A bulk-driven CMOS OTA with 68 dB DC gain

Abstract

An ultra-low voltage rail-to-rail operational transconductance amplifier (OTA) based on a standard digital 0.18 mm CMOS process is described in this paper. Techniques for designing a 0.8 volt fully differential OTA are discussed including bias and reference current generator circuits. To achieve rail-to-rail operation, complementary input differential pairs are used, where the bulk-driven technique is applied to reduce the threshold limitation of the MOSFET transistors. The OTA gain is increased by using auxiliary gain boosting amplifiers. This additional circuitry enables the OTA to operate at 0.8 volts, achieving an open loop gain of 68 dB while consuming 94 mW. The DC gain of the amplifier is the highest gain achieved to date in bulk-driven amplifiers. *

0.8 V bulk-driven operational amplifier

“…4 Effective transconductances of the input stage of the proposed amplifier and of the simple bulk driven differential pair as a function of V CM,in with V DD as parameter The worst case phase margin is 52°that is large enough concluding that the circuit is stable. Table 4 summarizes comparison results about the performance of the already published bulk-driven fully differential amplifier [3,27,29]. It should be noted here that the major number of the published bulk-driven amplifier is relevant to single-ended amplifier implementations.…”

“…The implementations of [27,29] used 0.35 and 0.18 lm triple-well processes, respectively, which are not actually standard CMOS process and less cost effective. They used parallel connected bulk-driven PMOS and NMOS differential pair in order to compensate the bulk-transconductance regarding the input common-mode level.…”

“…The input stage of the circuit employs a positive feedback loop in order to enlarge the inherent small bulk-transconductance. The usage of the positive feedback for voltage gain boosting of an amplifier has already been reported in the literature [3,[27][28][29]. In the aforementioned works, the input devices are simple bulkdriven transistors and consequently the input transconductance is still equal to the bulk transconductance.…”

“…Based in this technique the input signal is applied to the bulk terminal of the input devices featuring rail-torail input common-mode range [3,4,6,[18][19][20][21][22][23][24][25][26][27][28][29]. The main limitation of the amplifiers, which are based on the bulkdriven technique, is that the input transconductance is equal to the small bulk transconductance which is 3-5 times smaller than the gate-transconductance.…”

Bulk-Driven Current Differencing Transconductance Amplifier

“…Several LV active elements have been designed using this technique, such as voltage followers [11,43], op amps [27], and operational transconductance amplifiers (OTAs) [7,9,24,28].…”