83.3.2 - Small Signal Model of Current Mirror
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Introduction to Current Mirrors
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Today, we will explore current mirrors and specifically focus on the small signal model. Can anyone tell me what a current mirror is?
A current mirror is a circuit designed to copy a current from one active device to another, maintaining a constant current.
Exactly! Current mirrors are pivotal in circuits to maintain controlled currents. They can be created using transistors, either BJT or MOSFET. Today, we'll look specifically at their small signal models.
What does 'small signal' mean?
Good question! 'Small signal' refers to the condition where we analyze the circuit's behavior with small variations around a DC bias point, ignoring the larger DC levels. Let’s proceed and understand how we define small signal models.
Small Signal Model in DC Conditions
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Let's dive into the small signal model under DC conditions. For a current mirror not carrying any signal, we can simplify our analysis significantly. What happens to the components in this scenario?
The AC components would be ignored, and we treat the circuit as open?
Correct! When analyzing the small signal equivalent circuit under DC conditions, we focus on the resistance and dependent current sources. Each transistor operates at its DC bias without AC signals affecting our calculations.
So we consider no current flow in the AC analysis?
Exactly! This enables us to model the current mirror as a simple resistive element in many cases, making our calculations much more manageable.
Small Signal Model with Signal Currents
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Now, let’s look at the second case where the current mirror carries signal currents alongside the DC component. How would we analyze that?
I think we need to consider the AC signal superimposed on the DC current?
Right! In this case, we can model the input and output signals, leading to an expression for voltage gain. This requires understanding how AC signals affect the dependent current sources in the circuit.
Can you give an example of how we calculate that?
Sure! For a MOSFET-based current mirror with an input signal, the output voltage can be expressed as a function of the input current multiplied by the transconductance. Let’s move on to solidify this understanding.
Applications of Current Mirrors
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Current mirrors are extensively used in amplifier circuits. Can anyone think of specific applications?
They are used in differential amplifiers and to establish stable biasing in amplifiers.
Exactly! They help in improving linearity and gain in circuits such as common emitter amplifiers or common source amplifiers. In these circuits, the amplifying device operates ideally with well-matched currents.
So, using current mirrors can enhance the performance of our circuits?
Absolutely! By accurately reflecting currents, they play a crucial role in achieving desired performance metrics. Let's wrap up the session by summarizing our exploration of small signal models.
Comparative Analysis: BJT vs MOSFET
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Finally, let's compare the small signal models of BJTs and MOSFETs used in current mirrors. What do you think are the main differences?
I believe BJTs have a Bipolar nature while MOSFETs are field-effect devices? Does this affect their models?
Correct! This fundamental difference shapes how we model them. BJTs use transconductance in a base-emitter junction, while MOSFETs rely on gate-source voltage for current dependency. Let’s review the key aspects of each small signal model.
So we should remember both models when designing circuits?
Exactly! Understanding both types lets us choose the right tool for the job, ensuring that our designs are effective and reliable. Great job everyone today! Let’s summarize.
Introduction & Overview
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Quick Overview
Standard
The section elaborates on the small signal model of current mirrors, detailing their behavior under DC conditions and with signal currents. It reviews both BJT and MOSFET implementations and their respective small signal equivalent circuits, emphasizing their applications in amplifiers, particularly in differential amplifiers.
Detailed
In this section, we focus on the small signal model of current mirrors, which are crucial in various analog circuit applications. The discussion starts with the basic principles governing current mirrors in DC conditions and extends to scenarios where alternating signal currents are involved. We analyze two cases: first, where the current mirror operates without any signal (DC), showcasing the simplicity of its small signal model; second, where it accommodates signal currents, prompting a more complex interaction with the application circuit.
The MOSFET version of the current mirror is examined in tandem with the BJT version. The section conveys that the small signal equivalent circuit retains similar characteristics across these technologies, primarily involving dependent current sources and resistive elements. In practical applications, such as common emitter and common source amplifiers, the concepts discussed are essential for understanding how current mirrors enable enhanced performance. Their ability to reflect current accurately allows for improved linearity and gain across various circuits, linking the theoretical intricacies with real-world applications.
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Introduction to Small Signal Model
Chapter 1 of 5
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Chapter Content
To understand or to appreciate the effect of common current mirror in amplifier standard amplifier where, we normally talk about the linearized circuit whether it is common emitter or common source or common collector or common drain or for that matter even for differential amplifier.
Detailed Explanation
The purpose of studying the small signal model of a current mirror is to determine how it affects amplifier circuits. Depending on the specific type of amplifier (common emitter, common source, etc.), the small signal model can help us understand how the current mirror will respond to small variations in the input signal. This models the behavior under small perturbations around a DC operating point, allowing us to predict amplifier performance.
