Reliable Circuit Techniques for Low-Voltage Analog Design in Deep ...

22 Fayomi, Sawan and Roberts
2.1. Low-Voltage Operational Amplifier
One of the most important basic building blocks in
analog and mixed-mode circuits is the operational
amplifier. In a low-voltage (LV) opamp, the minimum
supply value is imposed by the differential pair of the
input stage and is equal to the threshold voltage (V th )
plus two overdrive voltages (V DSat ). In typical CMOS
processes, this value turns out to be around 1-V. On
the other hand, the main limitation of differential pairs
is the reduced input common-mode range (ICMR). In
order to minimize the supply requirements of the input
stage, both input terminals of an opamp must work
with potentials very close to one of the supply rails. To
overcome this limitation, several schemes for discretetime
(switched) operation with a single supply down to
1Vand large output signal swings have been recently
reported [2–10]. The switched-opamp (SO) technique
[2] has been shown to be a promising low-cost solution
to realize switched-capacitor (SC) circuits in standard
CMOS processes. The SO eliminates critical MOS
switches that set the minimum supply voltage to allow
sub-1-V operation of the SC circuits [4] and, thus,
does not have a reliability problem. Since then, a few
modifications have been proposed to improve the performance
of the SO techniques in terms of operation
speed and compatibility with existing SC circuits.
In switched applications, no input swing is required
for the opamp since both input terminals operate at
one of the supply rails. Many others 1 V opamps
are continuous-time for differential operation but use
switches for the common-mode. Those techniques will
not be covered in this paper. Most of continuous-time
techniques will be addressed and can be classified in
three categories. The first technique is mostly based
on level shifting of the common input signal to a
value capable of driving the input terminals [11–18].
Continuous-time and switched-capacitor techniques
have been developed to shift the input. The second
concept makes use of the bulk-driven MOS transistor
[19, 20] and current driven bulk [21] techniques, while
the local charge pump based approach is described in
[22]. The latter technique will not be addressed since it
is not widely used. Table 1 gives a simplified overview
of methods used for achieving almost rail-to-rail lowvoltage
opamp designs with their related performance.
Detailed description will be given below.
2.1.1. Common Mode Level Shifting Opamp Based
Method
In these approaches, the opamp input common-mode
voltage is set to a voltage closed to V SS or V DD .
In [11] a switched-capacitor technique is used
to level-shift down the common-mode input voltage
Table 1.
Trends in low-voltage CMOS opamp.
Threshold Gain UGB (MHz) Power
Method Authors Technology voltage (V) (dB) C 1 = load capacitor (µW) V DD (V)
Common-mode Fayomi et al. [11] 0.18-µm V th,p =−0.48 60 26.6 400 1
level shifting CMOS V th,n = 0.52 (C 1 = 5pF− R 1 = 20 k)
Karthikeyan et al. [13] 1.2-µm V th,p =−0.8 60 10 150 1
CMOS V th,n = 0.60 (C 1 = 20 pF − R 1 = 10 k)
Lee et al. [14] 1.2-µm V th,p =−0.8 58 2.2 400 1
CMOS V th,n = 0.60 (C 1 = 20 pF)
R-Angulo et al. [15] 0.8-µm V th,p ≈−0.85 – 13 70 1.2
CMOS V th,n ≈ 0.85 (C 1 = 10 pF)
R-Angulo et al. [16] 0.8-µm V th,p ≈−0.85 60 5 – 1.2
CMOS V th,n ≈ 0.85 (C 1 = 50 pF)
D-Carillo et al. [17] 1.2-µm V th,p ≈−0.75 87 1.9 410 1
CMOS V th,n ≈ 0.75 (C 1 = 15 pF)
Bulk-driven Blalock et al. [19] 2.0-µm V th,p ≈−0.80 44.8 1.3 287 1
CMOS V th,n ≈ 0.80 (C 1 = 22 pF)
Current driven Lehmann et al. [20] 0.5-µm V th,p ≈−0.60 62–69 2 40 1 or less
bulk CMOS V th,n ≈ 0.60 (C 1 = 20 pF)
DTMOS Annema [29] 0.35-µm V th,p =−0.65 – – 1.2 1 or less
CMOS V th,n = 0.65