Depletion-load NMOS gain stage

Improved performance comparing to the enhancement-load gain stage

 

 

Small-signal equivalent circuit:

 

 

 

Current law applied to the output node gives:

 

Voltage gain:

 

Denominator of the preceding expression is much smaller than that in the case of the enhancement-load gain stage - voltage gain can be much larger

 

Gains over 100 are possible if the substrate of the load device is separated from that of Q₁ (g_mb2 = 0)

 

Both gain stages are affected by Miller effect that can be deduced from the approximate equivalent circuit:

 

Transfer function:

 

 

C_in    ... input capacity      
             

R_s     ... source resistance

G_Leq ... total conductance

 

 

 

Result:

Capacitor C_gd1 behaves like a capacitance (1+|A_v0|) times its real size ... Miller effect

High-frequency gain seriously affected, bandwidth reduced