12-12-2012, 06:50 PM
Two Novel Fully Complementary Self-Biased CMOS
Differential Amplifiers
Two Novel Fully Complementary Self-Biased.pdf (Size: 319.81 KB / Downloads: 72)
INTRODUCTION
THIS brief paper presents two novel CMOS differential amplifiers. The first differential amplifier is intended for
applications in which the input common-mode range is relatively
limited; this amplifier is denoted a complementary
self-biased differential amplifier (CSDA) [ll. The second
differential amplifier is intended for applications in which
the input common-mode range is bounded only by the supply
voltages; this amplifier is denoted a very-wide-commonmode-
range differential amplifier (VCDA) [2].
The circuit configurations of both amplifiers differ from
those of conventional CMOS differential-amplifier configurations
in two important ways:
less sensitivity of active-region biasing to variations in
capability of supplying switching currents that are signifnominal
doubling of differential-mode gain ( + 6 dB).
processing, temperature, and supply;
icantly greater than the quiescent bias current;
These performance enhancements are particularly desirable
in comparator applications in commercial digital CMOS
VLSI integrated circuits, where precision, high speed, ease
of interfacing to ordinary logic gates, and consistently high
production yields are required. Both amplifiers have found
application in commercial CMOS VLSI integrated circuits as
precision comparators, as will be discussed below.
1) the amplifiers are completely complementary
VCDA
A. Theory of Operation
CMOS differential amplifiers with wide input dynamic
ranges have been reported [5]-171. All of these amplifiers are
externally biased, while none of them is entirely complementary.
In contrast, the VCDA is fully complementary and
entirely self-biased.
As with the CSDA, the operation of the VCDA may be
understood through its derivation. Fig. 3(a) illustrates two
folded-cascode differential amplifiers, each the complement
of the other. These amplifiers have greater dynamic ranges
than ordinary differential amplifiers as a result of the larger
drain-source voltage drop on the input pairs. This larger
voltage drop maintains the input pairs in the active region
even for very large swings of the input signal. While neither
VIN+ VIU
VBlAs
amplifier in Fig. 3(a) by itself is capable of covering the
entire input range from negative supply to positive supply, a
combination of the two amplifiers can cover this entire
range.
In the first step of the derivation, the loads of the two
amplifiers are deleted, and their outputs are connected together
to produce the fully complementary, but externally
biased, differential amplifier of Fig. 3(b). As in the case of
the circuit in Fig. l(b), the circuit of Fig. 3(b) cannot be
biased in a stable fashion, so it is not practical.
However, just as in the case of the circuit of Fig. l©, by
connecting all of the bias inputs to a single internal node,
VBlAs, the bias point of the amplifier is dramatically stabilized
through negative feedback. The resulting self-biased
amplifier is illustrated in Fig. 3©, which is the VCDA itself.
The differential-mode gain of the VCDA is given by (l),
just as for the CSDA.