Bad Circuit Design 9 - Oscillating Self-Biased References (BGRs and BMRs)
Without care a self-biased reference can be designed to oscillate. This topic is discussed on the bottom
of pages 625 and 630 in addition to other places in the book.
Below (click for a larger image) shows the BMR in Fig. 20.15 where we’ve added a capacitance across
R1. As mentioned on the bottom of page 625 if we bond out the resistor (connect the source of M2 to a
pad so that we can externally set the bias current with a resistor) the reference will oscillate. This is bad
design!!!
As another example, below (again, click for a larger image) is the bandgap reference (BGR), with
start-up, seen in Fig. 23.29a. We’ve, again, added a capacitance at the source of M2B and the circuit
oscillates. This could innocently occur if we tried to decouple the reference voltage or we connect a
significant amount of circuitry to the reference (that presents a large capacitance). See here for additional
information and pages 625 and 629-630.
Note that the BGR in Fig. 23.27 won’t become unstable with a significant capacitive load across VREF
since this additional load is outside the feedback loop.
