86.2.1 - Simple Current Mirror with MOSFET
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Introduction to Current Mirrors
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Today, we will dive into the workings of current mirrors, starting with the simple MOSFET configurations. Can anyone tell me what a current mirror does?
It replicates a current in another part of a circuit.
Exactly! Current mirrors maintain a constant current despite variations in power supply or load. They’re widely used in analog circuits. Let's explore the simple current mirror using MOSFETs.
What distinguishes a simple current mirror from a more complex one?
Simple current mirrors consist only of two transistors. More advanced versions might include additional transistors to improve accuracy and reduce non-ideal effects. Let’s look into some numerical examples.
Numerical Example Analysis
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Let’s examine a simple current mirror circuit. We know that we have a reference current. Can someone remind me of the formula for the output current in a current mirror?
Is it I_DS2 = K * (I_REF)?
Close! Remember K is the transconductance parameter, and it varies among the transistors. For our example, we have two MOSFETs with K factors of 1 mA/V² and 4 mA/V², respectively. What would be the output current if I_REF is 0.5 mA?
Wouldn’t it be 2 mA for the second transistor?
Correct! Now, let’s calculate the gate-source voltage for transistor 1 needed for this operation.
Saturation Conditions and Non-Ideal Effects
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A crucial aspect of MOSFET operation is ensuring they remain in saturation. What does that mean?
It means the drain-source voltage must be greater than the gate-source voltage minus the threshold voltage.
Exactly! For our MOSFETs in the example, can anyone determine the minimum V_DS for transistor 2?
If V_GS = 2.5V and V_th = 1.5V, then V_DS(min) would be 1V?
Exactly right! This is critical to ensure the current mirror functions as intended without distortion.
Impact of Channel Length Modulation
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Now, let’s discuss channel length modulation. Who recalls how it affects the output current?
It can cause the output current to vary based on the drain-source voltage.
Right! We adjust for this using the lambda parameter. If we calculate our output current with a lambda of 0.01 V⁻¹, how does it affect our mirroring?
I think it will slightly increase the output current, right?
Exactly, and understanding these nuances allows us to build better circuits.
Introduction & Overview
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Quick Overview
Standard
The section elaborates on the operation of simple current mirrors constructed using MOSFET transistors. Through a series of numerical examples, it explains how to calculate key parameters such as output current and necessary voltages while considering possible non-ideal effects including channel length modulation.
Detailed
Simple Current Mirror with MOSFET
In this section, we explore the operation of simple current mirrors created with MOSFET transistors. A current mirror is a crucial building block in analog electronics that allows for the replication of current through its branches, providing consistent operation for various applications.
Key Concepts:
- Current Mirror Configuration: The configuration involves at least two MOSFETs where one acts as a reference current source, and the other mirrors this current.
- Numerical Examples: The section presents detailed numerical examples, beginning with the calculation of voltages (V_DS) and output currents (I_DS) for various configurations and input conditions.
- Channel Length Modulation: It also discusses the impact of channel length modulation on the output current by introducing parameters such as the channel length modulation factor (λ).
- Design Considerations: The importance of ensuring that transistors remain in saturation for proper operation is emphasized with calculations that help determine minimum voltage requirements.
The section concludes with guidance on typical values used in current mirror circuits and their implications in design contexts.
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Understanding the Basic Structure of a Current Mirror
Chapter 1 of 4
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Chapter Content
A simple current mirror constructed by MOSFET, here we do have the example. So here we do have the example circuit where M1 and M2 are forming a current mirror. We do have a reference current here and then we do have the application circuit here.
Detailed Explanation
In a simple current mirror design, two MOSFETs (referenced as M1 and M2) are used to create a circuit that can replicate a certain reference current in another branch of the circuit. The primary role of M1 is to establish a reference current that M2 will then mirror. This type of circuit is crucial in analog signal processing as it allows for controlled current outputs that can be used in various applications.
Examples & Analogies
Think of a simple current mirror like a photocopy machine. Just as the machine can duplicate a document from one sheet to another, a current mirror duplicates (or 'mirrors') the current from one branch of the circuit to another. If you set a certain amount of current like setting the copy machine to copy at 10 sheets, the current mirror will 'copy' that reference current into its output.
Parameters of the MOSFETs
Chapter 2 of 4
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the of transistor-1 and transistor-2 along with the K factor, it is given. For transistor-1, we do have 1 mA/V^2. For transistor-2, we do have 4 mA/V^2. And let me assume that both the transistors are having equal threshold voltage of 1.5 V.
