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Understanding V-I Characteristics
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Today, we'll discuss the V-I characteristics of current mirrors. Can anyone tell me what V-I characteristics refer to?
Is it the relationship between voltage and current in the circuit?
Exactly! In current mirrors, we focus on how the output current changes as the output voltage varies, especially under practical conditions.
How does this change in current happen?
Great question! In BJTs, it’s due to the Early effect, where output current slightly increases with voltage due to changes in the device characteristics. Meanwhile, in MOSFETs, we deal with channel-length modulation.
So, the output can’t just remain constant as we change the voltage?
Correct! Let’s remember this with the acronym EVOC, which stands for 'Even Voltage, Output Changes.'
In summary, the relationship between output current and voltage is influenced by properties unique to each transistor type, which we will explore further.
Output Resistance
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Now, let's talk about output resistance, R_out. Can someone explain its significance?
Isn’t it important for ensuring that the current mirror behaves like a constant current source?
Absolutely! An ideal current source should have infinite resistance. In practice, BJTs and MOSFETs exhibit finite resistance due to various effects.
What about the differences between BJTs and MOSFETs regarding output resistance?
Good point! For BJTs, R_out primarily depends on the output transistor parameters and the Early effect. For MOSFETs, it is governed by channel-length modulation. Advanced configurations like the Wilson current mirror enhance this resistance further.
Can we visualize this somehow?
You could think of output resistance like a buffer in a pipe. A higher resistance means less flow change with varying pressures. Remember, R_out is vital for maintaining consistent current across varying loads.
In summary, R_out is crucial for the stability of the current supplied to the load. Next, we will discuss the minimum sustainable voltage.
Minimum Sustainable Voltage (V_ON)
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Finally, let's delve into the minimum sustainable voltage, V_ON. Why is it necessary?
It’s the minimum voltage required for the current mirror to work correctly?
Correct! If V_ON isn't met, the current mirror may not properly deliver its intended output current.
What determines V_ON for BJTs and MOSFETs?
For BJTs, it typically relates to the base-emitter voltage, approximately 0.7 V. For MOSFETs, it relates to the gate-source voltage. The design of mirrors like Wilson adds complexity since it results in higher V_ON due to additional transistors.
So, higher V_ON means less efficiency in driving loads?
Exactly! Always design with V_ON in mind for efficient current mirror applications. We can remember this with the mnemonic HOT, or 'Higher Output Threshold'.
In summary, V_ON is essential for understanding how low voltage can impact the performance of our current mirrors. This factor can limit the range of output voltage in real applications.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
The V-I characteristics highlight the relationship between output current and output voltage in current mirrors, describing their behavior under practical conditions, and discussing the implications of output resistance and compliance voltage (V_ON) in effective circuit design.
Detailed
V-I Characteristics: Output Resistance and Minimum Sustainable Voltage (V_ON)
Overview
This section covers the V-I characteristics of current mirrors, focusing primarily on two significant aspects: output resistance and the minimum sustainable voltage (V_ON). An ideal current mirror should ideally maintain a constant output current regardless of the output voltage across its terminals. However, practical implementations show variations due to certain phenomena.
Key Points
- Output Current Dependence: The output current (I_out) shows a reliance on the output voltage (V_CE for BJTs and V_DS for MOSFETs). In BJTs, the Early effect causes I_C to slightly increase as V_CE increases, while in MOSFETs, channel length modulation similarly affects I_D.
- Output Resistance (R_out): An important measure of a current mirror’s functionality as a constant current source. Practical output resistance values vary with the circuit design. For basic mirrors, R_out is dictated by the output transistor's characteristics and its output resistance due to the Early effect for BJTs and channel-length modulation for MOSFETs. Advanced designs like the Wilson and Widlar mirrors demonstrate higher output resistance, increasing performance.
- Minimum Sustainable Voltage (V_ON): The lower limit of the voltage drop necessary for the current mirror to function correctly without falling out of its operational region. For BJTs, this typically corresponds to V_BE(on) (around 0.7 V), while for MOSFETs, it equates to V_GS(on). Increasing voltage requirements exist in more complex configurations, notably in Wilson mirrors.
Importance
Understanding these characteristics is crucial in the design and implementation of current mirrors, allowing engineers to create circuits that ensure reliable performance over varied operational conditions.
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V-I Characteristics Overview
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The ideal current mirror should provide a constant output current (I_out) regardless of the voltage across its output terminals (V_CE2 for BJT, V_DS2 for MOSFET). However, in practical current mirrors, the output current shows some dependence on the output voltage.
Detailed Explanation
In an ideal current mirror, when you apply a certain reference current (I_ref), the output current (I_out) should remain constant no matter how much voltage is applied across its output. However, in reality, as the output voltage increases – due to factors like the Early effect in BJTs or channel-length modulation in MOSFETs – the output current often still increases slightly instead of staying completely flat. This demonstrates that while current mirrors aim to supply constant current, their operation does depend somewhat on the output voltage.
Examples & Analogies
Imagine a water hose that is supposed to keep a steady flow of water (the current). However, as you adjust the nozzle (the output voltage), it might slightly increase the water flow due to back pressure from the tank. So even though you are trying to maintain a constant flow, changes in the nozzle pressure (output voltage) can affect it.
