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Today, we'll discuss how a tail resistor in a BJT differential amplifier can be replaced by a current mirror. This helps in setting the tail current more effectively. Can anyone tell me why setting the tail current is essential?
Isn't it to ensure consistent operation of the amplifier?
Exactly! A steady tail current improves performance and impacts the amplifier's gain stability. Remember, a current mirror is used for biasing; it manages the tail current based on the reference current.
But how does the current mirror actually establish that reference current?
Great question! The reference current, driven by a voltage supply, flows through a diode-connected transistor, establishing a stable bias that reflects in the other transistor junctions within the mirror. Remember, βDC sets the stage for ACβ.
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Now, let's talk about why we transition from passive to active loads in differential amplifiers. Who can list a benefit?
Wouldn't it increase the gain?
Correct! Active loads allow us to achieve higher differential mode gains because they maintain impedance while reflecting the output signal effectively. Can anyone think of any other benefits?
Could it also help in improving the common mode rejection ratio?
Absolutely! While the common mode gain might increase, the ratio between differential and common mode gains often remains stable, enhancing overall performance.
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Now, letβs analyze the small signal model of our BJT differential amplifier. Can someone describe what we do first?
We need to derive the equivalent circuit, right?
Exactly! We replace BJTs with their small signal parameters. Who can remind us what those parameters might include?
We should include r_pi and different resistance values for nodes!
Spot on! Applying the small signal model will allow us to compute the differential mode gain. Remember, gain is influenced by both active load and tail current established via the current mirror. Keep this in mind as we move forward!
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Finally, let's discuss the impact of diode-connected transistors in our active load configurations. How do they affect signal performance?
They help improve signal strength, especially at the output?
Yes, it allows the signal at the output to be amplified, overshadowing weaker signals. But there's a balance. If the connection reduces the signal in certain operating conditions, what can we say about this configuration?
It can reduce distortion while retaining clarity in the main signal output!
Exactly! Thatβs why active loads and careful design are crucial in analog circuits. The interaction of these elements is key to achieving optimal results.
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In this section, we discuss the implementation of current mirrors in BJT differential amplifiers. Important insights are given on how the current mirror sets the tail current, the effects on common and differential mode gains, and modifications to improve circuit performance.
In this section, we delve into the usage of current mirrors in differential amplifiers constructed with BJTs. The focus is on replacing traditional passive elements with active components such as current mirrors to enhance performance. The tail current, critical for biasing, is established using a current mirror driven by a reference current sourced from a common supply.
Key concepts include:
1. Tail Current Generation: The reference current generates current bias for the BJT differential amplifier.
2. Small Signal Model Analysis: The small signal equivalent is analyzed to ascertain common and differential mode gains.
3. Active Loads: We also discuss how to replace the passive load with an active current mirror load, which leads to better gain performance.
4. Modified Circuit Design: The incorporation of active loads simplifies circuit design while improving signal amplification.
5. Common Mode Rejection: While the common mode gain is enhanced, the important ratio of differential to common mode gain remains stable, representing improved circuit efficiency without sacrificing signal quality.
This exploration aids in grasping the essential role of current mirrors in modern analog circuitry, particularly in precision amplification scenarios.
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Welcome back after the break. So, we are talking about the differential amplifier particularly constructed by MOSFET. Now we are going to see the differential amplifier using BJT, where we will be deploying the current mirror, corresponding current mirror using BJT and will see the similar kind of situation there.
In this section, we introduce differential amplifiers first using MOSFETs, then transitioning to BJTs. A differential amplifier is designed to amplify the difference between two input signals. Here, the concept of using a current mirror with BJTs is introduced to help maintain a consistent tail current in the differential amplifier design.
Think of a differential amplifier like a comparison tool at a school where they check the performance of two students. If we compare the scores of two students (input signals), we can see who performed better (the output signal). The method used to ensure both scores are accurately weighed is similar to the current mirror ensuring stable current.
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Here we do have the differential amplifier which is having till resistor as a passive element, and also the load part it is passive. Now, here instead of R_T, what we are using is transistor-3 which is getting a bias from transistor-4 and the R_BIAS circuit.
