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Introduction to Current Mirrors with BJTs
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Today, we will discuss current mirrors using BJTs and how they function in analog circuits.
What exactly is a current mirror?
A current mirror is a circuit that produces a constant currentβregardless of voltage fluctuationsβby 'mirroring' a reference current. It ensures that output current remains stable.
How is it constructed?
Typically, it uses two BJTs connected. The first BJT sets the reference current, and the second BJT tries to replicate that current.
Are there factors that can affect its performance?
Absolutely! Factors like early voltage, and base current need to be considered for precise operation.
What is early voltage?
Early voltage is associated with the output characteristics of a BJT that impacts the mirror's accuracy. A higher early voltage results in better output current mirroring.
In summary, current mirrors are vital for creating stable current sources, and we need to account for certain non-idealities in our designs.
Numerical Examples of BJT Current Mirrors
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Letβs analyze a numerical example. We have a current mirror comprising two BJTs, Q1 and Q2, with known parameters including base-emitter voltages.
What parameters are we provided here?
We're given the reverse saturation currents and the supply voltage. Can anyone calculate the reference current from this data?
I think we can use Ohm's law, but what would be the equation?
"Good question! The reference current (
Improving Current Mirrors Using a Beta-Helper Circuit
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Now that we understand the basics, letβs discuss how to improve our current mirror with a beta-helper circuit.
What does a beta-helper circuit do?
The beta-helper circuit helps reduce the base current loss in the mirror circuit, allowing for better current matching between Q1 and Q2.
How do we construct one?
Simply add a third transistor that allows for a more direct current flow, thereby reducing the base current drawn.
What should we expect in the output when using a beta-helper?
You can expect more consistent output currents that closely mirror the reference current, improving circuit performance.
In conclusion, including a beta-helper aids in maintaining output current consistency in BJT-based current mirrors.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
The section covers the theory and numerical examples of current mirrors built with BJTs. It discusses key concepts such as reference current, non-ideality factors due to early voltage, and base current loss, while showcasing practical applications in electronic circuits.
Detailed
In this section, we delve into the implementation of current mirrors using Bipolar Junction Transistors (BJTs). Current mirrors are pivotal in analog circuits, providing stable and precise current sources. The section outlines various complexities including the determination of reference current, derivation of output currents considering non-idealities, and explores the impact of early voltage and base current on performance. Numerical examples illustrate how to compute expected output currents in both ideal and non-ideal scenarios, emphasizing the importance of accurate calculations in designing effective current mirrors. Finally, enhancements such as the beta-helper circuit are introduced to mitigate loss due to base currents, showcasing practical approaches to improving current mirror performance.
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Introduction to Current Mirror using BJT
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In this example, we do have Q1 and Q2. Now it is forming the current mirror and in this case, just for a change, instead of giving a reference current, we are giving a resistor here, supply voltage it is given to us 12 V. This RBIAS resistor in resistance it is 22.8 kβ¦ and then we can assume that VBE(on) voltage for both the transistors are approximately 0.6 V. In addition to that, we also have the information about reverse saturation current of the 2 transistors. So, Q1 is having reverse saturation current of 9.5 Γ 10β14 A. On the other hand, for Q2 we do have reverse saturation current which is 2.85 Γ 10β13 A.
Detailed Explanation
In this chunk, we are presented with an example of a current mirror constructed using BJTs (Bipolar Junction Transistors). We start with two transistors, Q1 and Q2, which are configured to form the current mirror. Instead of supplying a reference current directly, a resistor is used, which plays a crucial role in establishing the current. The supply voltage for this setup is stated to be 12 V, and the resistor, R_BIAS, has a resistance of 22.8 kβ¦. Additionally, both transistors are assumed to have a base-emitter voltage of approximately 0.6 V. The reverse saturation current for Q1 and Q2 are given, indicating their respective starting point for current flow. This setup is fundamental in understanding how a current mirror operates using BJTs.
Examples & Analogies
Think of the current mirror as a water irrigation system. Here, Q1 and Q2 represent two separate pipes. Instead of directly pumping water (current) into each pipe (transistor), we use a control valve (the resistor, R_BIAS) to regulate how much water flows into each pipe. The main water supply (the 12 V voltage) and the characteristics of each pipe (the saturation currents) determine how effectively we can maintain a consistent flow across both pipes.
Calculating Reference and Output Currents
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To get thus this I reference current, we need to find what will be the IREF. IREF = VBIAS / RREF = 12 V / 22.8 kβ¦ = 0.526 mA. Now, if I consider a simple situation considering both the Ξ²βs are very high, early voltages they are also very high which means that non-ideality factor we can we are β 1. So, the current flow here IC2 is given by IC2 = C2 * IREF where C2 is the mirroring ratio. In our case, with the approximation that both BJTs operate identically, the mirroring ratio would typically be around 1:3, yielding an output current of 1.5 mA.