Output Resistance (R_out)
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Output resistance is a measure of how well the current mirror acts as a constant current source. An ideal current source has infinite output resistance.
- Basic BJT Current Mirror: The output resistance is primarily the output resistance of the transistor Q2, which is r_o2 (due to Early effect).
R_out = r_o2 = I_out / V_A - Basic MOSFET Current Mirror: The output resistance is primarily the output resistance of the transistor M2, which is r_o2 (due to channel-length modulation).
R_out = r_o2 = λI_out - Wilson Current Mirror: Significantly higher output resistance, typically orders of magnitude greater than the basic mirror.
For BJT Wilson mirror: R_out ≈ β r_o2.
For MOSFET Wilson mirror: R_out ≈ g_m3 r_o3 r_o2. - Widlar Current Mirror: The output resistance is also enhanced by the emitter resistor.
R_out = r_o2 (1 + g_m2 R_E)
Detailed Explanation
Output resistance tells us how well a current mirror can maintain a fixed output current despite changes in output voltage. If the output resistance is very high, the current will remain constant even as the voltage changes. In a basic BJT current mirror, this resistance comes from the transistor's output characteristics (r_o2). For MOSFETs, this resistance is influenced by channel-length modulation. In advanced designs, like the Wilson mirror, the output resistance is improved through feedback mechanisms, allowing for much higher output resistance compared to the basic mirrors. The Widlar mirror also enhances resistance by adding an emitter resistor, which increases the resistance faced by the current, allowing the mirror to better maintain the output current.
Examples & Analogies
Consider a good-quality battery-powered light bulb. If the voltage from the battery (output voltage) fluctuates a bit, the bulb stays bright (constant current). The bulb’s ability to maintain brightness despite fluctuations can be compared to the output resistance of a current mirror. In more advanced setups (like a better battery pack), additional features are included that help keep the brightness even steadier, similar to how the Wilson or Widlar mirrors enhance their output resistances.
Minimum Sustainable Voltage (V_ON)
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This is the minimum voltage drop required across the current mirror's output terminals for it to operate correctly and supply the intended current. Below this voltage, the output transistor comes out of saturation (for MOSFETs) or active region (for BJTs), and the mirrored current deviates significantly from the desired value.
- Basic BJT Current Mirror: For Q2 to be in the active region, V_C2 must be greater than V_B2−0.4V (approximately). Since V_B2=V_BE1, V_ON ≈ V_BE1(on).
- Basic MOSFET Current Mirror: For M2 to be in saturation, V_DS2 ≥ V_GS2−V_th. V_ON = V_GS1.
- Wilson Current Mirror: Requires higher V_ON due to the series connection of transistors.
V_ON ≈ V_BE1 + V_CE(sat)_Q3 ≈ V_BE + 0.2V. - Widlar Current Mirror: V_ON is approximately V_BE(on) plus the voltage drop across R_E.
Detailed Explanation
The minimum sustainable voltage (V_ON) is crucial because it determines the lowest output voltage a current mirror can have while still being able to supply the desired current. If the voltage falls below this threshold, the output transistor will not be able to remain in its active region, causing it to malfunction and reducing the output current. Different types of mirrors have different V_ON requirements due to their configurations, with basic designs needing just enough voltage for the transistors to operate correctly, while more complex designs may need additional voltage due to extra components influencing the operation zone.
Examples & Analogies
Think of V_ON like the minimum engine power needed for a car to maintain a certain speed uphill. If you don't have enough power pushing the car (voltage), it can't maintain its speed and starts to slow down (current drops). Just like a car needing more power to go up a steeper hill, a complex current mirror might need a higher V_ON to function correctly compared to a simpler model that can go up easier slopes.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Output Current Dependence: The output current varies with the output voltage due to effects in BJTs and MOSFETs.
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Output Resistance (R_out): A measure of the constant current source capability; higher values indicate better performance.
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Minimum Sustainable Voltage (V_ON): The necessary voltage required for current mirrors to perform correctly, influencing design considerations.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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The output current of a BJT current mirror can be graphed as I_C2 vs. V_CE2, showing an increase in I_C2 with increasing V_CE2 due to the Early effect.
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In a MOSFET current mirror, I_D2 vs. V_DS2 would show a slight increase as V_DS2 increases due to channel-length modulation.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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In mirrors, V_ON must stay; keep the current bright today.
📖 Fascinating Stories
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Imagine a water fountain. The water level (V_ON) must be maintained to keep the flow (current) continuous. If it drops too low, the water above can't push downer, and the fountain won't work properly.
🧠 Other Memory Gems
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Use 'VEC' to remember: Voltage must be enough to keep the Current flowing.
🎯 Super Acronyms
R.O.V.= Right Output Voltage means the current stays consistent.
Flash Cards
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Glossary of Terms
Review the Definitions for terms.
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Term: Output Resistance (R_out)
Definition:
A measure of how well the current mirror maintains a constant output current despite changes in output voltage.
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Term: Minimum Sustainable Voltage (V_ON)
Definition:
The minimum voltage drop required across the current mirror's output terminals to maintain proper operation and deliver the intended current.
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Term: Early Effect
Definition:
A phenomenon in BJTs where the output current increases with an increase in collector-emitter voltage due to changes in base width.
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Term: ChannelLength Modulation
Definition:
A phenomenon in MOSFETs where the output current varies with the output voltage due to changes in the effective channel length.