DC current source

 

• Current mirror

 

 

Input resistance of Q₁ is low (shorted gate/drain terminals)

Q₁ is forced to carry a current I₁

Q₁ and Q₂ in saturation

I₁ and I₂ determined by V_GS

Since V_GS1 = V_GS2

And if both transistors have same l, k', and V_T, then:

 

Current I₁ is mirrored in I₂

 

 

Small-signal equivalent circuit of the MOS current mirror:

 

r₀ ... incremental resistance of the current source I₁

v_in ... test voltage for measuring the circuit output impedance

 

Small-signal output impedance:

 

• Wilson's current source

Increased output impedance ... performance more like an ideal current source

 

Schematics:

 

 

Small-signal equivalent circuit of Wilson's current mirror:

 

 

 

 

Output impedance:

 

 

• Cascode current source

 

 

Output impedance: