Bad Circuit Design 4 - Self-Biased References
We used the beta-multiplier reference (BMR) seen below throughout the book to provide biasing
for our analog circuits (see the nano-CMOS version in Fig. 20.22). We use this topology because
it provides ďsupply independent biasing.Ē Further, as indicated on page 625 the gm of the
MOSFETs can be controlled. The threshold voltage of M1 and M2 subtract out (donít affect the
bias current) making the BMR work well with process shifts. Finally, the decrease in the MOSFETís
mobility with increasing temperature is compensated by the increase in the resistorís value (see Eq.
[20.37]) making the BMR-generated reference current stable with temperature.
There are other self-biased references that can be implemented in a CMOS process. For example,
the circuit seen below is a threshold voltage-referenced self-biasing circuit. M1 is made relatively
wide so that itís gate-source voltage is a threshold voltage. The current that flows in either side is then
VTHN/R. Why is this a bad design for providing MOSFET biasing? It is supply independent biasing
afterall. Well, first off notice that as temperature increases the threshold voltage drops and the
resistor value goes up resulting in the current having a large negative TC. Next, notice that the
threshold voltage doesnít subtract out (this is a big problem). The result is the MOSFET biasing
conditions will vary all over the place with process, temperature, and voltage shifts. Donít use this
circuit for biasing. Itís bad design! Use the BMR.
Other examples of self-biased reference circuits are the diode-referenced self-biased circuit (Fig. 23.22)
and the thermal voltage referenced self-biased (Fig. 23.24). The MOSFET threshold voltage doesnít
affect the operation of these circuits and the diode forward voltage drop doesnít vary a significant amount
so why arenít these good circuits to provide biasing. To begin, the diode-referenced bias circuit has a
large negative TC (diodeís forward drop decreases with increasing temperature while the resistorís value
goes up with increasing temperature). Why this (large negative TC) is undesirable, the fact that the
referenceís current isnít directly related to any MOSFET parameters doesnít result in stable biasing
conditions. The latter point is also applicable to the thermal-voltage referenced self-biasing circuit (though
this reference has a better TC).