Comparison of quasi-/pseudo-floating gate techniques and low-voltage applications

The quasi- or pseudo-floating gate (QFG) technique addresses a key issue with the floating-gate MOS transistor technique, by using ultra-high resistances to provide dc paths to otherwise floating nodes. Several ways have been suggested to implement the quasi-infinite resistors (QIRs). In this paper, basic QIR structures are analyzed and compared, and three sources of error, dc offset, signal distortion, and signal-dependent offset, are defined. Then, through simulations and experiments, the suitability of several QIR implementations for use in various applications is compared. A particular QIR implementation is found to minimize dc offset, but requires voltage swings to be limited to less than a diode turn-on voltage. Some application circuits using quasi-floating gate are presented, including a QFG translinear geometric-mean circuit and QFG low-voltage fully-differential amplifiers with QFG common-mode feedback using several QIR structures. Measurements on current-mode QFG circuits exhibit large offsets and very long turn-on transients, which could limit practical application of this technique.