Home > Analog CMOS Design > CMOS Differential Amplifier > Current Mirrors

The current mirror circuits are simple current sources which gives constant current. The current mirror circuits are based on the principle that, if the gate to source voltage of two identical MOSFETs are equal then the drain current flowing through them is equal. The basic current mirror circuit is shown in Figure below.

By neglecting the channel length modulation of the two transistors the drain currents can be given as :

I_REF = m_n C_ox (V_GS - V _TH)²

and I_out = m_n C_ox (V_GS - V _TH)²

If we take ratio of two equations, we get,

= i.e. I_out = I_REF

The key property of this circuit is that it allows precise copying of the current with no dependence of process and temperature. The ratio of I _out and I_REF is given by the ratio of device dimensions of two transistors.

In basic current mirror circuit we have neglected the channel length modulation. In practice for short channel devices it results in significant error in mirroring the currents.

By considering the channel length modulation effect for the basic current mirror circuit the current equations for both the transistors can be given as,

I_D1 = m_n C_ox (V_GS - V _TH)² (1 + l V_DS1)

and I_D2 = m_n C_ox (V_GS - V _TH)² (1 + l V_DS2)

\ = ×

Here V_DS1 = V_GS1 = V_GS2 But, V _GS2 ¹ V_DS2 which results in error while copying the current from M₁ to M₂.

In order to suppress the effect of channel length modulation a cascode current mirror circuit is used which is as shown in Figure below.

If V₁ and V₂ voltages are equal then I_out closely tracks the I_REF i.e. I_out » I_REF. In order to ensure this equality we must guarantee that

V_GS4 + V₂ = V_GS3 + V₁

Thus, if = then

V_GS3 = V_GS4 and V₁ = V₂ which gives I_out = I_REF