Examples & Analogies
Imagine tuning a musical instrument: your adjustments based on how the instrument sounds represent the small signals. Similarly, the small signal model fine-tunes how we understand the current mirror’s behavior in amplifiers, ensuring that we achieve the smoothest and most expected performance.
DC Condition Small Signal Equivalent Circuit
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Chapter Content
We need to understand the small signal model of current mirror under DC condition. For small signal model this is DC current so; obviously, we have to make the current here it is since it is 0. So, we can say it is this circuit is open.
Detailed Explanation
In the first case, when there is no AC signal (only DC), the small signal model becomes simpler. All signals are treated as zero, and the small signal equivalent circuit behaves like an open circuit because the DC current is constant and does not vary. This means that any change in input signal does not influence the behavior of the current mirror under these DC conditions.
Examples & Analogies
Think about a water faucet that has been turned off. No matter how much you adjust the faucet knobs (signal changes), no water flows (current). This is similar to the way the current mirror behaves under DC conditions—it doesn’t change until you open the faucet again (introducing an AC signal).
MOSFET Small Signal Model Analysis
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Chapter Content
When discussing the small signal model of current mirror implemented by MOSFET, we do have the small signal model for transistor-1, it is having g m v gs1 voltage dependent current flow.
Detailed Explanation
For a MOSFET-based current mirror, we analyze small signals by focusing on the transconductance parameter (g_m), which shows how changes in gate voltage (v_gs) affect drain current. This relationship allows us to define a more dynamic model that includes changes in both current and voltage. The small signal equivalent model takes into account how the signal influences the current mirror's output.
Examples & Analogies
Consider a volume knob on a stereo. Adjusting the knob changes the sound output dynamically. The g_m in our equation is like this knob—small changes in the input voltage (volume adjustment) result in proportionally larger changes in the output (sound volume).
Current Mirror with AC Signal
Chapter 4 of 5
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Chapter Content
Situation when the current mirror may have a signal part namely say i_in, it is also having signal current i_in. So this signal current it is again it is coming from either DC you are whatever it is, but finally, at the collect the drain node of the MOSFET it is arriving and it may be producing a voltage here.
Detailed Explanation
In situations where the current mirror carries a signal current, we need to modify our analysis. The presence of AC signals alters the behavior of the circuit, introducing variables dependent on this signal current. It allows us to utilize the relationship between the input signal current and the output voltage generated by the current mirror, yielding a much richer model that reflects real-world usage.
Examples & Analogies
Think of a busy restaurant where the waiter takes orders (the AC signal) that change frequently. The kitchen (current mirror) needs to adapt and respond to each order quickly, just like the current mirror adapts to changes in signal current. Just as the waiter keeps track of orders, we must track how signal currents affect our circuit performance.
BJT Small Signal Model Analysis
Chapter 5 of 5
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Chapter Content
Here we do have the small signal equivalent circuit of the application module. So, this is small signal equivalent circuit of the application module and then only r is there.
Detailed Explanation
Just like the MOSFET analysis, we can also model current mirrors using BJTs. The BJT small signal model incorporates additional elements such as base-emitter resistance and transconductance similar to what we discussed for MOSFETs. Despite the differences in device behavior, the analytical approach follows the same logic of evaluating small signal changes.
Examples & Analogies
Consider a performance by an orchestra where two different conductors are leading different sections (BJT and MOSFET). While each conductor has a distinct style (device behavior), the overall music produced depends on their ability to adapt to the orchestra's performance (small signal response). Both lead their sections in a responsive manner similar to how BJTs and MOSFETs adjust to small signals.
Key Concepts
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Small Signal Model: Analytically represents circuit behavior under small variations.
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BJT vs MOSFET: Different transistor types influence small signal characteristics.
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Signal Analysis: Distinguishes behavior of circuits carrying AC signals.
Examples & Applications
A BJT current mirror with a stable reference current versus one with signal input.
A differential amplifier using a current mirror for improved biasing and performance.
Memory Aids
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Rhymes
When currents flow, and mirrors show, the stable path is what we know.
Stories
Imagine a fountain where one spout directs water to others below; current mirrors ensure each receives an equal flow, creating harmony in circuits.
Memory Tools
To remember characteristics of BJTs and MOSFETs, think: B-JT, for 'Base-Junction Transistor' and M-OSET, for 'Metal Oxide Source Effect Transistor'.
Acronyms
MIRROR
Maintain Input
Reflect Resistor Output.
Flash Cards
Glossary
- Current Mirror
A circuit that reflects an input current through a replicated current source.
- Small Signal Model
An approximation of a circuit's behavior under small AC signal variations around a DC operating point.
- BJT (Bipolar Junction Transistor)
A type of transistor that uses both electron and hole charge carriers.
- MOSFET (Metal Oxide Semiconductor FieldEffect Transistor)
A type of field-effect transistor that uses an electric field to control current.
- Transconductance (g_m)
A measure of the rate of change of the output current with respect to a change in input voltage.
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