A robust feedforward compensation scheme for multistage operational transconductance amplifiers with no Miller capacitors

Abstract

A multistage operational transconductance amplifier with a feedforward compensation scheme which does not use Miller capacitors is introduced. The compensation scheme uses the positive phase shift of left-half-plane (LHP) zeroes caused by the feedforward path to cancel the negative phase shift of poles to achieve a good phase margin. A two-stage path increases further the low frequency gain while a feedforward single-stage amplifier makes the circuit faster. The amplifier bandwidth is not compromised by the absence of the traditional pole-splitting effect of Miller compensation, resulting in a high-gain wide-band amplifier. The capacitors of a capacitive amplifier using the proposed techniques can be varied more than a decade without significant settling time degradation. Experimental results for a prototype fabricated in AMI 0.5-m CMOS process show dc gain of around 90 dB and a 1% settling time of 15 ns for a load capacitor of 12 pF. The power supply used is 1.25 V.

A selectable-bandwidth 3.5 mW, 0.03 mm2 self-oscillating Sigma Delta modulator with 71 dB dynamic range at 5 MHz and 65 dB at 10 MHz bandwidth

“…However to obtain a power efficient design, we avoided power hungry com- pensation techniques such as Miller compensation. Instead we used a feedforward compensated opamp [10]. A signal flow diagram of the opamp is shown in Fig.…”

“…Note that this compensation strategy has the side-effect that the closed-loop transfer function will have a doublet at f c . This is undesirable in switched capacitor applications because it gives rise to a slow settling component in the step response [10]. However this is not an important issue in a continuoustime sigma delta modulator.…”

A Low Power, Fully Differential Bulk Driven OTA in 180 nm CMOS Technology

“…1 [7]. In the figure, g m1 and g m2 are fully differential transconductors forming the forward path of the OTA and g m3 , also a fully differential transconductor, forming the feed-forward path.…”

“…When such a OTA is used to realize a filter, the realizable filter bandwidth is limited by the OTA gain-bandwidth. Feed-forward compensation scheme is found to be effective in pushing the gain-bandwidth product of the OTA to a higher value [5]- [7].…”

A Highly Linear CMOS Baseband Chain for Wideband Wireless Applications

“…To minimize the effect of the imperfect pole-zero cancellation, the cancellation must occur at high frequencies and the second and feed-forward stages should not include a non-dominant pole until the arrival of the overall unity gain bandwidth product [5]. The second and feed-forward stages must be optimized for a high bandwidth, and the first stage needs to be designed to have a high gain and small load capacitance.…”