Variants of Current Mirrors: Wilson, Widlar, etc.
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Overview of Current Mirrors
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Today, we're discussing current mirrors. Can anyone explain what a current mirror does?
Isn't it a circuit that replicates a current from one part to another?
That's correct! A current mirror uses matched transistors to copy a reference current. Remember, a mnemonic to recall this is 'Copy Current Technology' or CCT.
What are the benefits of using current mirrors?
They provide stable bias currents and can replace resistors in amplifier stages for better efficiency.
So, are there different types of current mirrors?
Yes, indeed! Let's delve into some of the variants like the Wilson and Widlar current mirrors.
Wilson Current Mirror
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First up is the Wilson current mirror. Can anyone describe its main feature?
It has three transistors, right? But what does that do?
Exactly! The extra transistor provides feedback which enhances the output resistance. Remember, 'Three's a Charm' can help you recall its three-transistor structure.
What about its limitations?
It requires a higher minimum output voltage, which might limit its application. Can you think of a scenario where this might be a concern?
If we're working with low supply voltages?
Exactly!
Widlar Current Mirror
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Now, letβs talk about the Widlar current mirror. What is its main advantage?
It generates small currents without needing large resistors!
Correct! By adding a resistor in the emitter path, it reduces output current significantly. Remember, 'Less is More' for small current outputs.
Are there any drawbacks to this configuration?
Yes, it can be sensitive to temperature variations. It's important to design accordingly.
Comparison of Current Mirrors
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Letβs compare the Wilson and Widlar mirrors. What stands out?
The Wilson has higher output resistance, but what about Widlar?
Widlar is great for low currents. Think of applications like low-power analog circuits.
Can we use both in a circuit?
Absolutely! You can design circuits that utilize both depending on the required performance characteristics.
This is really interesting how they complement each other.
Indeed, balancing performance and efficiency is crucial in circuit design.
Introduction & Overview
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Quick Overview
Standard
The section explores key variants of current mirrors, specifically the Wilson and Widlar configurations. The Wilson current mirror enhances output resistance and reduces base current errors, while the Widlar current mirror allows for the generation of small output currents using smaller resistors. It highlights the advantages and limitations of each design.
Detailed
Variants of Current Mirrors
In this section, we discuss two significant variants of current mirrors β the Wilson current mirror and the Widlar current mirror. These designs aim to address limitations found in the basic current mirror configurations, such as output resistance and current scalability. The Wilson current mirror consists of three transistors arranged to provide greater output resistance and minimize the influence of base currents on output accuracy. This is accomplished through negative feedback mechanisms inherent in its design.
In contrast, the Widlar current mirror is specifically engineered for applications requiring low output currents from a larger reference current. It does this by introducing a resistor in the emitter path of the output transistor, which allows the output current to be significantly less than the reference current without needing large resistors. Though advantageous for generating small currents, the Widlar mirror's sensitivity to temperature can be a drawback. This section illustrates the operational principles, advantages, and disadvantages of each variant, thereby emphasizing their roles in improving the precision and adaptability of current sources in integrated circuit design.
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Widlar Current Mirror
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The Widlar current mirror is specifically designed to create small output currents (I_out) from a larger reference current (I_ref) without requiring large resistors. This is particularly useful for low-power applications.
Advantages:
- Generates Small Currents: Can produce output currents that are significantly smaller than the reference current by adding a resistor in the emitter of the output transistor.
- Lower Resistance Values: Avoids the need for very large (and space-consuming) resistors to generate small currents.
Operation:
A resistor (R_E) is added in the emitter of the output transistor (Q2). This creates a voltage drop across R_E, reducing V_BE2 relative to V_BE1. Since I_C is exponentially related to V_BE, even a small difference in V_BE can lead to a large difference in I_C.
Key Equations (Widlar Mirror):
Assuming matched transistors and neglecting base currents for simplification:
- V_BE1βV_BE2=I_E2R_EapproxI_C2R_E=I_outR_E
- Also, V_BE1βV_BE2=V_Tln(frac{I_C1}{I_C2})=V_Tln(frac{I_ref}{I_out})
- Equating these: Iout R_E = V_T ln(I_out/I_ref). This equation must be solved iteratively or graphically to find I_out for a given I_ref and R_E. This allows I_out to be much smaller than I_ref.
Disadvantage:
- Sensitivity to Temperature: The V_T term makes the output current more sensitive to temperature variations.
- Slightly Lower Output Resistance: The output resistance is typically dominated by r_o of the output transistor and the feedback effect of R_E.
Detailed Explanation
The Widlar current mirror specializes in generating low output currents. It does this by strategically adding a resistor in the emitter of the transistor that delivers the output current. This setup ensures that a small change in voltage (due to the added resistor) can result in a larger change in current output. This property is especially useful in circuits where small bias currents are required, like in low-power applications. However, it does have some drawbacks, such as being sensitive to temperature, which can affect the performance of the current mirror.
Examples & Analogies
Imagine you need to water a small plant. Instead of using a large watering can (large current source), you could use a small spray bottle with a nozzle that allows you to control the flow (Widlar current mirror). This way, you can carefully give your plant just the right amount of water it needs without wastage, but if someone bumps the nozzle, it might spray differently due to variations in pressure.
Key Concepts
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Wilson Current Mirror: A configuration that enhances output resistance using an additional transistor.
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Widlar Current Mirror: A circuit that generates small output currents suitable for low-power applications.
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Output Resistance: Vital for a current mirror's function as it indicates its fidelity in constant current delivery.
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Base Current Error: The deviation caused by the base current affecting the output current accuracy.
Examples & Applications
An example of a Wilson current mirror can be used in precision analog circuits where high output impedance is essential, such as in op-amp circuits.
A Widlar current mirror may find its application in biasing small signal transistors in analog signal applications.
Memory Aids
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Rhymes
For current mirrors that truly shine, the Wilson's output resistance is divine.
Stories
Imagine two friends at a fair, the Wilson mirror is strong, standing tall, while the Widlar friend is careful and small, generating small outputs with ease in all.
Memory Tools
In a Wilson mirror β W stands for 'Wonderfulβ output resistance, while Widlar β W for 'Weak' currents.
Acronyms
CW - Current for Wilson, W for Wider and smaller currents for Widlar.
Flash Cards
Glossary
- Current Mirror
A circuit that duplicates a current from one branch to another, typically using matched transistors.
- Wilson Current Mirror
A three-transistor configuration designed to provide greater output resistance and improve accuracy.
- Widlar Current Mirror
A current mirror variant that generates small output currents by adding resistors to the emitter of the output transistor.
- Output Resistance
The resistance seen by the load at the output of a device or circuit; important for analyzing current sources.
- Base Current
The current flowing into the base terminal of a transistor, which can affect the overall output current in mirrors.
- Temperature Sensitivity
The response of a circuit's performance to changes in temperature, relevant in the context of the Widlar mirror.
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