Detailed Explanation
In this section, we consider important characteristics of the MOSFETs used in the current mirror. Each MOSFET has a transconductance parameter (K factor), which indicates how much the current through the device will change with variations in voltage. We have two transistors: Transistor 1 has a K factor of 1 mA/V², while Transistor 2 has a K factor of 4 mA/V². Additionally, the threshold voltage, which is the minimum gate-to-source voltage needed for the transistor to conduct, is assumed to be the same for both, set at 1.5 V. This equivalence simplifies our calculations and analysis.
Examples & Analogies
Imagine you have two water hoses with different diameters. Hose 1 (transistor 1) allows a smaller flow of water compared to Hose 2 (transistor 2), which can push a much larger flow due to its diameter. When you set a certain water pressure at the entrance (akin to the threshold voltage), both hoses will have differences in water output depending on their size (represented by the K factor).
Calculating the Output Currents
Chapter 3 of 4
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To start with, let we go simpler version, ignoring lambda effect considering both the λ’s are very small and let we try to find the values of V_GS1 and I_DS2. Of course, I_DS1 is same as I_REF, specifically, 0.5 mA.
Detailed Explanation
In our calculations, we simplify our analysis by ignoring the lambda effect, which relates to the output resistance of MOSFETs. We begin our calculations with the reference input current (I_REF) being set at 0.5 mA. Since I_DS1 (the drain current of M1) is equal to I_REF, it is also 0.5 mA. Our next step involves calculating the gate-source voltage (V_GS1) for M1, and then using the K factor of each transistor to determine the output current of M2 (I_DS2). This current differs from I_DS1 based on the K factors of the two transistors.
Examples & Analogies
Think of a chef using different sizes of bowls to serve food. If one bowl (transistor M1) is set to hold a specific amount of food (0.5 mA), and another larger bowl (transistor M2) is meant to hold a proportional amount based on its size (K factor), then depending on the bowl size, the amount of food served will vary. Here, we calculate that output food quantity using the sizes (K factors) of both bowls!
Condition for Proper Operation
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Chapter Content
we need to find what is the minimum value of V_DS2 for proper operation of the circuit. Namely, the current mirror output current can be well defined by this equation only when transistor-2 also in saturation.
Detailed Explanation
To ensure that the current mirror functions properly, it's essential that both transistors operate in saturation mode. This means that the drain-source voltage (V_DS2) for transistor M2 must exceed a particular threshold to keep it in saturation, allowing it to effectively mirror the current without distortion. In this instance, for M2 to remain in saturation, the V_DS2 must be greater than V_GS2 - V_th (the gate-source voltage minus the threshold voltage), influencing how we set up the circuit.
Examples & Analogies
Think of a bridge that allows vehicles to cross only if it’s extended fully. Just like the bridge requires a minimum height from the water level (akin to V_DS2) to function properly, transistor M2 requires a sufficient voltage difference (V_DS) to operate correctly. If the bridge isn't high enough, cars can't cross safely, just like the current can't flow through the transistor effectively if the voltage isn’t sufficient.
Key Concepts
-
Current Mirror Configuration: The configuration involves at least two MOSFETs where one acts as a reference current source, and the other mirrors this current.
-
Numerical Examples: The section presents detailed numerical examples, beginning with the calculation of voltages (V_DS) and output currents (I_DS) for various configurations and input conditions.
-
Channel Length Modulation: It also discusses the impact of channel length modulation on the output current by introducing parameters such as the channel length modulation factor (λ).
-
Design Considerations: The importance of ensuring that transistors remain in saturation for proper operation is emphasized with calculations that help determine minimum voltage requirements.
-
The section concludes with guidance on typical values used in current mirror circuits and their implications in design contexts.
Examples & Applications
Calculation of output current using known reference current and K factors.
Determining minimum voltage requirements for ensuring MOSFET saturation.
Memory Aids
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Rhymes
In a current mirror so fine, currents stay in line; with MOSFETs they shine, creating output divine.
Stories
Once in an electrical circuit land, two transistors took a stand – one for reference, one to mirror, keeping currents steady, making designers shiver!
Memory Tools
Remember: 'MIRROR' - MOSFETs, Input voltage, Replicate, Reference current, Outputs.
Acronyms
SIMPLE - Saturation, Input-output, MOSFET, Parameters, Length modulation, Effects.
Flash Cards
Glossary
- Current Mirror
A circuit that replicates a current in a different branch, maintaining constant current outputs.
- MOSFET
Metal-oxide-semiconductor field-effect transistor, a type of transistor used to switch or amplify electronic signals.
- K Factor
The transconductance parameter of a MOSFET, indicating the output current per unit voltage.
- Saturation
A region of operation for a MOSFET where it behaves like a current source, important for accurate current mirrors.
- Channel Length Modulation
The effect where the output current of a MOSFET varies with the output voltage due to changes in the channel length.
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