DC voltage source

Two voltage supplies available (V_DD > 0; V_SS ≤0)

Voltages V_i where V_SS < V_Bias < V_DD can be obtained by voltage division

• Resistive divider

Seldom used in MOS technology

• Large silicon area required
• Depends on V_DD

 

Output voltage (no charge impedance)

 

 

• MOS voltage divider

n-channel transistor in saturation

V_SS = 0

"Totem-pole" configuration

Voltages V₀₁, V₀₂, ... , V_0n in the range from 0 to V_DD

 

 

For all devices: v_GS = v_DS

The condition for saturation v_DS > v_GS - V_T is thus satisfied

Drain current:

 

 

Disadvantage: All voltages and currents depend on the supply voltages V_DD and V_SS.

 

• Improved circuit:

 

 

Depletion-mode device Q₂ acts as current source

Q₁ is forced to carry the drain current I_bias

Bias voltage V₀₁ is thus stabilised

Only one bias voltage

• Another solution:

 

 

Transistors Q₃ and Q₄ are matched

Equal currents I_bias independent of V_DD

Equal Bias voltage V₀₁ independent of V_DD