Current mirrors are transistor circuits that copy a reference current to another branch. In Electrical Circuits and Systems I, they show up as biasing blocks that keep analog circuits working consistently.
A current mirror in Electrical Circuits and Systems I is a circuit that takes one reference current and reproduces it in another branch. Instead of trying to hold a voltage steady, it is designed to hold current steady, even when the load voltage changes within the circuit's operating range.
The basic idea is simple: one transistor is set up to carry a known reference current, and a matched second transistor is forced to carry nearly the same current. If the devices are well matched, the output branch copies the reference branch. That is why the term "mirror" fits, it is supposed to reflect the current from one side of the circuit to the other.
You will see current mirrors built with BJTs or MOSFETs. With BJTs, matching base-emitter behavior matters. With MOSFETs, matching threshold behavior and device geometry matters. In both cases, the quality of the mirror depends on how closely the transistors match and how much the output current changes when the output voltage changes.
That last part is where output impedance comes in. An ideal current mirror would have infinite output impedance, which means the current would not budge no matter what load you connect. Real circuits are not perfect, so the current shifts a little when the output voltage moves. Better designs, like Wilson or cascode mirrors, reduce that problem and make the current closer to constant.
In practice, current mirrors often act as bias sources for analog blocks. For example, they can set the operating current for an op-amp stage or for a chain of transistors that need the same bias current. In a circuit problem, you may be asked to find the mirrored current, check whether the transistors stay in the right operating region, or explain why the current changes slightly when the load changes.
A useful way to think about them is this: voltage division splits voltage across resistors, while a current mirror is a controlled way to steer current into a branch that needs a predictable value.
Current mirrors show up anywhere this course moves from simple resistor networks into real analog circuit design. They are one of the main ways engineers create a stable bias current without using a separate resistor for every branch, which makes circuits more consistent and easier to scale.
They also connect several ideas from the class at once. To analyze a current mirror, you use transistor behavior, check output impedance, and think about whether the load effect is small enough to ignore. That means the term sits right at the intersection of device models and circuit analysis.
Once you understand current mirrors, op-amp input stages, differential pairs, and other analog blocks make a lot more sense. Many of those circuits depend on a mirror to keep currents balanced or to feed a known current into the right node. If the mirror shifts too much, the whole bias point shifts with it.
They are also a clean example of how a circuit can control current indirectly. Instead of forcing current with an ideal source, the mirror uses transistor matching and feedback-like behavior to approximate that source in a practical way.
Keep studying Electrical Circuits and Systems I Unit 3
Visual cheatsheet
view galleryBJT (Bipolar Junction Transistor)
BJT-based current mirrors use matched transistor pairs to copy a reference current. In this version, the base-emitter voltage relationship is what ties the two branches together, so matching matters a lot. If the BJTs are poorly matched or leave their active region, the mirrored current stops tracking cleanly.
MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor)
MOSFET current mirrors are common in integrated analog circuits because they are easy to match on a chip. The drain current depends on gate-source voltage and device sizing, so the mirror often uses transistor dimensions to scale the copied current. This makes MOSFET mirrors useful for bias networks and compact design.
Output Impedance
Output impedance tells you how much the mirror current changes when the output voltage changes. A high output impedance is what makes a current mirror act like a steady current source instead of a resistor-like element. If the output impedance is low, the current is much easier to disturb.
loading effect
The loading effect is one reason a real current mirror is not perfect. When the load at the output branch changes, it can pull the current away from its target value. A good mirror is designed so the load has only a small effect on the output current.
A problem set question usually asks you to identify the mirrored current, compare it to the reference current, or decide whether the transistor is staying in the right region of operation. You may also have to explain why the current is not perfectly constant when the load changes, which is where output impedance and loading effect come in.
In a lab, you might build a simple BJT or MOSFET mirror and measure how close the output current is to the reference current as you vary the load resistance. If the current stays nearly fixed, the mirror is working well. If it drifts a lot, that usually points to poor matching, the wrong operating region, or limited output impedance.
For quiz-style questions, the fastest move is to connect the circuit picture to the behavior: one branch sets the current, the other copies it. Then check whether the device type, biasing, and load all support that copy.
A current source is the broader idea of a circuit that supplies a steady current. A current mirror is one common way to build that behavior using transistors and a reference branch. So the mirror is a circuit technique, while the current source is the function it is trying to create.
A current mirror copies a reference current into another branch of a circuit.
In Electrical Circuits and Systems I, it is mainly used as a stable biasing element for analog circuits.
The best mirrors use matched transistors, usually BJTs or MOSFETs, so the output current closely tracks the reference current.
Real mirrors are not perfect, because the output current can still change when the load voltage changes.
Higher output impedance and better mirror designs, like Wilson or cascode versions, make the current more stable.
It is a transistor circuit that copies one reference current into another branch. In this course, you use it as a practical way to create stable bias currents for analog circuits. The output is meant to stay nearly constant even when the load changes.
One transistor is set up with a known reference current, and a matched second transistor is arranged to carry the same current. The device matching and biasing make the output branch "mirror" the first one. The better the match, the closer the copied current is to the reference current.
A current source is the function, a circuit that delivers a steady current. A current mirror is one way to build that function using transistor matching. In other words, the mirror is a design method, while the source is the behavior you want.
If output impedance is high, the output current changes very little when the output voltage changes. That makes the mirror behave more like an ideal current source. Low output impedance means the load can disturb the current much more easily.