What is a Current Mirror : Circuit & Its Working

For designing the monolithic integrated circuit, the most popular technique is used namely the current mirror. So in this method, the designing of the circuit can be done to copy the flow of current throughout one energetic device to another including the feature of current control. Here, the flow of current can be copied in the form of inverting from device to device. Once the flow of current within the first active device is altered then the reflected output current from the other active device will also be changed. Therefore, the current mirror circuit is frequently referred to as a CCCS (Current Controlled Current Source). This article discusses an overview of the current mirror circuit and its working.

What is a Current Mirror?

The circuit is used to copy the flow of current in one active device and controlling the flow of current in another device by maintaining the output current stable instead of loading is known as a current mirror. Theoretically, a perfect current mirror is an inverting current amplifier. The main function of this amplifier is to reverse the direction of the flow of current. The main function of the current mirror is to provide active loads as well as bias currents to circuits and also used to form a more practical current source.

Current Mirror Circuit

Generally, the designing of current mirror circuits can be done with two main transistors and even though other devices like FETs also used. Some of these circuits may utilize the above two transistors for allowing the performance level to be enhanced. As the name suggests, it copies the flow of current in one active device whereas, in another active device, it maintains the output current stable instead of loading. The copied current is a constant current.

The current mirror circuit diagram is shown below. This circuit can be built with two transistors, where one of the transistors base and collector terminals are connected whereas in other it doesn’t. In the circuit, both the transistor’s base terminals are connected whereas the emitter terminals are given to GND. In this circuit, both the transistors work similarly.

In the circuit operation, the base-emitter terminals of the first transistor (TR1) work as a diode because the collector and base terminals are connected together.

The flow of current toward the TRI transistor is externally set through other components, and consequently, there is a specified voltage developed across the BE junction of the first transistor. When the BE voltage on these two transistors is the same, then the flow of current within one transistor will accurately mirror the second transistor. So, the flow of current into the first transistor will be reflected into the second transistor & therefore into the R1 load.

Wilson Current Mirror

The variation of the current mirror circuit is known as Wilson current mirror circuit because this transistor includes another transistor-like TR3. The circuit diagram of Wilson’s current mirror is shown below. In the circuit, the TR3 transistor keeps the collector terminal of the first transistor (TR1) at a voltage equal to two diodes that fall under the Vcc.

This circuit overcomes the earlier effect and it is extremely useful, particularly in ICs or integrated circuits. The different components can be used simply in the circuit design. They allow unbiased currents to flow into the circuits such as differential pairs & this makes sure that their function is enhanced further. These mirrors are not extensively used external IC technology in view of the extra number of components necessary, however, the principles are similar in both discrete form & once used in ICs.

Limitations of Current Mirror

The current mirror circuit using two transistors which are discussed above can be frequently quite sufficient for most of the applications. But it includes some limitations below several conditions like the following.

Current Changes with Change within Output Voltage

The flow of current will be changed when the output voltage changes due to the o/p output impedance is not unlimited because; there is a small difference of ‘Vbe’ through the collector voltage at a specified current within the TR2 transistor. Frequently the flow of current may change with 25% of the output compliance range.

Current Matching Lies on Transistor Matching

The matching of current mainly depends on the transistor matching. Frequently the transistors require to be on a similar substrate if they are to precisely reflect the current. These issues can be solved by using advanced current mirror circuits.

Current Mirror Circuit using MOSFET

This current mirror circuit can be implemented with two MOSFET transistors. This circuit working is similar to the previously discussed mirror circuit. The current mirror circuit using MOSFETs is shown below. In the following circuit, the two MOSFETs are considered as M1 and M2.

The first MOSFET like M1 is in the saturation region because of the VDS ≤ VGS whereas the second MOSFET like M2 is in the saturation region if the output voltage is higher as compared to the saturation voltage. So the input current of the first MOSFET can control the o/p current of the second MOSFET.
The function of MOSFET is, the drain current of this transistor replicates the function of the G to S and D to G voltage. So, by using the following function, the formula can be written.

