Load-resistor differential pair

[harry456]

In the standard differential pair that uses nmos/pmos transistors as switches the load is also realized by a MOS-transistor (e.g. a PMOS in the linear-region (i.e. the mos's behavior is similar to a resistor)) but it seems that there are quite a number of different ways to realize the load needed for the differential pair. What are the downsides of a simple PMOS/NMOS in triode region in comparison to more complicated loads such as ?

 

[erikl]

With a simple PMOS/NMOS in triode region, the total current resp. its resistance is fixed, and you probably have to adjust or regulate it from the other side of supply. With the current mirror solution you have the possibility to control the total current through the differential pair without the necessity to control the load-side transistors.

Actually this shifts the load-side control to the source-side, and maybe is more flexible regarding (switching-)speed adjustment.

If you intended to control a single source-side transistor's resistance anyway, you practically use again the mirror configuration.