In this part, we discuss replacing the passive tail resistor of the differential amplifier with a transistor that receives its bias from another transistor. This helps improve the current flow consistency and overall circuit performance. The signalling aspect also implies that the circuit can respond better compared to using resistors.
Imagine a car engine where the fuel supply (tail resistor) is replaced by a smart automated fuel injector (transistor). This ensures that the fuel flow (current) is regulated based on the engine's needs, enhancing the car's performance.
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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. Now in this case, the reference current I_REF, it is coming from V_CC.
The current mirror is a critical component that allows the design to maintain a stable bias current across the transistors. The reference current sourced from V_CC is essential in deriving the operational currents needed for the amplifierβs function. This component works to βmirrorβ the current accurately from one part of the circuit to another.
Think of a current mirror like a shadow β if you have a strong light source (V_CC) illuminating an object (the reference current), that object's shadow is consistently cast, mirroring its shape and intensity in another area (the output current).
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Now again similar to the previous case here if you analyze we can find the common mode gain. And that can be well approximated by ...
In this part, we learn to calculate the common mode gain of the differential amplifier. This gain allows us to understand how much of the unwanted common signal is being processed. The analysis involves determining how the differential signal interacts with the circuit when both inputs are experiencing similar signals.
Consider two people chatting in a crowded room. The common mode gain is like muffled background noise; it represents the sounds that both persons hear together, while the differential mode is their actual conversation. By maximizing the differential mode (the conversation), we minimize the impact of the background noise.
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Now similar to the previous case, this part can also be replaced by active load, and that load current of course, should be consistent with whatever the current we do have flowing through transistor-3.
When we replace the passive load with an active load in this amplifier design, we improve the performance of the amplifier because it allows for better control of the load current. This matches with the current flowing through transistor-3, ensuring that the circuit operates efficiently.
Using an active load is like using a smart thermostat for temperature control instead of an old radiator. The thermostat adjusts the heating level actively based on the room temperature (current), instead of allowing the heat to radiate passively at a constant rate.
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So, to summarize we have replaced both the active load part and the tail part it is getting in fact, current mirror. So, that makes the differential mode gain and common mode gain getting improvised.
This conclusion draws together the previous points, showing that implementing both an active load and using a current mirror improves differential and common mode gains. Improved gains mean the differential amplifier can effectively amplify the desired signal while reducing noise interference.
It's like upgrading your smartphone's camera from a basic model to a high-end one. The high-end camera improves focus (differential gain) while also minimizing flash overexposure (common mode gain), giving you clearer, more detailed photos.
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
Current Mirror: A configuration that allows for biasing and current sourcing in differential amplifiers.
Tail Current: This is critical for setting the operating point and must be stable for an amplifier to function effectively.
Differential and Common Mode Gain: Understanding the difference is crucial for ensuring desired signal amplification in various conditions.
See how the concepts apply in real-world scenarios to understand their practical implications.
In a differential amplifier circuit, swapping a passive resistor for a current mirror allows increased gain without imposing significant voltage drops.
Utilizing active loads, such as a current mirror, allows for better matching and signal propagation, particularly in noisy environments.
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
A mirror reflects, but not in sight, it helps set currents, keeping them just right.
Imagine a tightrope walker who must stay balanced above a crowd. The current mirror acts like a steady hand, ensuring the tightrope's tension remains even on both ends, reflecting balance through stable currents.
Remember RACE for current mirror fundamentals: Reflect, Apply, Control, Extend.
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Review the Definitions for terms.
Term: BJT
Definition:
Bipolar Junction Transistor, a type of transistor that uses both electron and hole charge carriers.
Term: Current Mirror
Definition:
A circuit configuration that produces a constant current through one active device that is set by another device.
Term: Tail Current
Definition:
The biasing current through the active device that sets the operating point of the amplifier.
Term: Common Mode Gain
Definition:
The gain of the amplifier when input signals are applied equally to both inputs, effectively reflecting the common component.
Term: Differential Mode Gain
Definition:
The amplifier gain measured when equal and opposite signals are applied to the inputs.