I_D = f (V_GS, V_DG)

Because of this, the M1 input current can be mirrored toward the drain current. The input current can be provided through the bias resistor. If the VDG is 0 for the M1, then the drain current of M1 will be

I_D = f (V_GS, V_DG=0)

Thus, f (V_GS, 0) = I_IN so, I_IN will fix the VGS value. The same VGS can be reflected across the second MOSFET.

I_OUT = f (V_GS, V_DG=0) is true.

So the o/p current can be mirrored like the i/p current, I_OUT = I_IN

Further, the VDS can be introduced like V_DS = V_DG + V_GS. By changing this, the Shichman-Hodges model gives the estimated answer for the f(V_GS, V_DG). So this function can be expressed like the following.

I_D = f (V_GS, V_DG)

I_D = ½ Kp (W/L) (V_GS-Vt_h)² (1+λV_DS)

I_D = ½ Kp (W/L) (V_GS-Vt_h)² (1+λ(V_DG + V_GS))

The o/p resistance is calculated when the output resistance is limited

R_OUT = ((1/ λ) +V_DS)/I_D as R = V/I

From the above equation,

Where ‘K_P’ is a transistor technology-related constant

‘W/L’ is the ratio of Width & Length

‘λ’  is mainly used for the channel length’s modulation constant.

‘V_GS’ is the gate to source voltage

‘Vth’ is the threshold voltage

‘V_DS’ is a drain to source voltage

The compliance voltage, where the VDG = 0 & the resistance of MOSFET is high, the current mirror works within the less o/p voltage. This voltage can be measured by deriving the situation.

V_CV = V_GS (I_D at V_DG = 0)

Otherwise, f^-1 (I_D) once the V_DG = 0

Specifications

The characterization of a current mirror circuit can be done by the following specifications.

Current Transfer Ratio

A current mirror circuit is used to copy one active device’s input current to the output of other active devices. This kind of circuit is also known as an ideal current amplifier including the inverting design that can overturn the direction of current flow. Thus, the current transfer ratio is a significant factor for a current amplifier.

AC Output Resistance

Resistance includes a VI relationship according to the ohms law. So, the resistance of AC o/p plays a key role within the output current’s stability with respect to changes in voltage.

Voltage Drop

The voltage drop of a working mirror circuit across the output is less. A voltage range where this circuit can work is known as the compliance range & the lowest to the highest voltage within this compliance range is known as compliance voltage. The smallest amount of voltage is necessary to maintain the transistor active, so the least voltage mainly depends on the specifications of the transistor.

Thus, this is all about an overview of current mirror circuits like working and application of current mirror circuits.

Current Mirror Circuit using BJT

The current mirror circuit using BJTs is shown below. Assume that these transistors are equal so both the transistors operating temperatures as well as device parameters are similar. Since the VBE( base-emitter voltage) is the same for these transistors, then the IC1(reference current) for the Q1 transistor can be mirrored over the Q2 transistor that is IC2 = IC1 once the base currents are ignored. This permits us to modify IC2 irrespectively of the VC2 voltage through simply modifying the R1, as

I_C1 = V_CC-0.7V/R1

If both the base currents like I_B1 & I_B2 are used, then

I_C2 = I_C1/ (1+(2/β))

Practically, the IC2 is not fairly independent of VC2 as the ‘Q2’ transistor includes a limited output resistance because of the early effect. Therefore, IC2 can be found to amplify slightly through VC2.
The IC2 performance as compared to VC2 can be explained in terms of the early voltage like

I_C2 = Is e^V_BE2/V_T (1+V_CE2/V_A)

From the above equation, ‘VA’ is the Early voltage. So, the current mirror’s o/p resistance can be written as

Ro = V_A/I_C2

Lastly, the BJTs should be in active mode to work the current mirror accordingly.

Thus, this is all about an overview of the current mirror circuit and its working using BJTs, MOSFETs, Specifications, etc. Here is a question for you, what are the applications of the current mirror circuit?