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 “supply independent biasing.” Further, as indicated on page 625 the gm of the

MOSFETs can be controlled. The threshold voltage of M1 and M2 subtract out (don’t affect the

bias current) making the BMR work well with process shifts. Finally, the decrease in the MOSFET’s

mobility with increasing temperature is compensated by the increase in the resistor’s value (see Eq.

[20.37]) making the BMR-generated reference current stable with temperature.

 

http://cmosedu.com/cmos1/bad_design/bad_design4/snap1.jpg

 

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 it’s 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 doesn’t 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. Don’t use this

circuit for biasing. It’s bad design! Use the BMR.

 

 

http://cmosedu.com/cmos1/bad_design/bad_design4/snap2.jpg

 

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 doesn’t

affect the operation of these circuits and the diode forward voltage drop doesn’t vary a significant amount

so why aren’t these good circuits to provide biasing. To begin, the diode-referenced bias circuit has a

large negative TC (diode’s forward drop decreases with increasing temperature while the resistor’s value

goes up with increasing temperature). Why this (large negative TC) is undesirable, the fact that the

reference’s current isn’t directly related to any MOSFET parameters doesn’t 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).

 

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