Opamp Input Stage Research Articles

Common-Mode Response Overlapping vs. Shaping in Rail-to-Rail Op-Amp Input Stages

In this paper, a circuit technique—common-mode (CM) response overlapping—for maintaining the small-signal behavior of rail-to-rail amplifiers nearly constant over the whole input CM range, is introduced. This technique modifies the CM response of the rail-to-rail input stage adequately by means of two constant input floating voltage sources, which are adjusted by using a static tuning section. Its performance is compared with a second technique—CM response shaping—, which is also based on two variable input floating voltage sources, but relies on a dynamic feedback tuning loop. Experimental results obtained from two 3-V 0.8-μm CMOS single-stage rail-to-rail amplifiers, which operate with CM response overlapping and shaping, respectively, are provided.

Design of a CMOS opamp input stage immune to EMI

In this paper the behaviour of a complementary nMOS–pMOS differential pair in the presence of RF interference superimposed on the input terminals is analysed. An intrinsic nonlinearity cancellation mechanism in this structure is demonstrated, and proper design criteria are presented to exploit this mechanism in order to achieve a very high immunity to RF interference. A high-immunity complementary differential pair has been employed as an input stage of a folded cascode operational amplifier and its improved behaviour in the presence of RF interference has been verified by computer simulations.

Robust Design of LV/LP Low-Distortion CMOS Rail-to-Rail Input Stages

Two robust CMOS rail-to-rail OpAmp input stages are presented for low voltage (≤ 3 V) applications. The robust input stages are implemented using two recently reported universal approaches to achieve constant transconductance. Transconductance control circuit is also introduced to compensate for K_{p}, K_{n} mismatch of PMOS and NMOS differential pairs in the input stage. The input stages are designed for operation in the strong inversion and have a rail-to-rail common mode input voltage range. Compared with an OpAmp with simple complementary input pairs, a two stage rail-to-rail OpAmp design example exhibits lower total harmonic distortion (THD) levels over the entire common mode input voltage range.

Constant-g/sub m/ rail-to-rail CMOS op-amp input stage with overlapped transition regions

Conventional techniques to achieve a constant-g/sub m/ rail-to-rail complementary N-P differential input stage require complex additional circuitry. In addition, the frequency response and common-mode rejection ratio (CMRR) are degraded. An economical but efficient design technique to overcome these problems is proposed. The proposed technique strategically overlaps the transition regions of the tail currents for the n- and p-pairs to achieve constant overall transconductance. Experimental results demonstrate that g/sub m/ variation can be restricted to within /spl plusmn/4% with improved CMRR and frequency response.

Universal constant-g/sub m/ input-stage architectures for low-voltage op amps

In this paper, a novel design technique for low-voltage, constant transconductance (g/sub m/) op amp input stages is presented. The new technique which uses current-mode circuits is based on processing signal currents, rather than handling DC tail currents, to achieve a constant-g/sub m/. Two cases are developed. One is based on processing signal currents (the AC case) while the other is based on processing total instantaneous currents (the TIC case). The adopted design strategy in both cases is universal in that it is independent of the input stage transistor types (FET or bipolar) and their operating regions. It also considerably simplifies the design procedure of low-voltage op amps. To demonstrate the new concepts, universal op amp input stage architectures have been developed and their performances have been verified in both MOS and bipolar design examples. The MOS designs have been verified in both weak and strong inversion. The proposed universal implementations achieve almost constant-g/sub m/, independent of the common mode input voltage range from rail-to-rail.