# Current Mirror working principle and application

Here it is explained what a current mirror circuit is, why it is useful, where it is used with some application examples.

A current mirror, also called current regulator circuit, is an electronic circuit that produces duplicate or copy of current flowing through one active device into another. It is a simple and useful building block in analog circuits. The current mirror circuit typically consists of a pair of bipolar junction transistors(BJT) or field-effect transistors (FETs), where one transistor is used as the input device, and the other as the output device. The input transistor's current flows into the load resistor, which is connected to the output transistor. The current flowing through the load resistor is replicated by the output transistor, providing a mirrored current. This makes the output current proportional to the input current, allowing it to be adjusted by changing the load resistor value.

Usually, a current mirror circuit  is used to create a  DC current source at a point in the circuit. This is done often to bias a transistor so that the Q-point is stable and in the active region(usually). For example, in mobile phones the DC current may deviate due to changes in temperature. Such DC current bias using current mirror circuit is generally used in integrated circuits(IC) but can also be implemented in discrete component based analog or digital circuits.

The following shows circuit diagram of a BJT based current mirror.

We will now show that it is a current mirror circuit. That is, the output current Io is the mirror of the input current IR. That is, Io=IR

In the circuit above, transistor T1 is a bipolar junction transistor (BJT) which is diode-connected, meaning that there is a direct connection between its collector and base terminals with no voltage drop. This unique configuration means that T1 can only operate in active mode or be in cutoff mode, as it can never work in saturation conditions.

To ensure that T1 and T2 have similar characteristics, they must be manufactured using the same procedures and on the same substrate. This is referred to as "matching" the transistors. In order to guarantee twin characteristics, it is important to keep the Î²s (the current gain) as close as possible. So we assume that,

beta1=beta2=beta

Vbe1=Vbe2=Vbe

Ib1=Ib2=Ib

And it follows from Ic=beta*Ic that, Ic1=Ic2=Ic

Applying KCL in the transistor T1 collector loop we have,

IR = Ic1+Ib1+Ib2=Ic+2*Ib=Ic+2*Ic/beta

Since, Io=Ic2 and Ic=Ic2 we can rewrite,

Io=IR/(1+2/beta)

Assuming beta>>1 we have,

Io=IR

hence proved because in this circuit, the current injected into the transistor T1, IR is copied at the output by creating s” a current source into the collector of T2. The value of the copied current can be determined by,

IR = (Vcc-Vbe)/R

Current mirrors are used in a wide range of applications, including current amplification, current limiting, precision current sources, and other analog circuits. The concept of current mirroring is also used in many integrated circuits, including operational amplifiers, voltage regulators, and power amplifiers.