14.4. Current-Source Load for the Differential Stage
The load resistor (or bias resistor) of the differential stage will now be replaced with a current-source load. In addition to eliminating the need again for a resistor, it provides for much better dc stability and a significant gain improvement, including the elimination of the factor of ½ associated with the differential stage gain as in (8.15) and (8.17).
As shown in the new circuit in Fig. 14.3, the resistor RD2 is replaced by transistors M4 and M5. Transistor M5 provides a current-source load for transistor M2. The diode-connected transistor, M4, provides the dc reference voltage for M5. The reference current is ID1 = ID4.
Figure 14.3. Differential amplifier stage with current-source ...
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