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Today, we're going to discuss current mirror circuits, both BJT and MOSFET versions. Who can tell me what a current mirror does?
A current mirror is used to provide a constant current in a circuit, mimicking a reference current!
Exactly! It uses a reference current to generate an output current that ideally mirrors this reference. Can anyone explain how this works for a MOSFET current mirror?
In a MOSFET current mirror, the first transistor is diode-connected, maintaining a voltage that is passed to the second transistor.
Great point! We set the VGS of the first transistor to establish a reference for the second. This reference current is paramount in determining the output current I2.
And it sounds like if the output voltage changes, the current mirror still tries to keep the I2 stable, right?
That's right! This leads us to the concept of output resistance, which we will discuss next. Let's recap: current mirrors help in providing a constant current by mirroring a reference current. Any questions before we move on?
Let's derive the output current expression for the MOSFET current mirror. Can anyone remind me how we start this derivation?
We assume that the transistor is in saturation and that VGS is equal for both transistors.
Correct! The output current I2 ultimately depends on the reference current I1 and the aspect ratio of the transistors. What do we need to consider about λ or the channel length modulation?
It adjusts the current based on the effect of VDS on the output, right? The output current gets influenced by the non-ideality factor.
Exactly! This non-ideality factor can increase or decrease the current based on how VDS2 relates to VDS1. So, the expression for I2 becomes I1 times this factor. What is this factor again?
It’s (1 + λ(VDS2 - VDS1)).
Perfect! Remember that if VDS2 is smaller than VDS1, this factor can shrink, affecting our output current. Any questions on this derivation?
Now, let's talk about output resistance, denoted as R_out. Why is this parameter important for the current mirror?
Higher output resistance means the current is less dependent on changes in output voltage.
Exactly! A high output resistance helps maintain a stable I2 despite varying load conditions. Can anyone explain how we achieve high output resistance?
By keeping the transistors in the saturation region and ensuring proper voltage levels are maintained, like VDS being greater than V(th).
Well said! Thus, the circuit needs to be designed to maintain an adequate voltage across the mirrors to keep them operational in saturation. Recapping, a higher R_out means better current stability. Questions?
Let's shift gears to BJTs. How does the BJT current mirror compare to the MOSFET current mirror we've just discussed?
They both aim to mirror currents, but the BJT versions have to consider base currents which affect the equality of I1 and I2.
Right! In BJTs, I2 is not directly equal to I1 due to base current losses. What can we write to account for this in our expressions?
We should include the base currents in our calculations, which can be expressed in terms of I2 and I1.
Exactly! This complicates the model slightly but allows us to maintain similar current mirroring principles. Remember, the goal is to keep the transistors ideally in their active region. Good recap here!
Circuits that replicate a reference current for consistency in outputs.
A critical measure reflecting the default performance of the mirror under varying loads.
Indicates how real-world conditions modify the intended mirror current.
The higher it is, the more stable the current output under electrical load conditions.
Affects the performance of MOSFET mirrors at different signal levels.
Current mirrors provide a steady flow, With output resistance that helps it grow.
Imagine a factory where every machine needs the same amount of power. Just like a manager ensures every team runs on the same energy, current mirrors do the same for transistors, ensuring a steady supply regardless of changes in demand.
For Remembering outputs: "More Current Requires Good Resistance" – denotes the need for higher output resistance for stable current.
CORN - Current Output Requires Non-ideality factor to be minimized.
Term
What is the non-ideality factor?
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Term
Define output resistance.
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A circuit designed to copy a current through one active device by controlling the current in another active device.
The current delivered by the current mirror, ideally equal to a reference current, adjusted by non-ideality factors.
A term which accounts for real-world deviations from the ideal behavior in current mirrors, typically represented as (1 + λ(VDS2 - VDS1)).
The dynamic resistance seen by the load connected to the output of the current mirror, indicating current stability.
A phenomenon where output current changes with varying drain-source voltage due to the finite size of the MOSFET.
This section explores the output current and resistance of current mirror circuits, detailing the non-ideality factor and how it relates to the performance of BJT and MOSFET versions.
In this section, we delve deeply into current mirror circuits, specifically focusing on the output current expressions, output resistance characteristics, and the non-ideality factor. We establish expressions that describe these relationships for both MOSFET and BJT configurations, illustrating how changes in voltage can affect current output in practical applications.
In this section, we analyze current mirror circuits constructed with both BJTs and MOSFETs. The output current, noted as I2, can be expressed in terms of a reference current I1 and the aspect ratio of the transistors involved. For MOSFETs, the output current expression assumes the device is in saturation and is influenced by the transconductance and various voltage parameters, leading to the concept of the non-ideality factor.
In conclusion, this section highlights both theoretical foundations and practical considerations in designing robust current mirrors, emphasizing the importance of maintaining transistors within their saturation regions.