Super class AB OTA without open‐loop gain degradation based on dynamic cascode biasing

Abstract

A scheme to achieve simultaneously extremely high slew-rate improvement and avoiding open-loop gain degradation in one-stage super class AB op-amps is introduced. It overcomes the serious shortcoming of super class AB operational transconductance amplifiers that shows very high-output current enhancement factors at the expense of degrading the open-loop gain. The proposed scheme uses dynamically biased cascode transistors to avoid gain and slew-rate degradation. Experimental results of a super class AB operational transconductance amplifier in 180-nm complementary metal-oxide semiconductor technology with open-loop gain of 67 dB, a factor 2 improvement in GBW, and a current enhancement factor of 270 verify the proposed scheme.

Class AB amplifier with enhanced slew rate and GBW

“…There are also some reported single‐stage class AB topologies that provide higher FoM L and/or FoM S , but they achieve it at the expense of PM degradation even using large capacitive loads. This is typically because they require increasing the impedance level at internal nodes (eg, using local common‐mode feedback in Lopez‐Martin et al and Pourashraf et al, adaptive loads in Sutula et al, or extra signal paths in Lopez‐Martin et al and Garde et al) which lead to nondominant poles at lower frequencies. This issue may preclude the use of these topologies for small load capacitors without using techniques such as output lead compensation .…”

“…First, the input current is sensed at the source of M CN1 ‐M CN2 , in order to benefit from the reduced input resistance at this node due to the FVF feedback loop. Second, the quasi‐floating gate (QFG) technique is applied to achieve class AB operation. It is based on including two capacitors C bat and two high‐resistance pseudo‐resistors M PR .…”

“…This latter effect is especially relevant for transistors M N1 , M N2 , M P1 , and M P2 because cascode transistors M CN1 , M CN2 , M CP1 , and M CP2 limit their V DS voltage and can make them enter triode mode for large dynamic currents. To mitigate this issue, dynamic biasing of the cascode transistors can be implemented using the QFG technique …”

High‐gain, high‐CMRR class AB operational transconductance amplifier based on the flipped voltage follower

“…Achieving significant gain in advanced CMOS processes is also difficult, and single‐stage cascode structures and two‐stage conventional Miller amplifiers cannot achieve sufficiently high gain for high‐accuracy operation. The alternatives are two‐stage amplifiers with cascoded stages, single‐stage gain‐boosting topologies, multistage Miller amplifiers, and positive feedback structures .…”

1-V 15-μW 130-nm CMOS Super Class AB OTA

“…To date super class AB OTAs have been implemented using symmetrical (current mirror) [3], [5]- [7] and recycling folded cascode (RFC) [8]- [11] topologies. In this paper we present a new super class AB OTA outperforming the previous ones in terms of average FoMAVG=(FoMS•FoML) 1/2 thanks to a new way of exploting the triode-saturation transition and cascode biasing to improve SR, and a different use of current steering.…”