Detailed Explanation
Next, we delve into calculating the reference current (I_REF), which is essential to understanding the current mirror's performance. The reference current is determined by the voltage across the resistor (V_BIAS) divided by its resistance (R_BIAS). In this scenario, we find that I_REF equals approximately 0.526 mA. Assumptions are made that the beta (Ξ²) values of both transistors are significantly high, leading to a high Early voltage which implies a negligible non-ideality factor. With this, we can conclude that the output current (I_C2) produced by Q2 can be calculated by multiplying the reference current (I_REF) by the mirroring ratio, which is 1:3. This means for every 1 mA flowing into Q1, about 3 mA can be expected to flow out from Q2, resulting in an output current of approximately 1.5 mA.
Examples & Analogies
Returning to the irrigation analogy, the I_REF is like measuring the amount of water being fed to our control valve (R_BIAS). If we know how much water (voltage) is available and how tight the valve is (resistance), we can determine how much water will flow through the pipes. The mirroring ratio could be seen as the efficiency of the pipes: if one pipe can carry three times more than the other, then we can expect that for every liter supplied, three liters will flow through that more efficient pipe.
Impact of Beta and Early Voltage
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Now, let us consider the effect of Ξ² namely, the current loss due to whatever the currents are flowing here. This means, this is no more β 1 and to get this non-ideality factor you may recall that this part depends on Ξ²1 and Ξ²2 which represent the current gains of Q1 and Q2 respectively. If for simplicity Ξ²1 = 150 and Ξ²2 = 50, the non-ideality factor would be calculated to give a correction to the output current, which may decrease to around 1.456 mA from the ideal 1.5 mA.
Detailed Explanation
In this section, we discuss the influence of beta (Ξ²), which represents the current gain for each transistor in the current mirror. It is vital to recognize that lower Ξ² values indicate a more significant loss of current because not all of the base current can be supported by the collector current. When we apply real-world values such as Ξ²1 = 150 for Q1 and Ξ²2 = 50 for Q2, we find that our previously calculated output current must be adjusted downward due to this effect. This results in a new output current of approximately 1.456 mA, showcasing how real-world imperfections affect ideal outputs.
Examples & Analogies
Think of water flow in a garden hose. If the hose diameter is sufficient (high Ξ²), water flows easily with minimal leaks (base current loss). However, if the hose has a narrow diameter (low Ξ²), then despite your strong water supply, much of it will leak out along the way, resulting in less water reaching the flower beds (the output). Thus, understanding the limitations due to Ξ² is akin to knowing the efficiency of how our water flows.
Considering Early Voltage Impact
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As seen before, if we consider the Early voltage and if you observe the VCE voltage difference, we can factor that into our currents. Accounting for a situation where the Early voltage is significantly high, the non-ideality factor comes into play, leading to adjusted current outputs. The performance may further diminish the values from the nominal amount because differences in Early voltage may result in non-ideal behaviors leading to discrepancies in output current.
Detailed Explanation
In this chunk, we shift our focus to Early voltage, a crucial parameter affecting how BJTs operate under various conditions. It relates to how effectively a transistor can maintain its current characteristics as the collector-emitter voltage (V_CE) changes. When Early voltage is considered, we can introduce additional corrections to our output current calculation. Higher differences in Early voltages between Q1 and Q2 can introduce further non-ideality (non-ideality factors), which impacts the measured output current negatively. This means real-world devices often perform worse than theoretical predictions, and understanding this helps design better circuitry.
Examples & Analogies
Imagine the growth of plants which need consistent watering to thrive. If you are using two hoses (transistors) with different pressures (Early voltages), one hose delivers water smoothly while the other has fluctuations based on how the pressure shifts. Just like plants respond better with steady water supply, circuits perform more reliably with consistent current flows. A higher Early voltage translates to less fluctuation and more stability.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Current Mirror: A circuit that replicates a current.
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Reference Current: Essential current for mirroring.
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Non-ideality Factors: Factors that impact accuracy.
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Beta-helper: A method to enhance current mirror functionality.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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A common example of a current mirror is the use of two BJTs where one sets the reference current, and the other replicates it under varying load conditions.
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Another example involves calculating output currents of BJTs considering their beta ratios and how to apply the beta-helper method to reduce errors.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
π΅ Rhymes Time
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Mirrors of current, steady and bright, in circuits they shine, ensuring it's right.
π Fascinating Stories
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Imagine two friends, one with a treasure of exact jewels. The first friend wants to give his treasure to the second without loss, and so they create a perfect scheme to ensure the second friend always receives the same consistent treasure.
π§ Other Memory Gems
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Remember the acronym 'CRISP' to recall factors in current mirrors: Current, Reference, Ideal, Saturation, Precision.
π― Super Acronyms
BJT
- Base Junction Transistor - the foundation of creating efficient current mirrors.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Current Mirror
Definition:
A circuit designed to provide a stable output current that mirrors a reference current.
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Term: Early Voltage
Definition:
A parameter that indicates how much the collector current changes with changes in collector-emitter voltage, affecting output current accuracy.
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Term: Betahelper Circuit
Definition:
An additional transistor that reduces base current losses in a current mirror, improving current accuracy.
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Term: Reference Current
Definition:
The original current that a current mirror is designed to replicate.
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Term: Nonideality Factor
Definition:
A parameter reflecting discrepancies in output current due to factors like base current losses.