derive expressions for both types of gains, which are important in evaluating amplifier performance.
Think of an orchestra where every musician plays the same note together (common mode) versus when they play different notes (differential mode). The conductor (the amplifier) must ensure that the sound produced is balanced regardless of whether the musicians are playing in unison or harmony. The gains help measure how well the conductor achieves this balance.
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Key Concepts
Current Mirror: A circuit that produces a constant current.
Differential Amplifier: Amplifies the difference between two input signals.
Active Load: Replacing a passive load with an active current source to enhance performance.
Gain: A measure of amplification in circuits, critical for amplifier performance.
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A BJT differential amplifier with a current mirror configuration achieves better gain than traditional resistor-loaded amplifiers.
In a common mode rejection application, the current mirror allows effective suppression of noise and interference in signals.
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In a mirror, the current stays, helping circuits in many ways.
Imagine two identical twins (BJTs) with a constant allowance (current) that they always share, helping them keep equality.
Remember C.A.R for Current Mirror: Constant, Accurate, Reliable.
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Review the Definitions for terms.
Term: Current Mirror
Definition:
A circuit designed to maintain a constant current that is independent of voltage across it.
Term: Differential Amplifier
Definition:
An amplifier that amplifies the difference between two input voltages.
Term: BJT (Bipolar Junction Transistor)
Definition:
A type of transistor that uses both electron and hole charge carriers.
Term: Tail Current
Definition:
The current flowing through the differential amplifier, typically established by a current mirror.
Term: Common Mode Gain
Definition:
The gain of a circuit when the same signal is applied to both inputs.
Term: Differential Mode Gain
Definition:
The gain of a circuit when opposite signals are applied to the inputs.