Constant current source (Current Mirror)

Constant current source (Current Mirror):

A constant current source makes use of the fact that for a transistor in the active mode of operation, the collector current is relatively independent of the collector voltage. In the basic circuit shown in fig 1 and collector characteristics of a CE Transistor as in fig.2

Transistors Q₁&Q₂ are matched as the circuit is fabricated using IC technology. Base and emitter of Q₁& Q₂ are tied together and thus have the same VBE. In addition, transistor Q₁ is connected as a diode by shorting its collector to base. The input current Iref flows through the diode connected transistor Q₁ and thus establishes a voltage across Q₁. This voltage in turn appears between the base and emitter of Q₂ .Since Q₂ is identical to Q₁, the emitter current of Q₂ will be equal to emitter current of Q₁ which is approximately equal to Iref. As long as Q₂ is maintained in the active region ,its collector current I_C2=I_o will be approximately equal to I_ref . Since the output current Io is a reflection or mirror of the reference current I_ref, the circuit is often referred to as a current mirror.

Analysis:

The collector current I_C1  and I_C2 for the transistor Q₁ and Q₂ can be approximately expressed as

Where I_ES is reverse saturation current in emitter junction and VT is temperature equivalent of voltage.

From equation (1) & (2)

Since V_BE1=V_BE2 we obtain I_C2=I_C1=I\_C=I_O

Also since both the transistors are identical, I_C1= I_C2

KCL at the collector of Q₁ gives

I_ref= I_C1+I_B1+I_B2

From Eq.5 for β/ [β +1] >>1, is almost unity and the output current I0 is equal to the reference current, ref which for a given R₁ is constant. Typically I_o varies by about 3% for 50 ≤ β ≤200.

The circuit however operates as a constant current source as long as Q₂ remains in the active region.

Widlar current source:

Widlar current source which is particularly suitable for low value of currents. The circuit differs from the basic current mirror only in the resistance R_E that is included in the emitter lead of Q₂. It can be seen that due to R_E the base-emitter voltage V_BE2 is less than VBE1 and consequently current Io is smaller than I_C1

The ratio of collector currents I_C1&I_C2 using

A relation between I_C1 and the reference current I_ref is obtained by writing KCL at the collector point of Q1

I_ref = I_C1 + I_B1 + I_B2

I_ref = I_C1 + I_C1/β + I_C2/β

Neglecting I_C2/β,

I_ref = I_C1 (1 + 1/β)

I_ref = [ Vcc - V_BE ] /R₁

When β>> 1, I_C1 = I_ref

Wilson current source:

The Wilson current source shown in figure

It provides an output current I0 which is very nearly equal to Vref and also exhibits a very high output resistance.

Analysis

But output resistance is greater than Widlar source.