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.
Recently, the quasi-or pseudo-floating gate (QFG) technique has been suggested by several authors to implement many of the functions of the widely used floatinggate MOS transistor technique by using ultra-high resistances to isolate a floating node. Several different ways have been suggested to implement these quasi-infinite resistors (QIRs). In this paper, several 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 are compared. One implementation is shown to minimize dc offset, but voltage swing is limited to less than 0.7 V PP . Configurations using parallel connections of QIRs improve signal distortion and signal-dependent offset, but still suffer from dc offset and limited voltage swing. Series-connected QIR structures allow increased voltage swings, but typically lead to hard-to-predict offsets of up a few hundred mV because of well-substrate leakage currents. Other circuits are also analyzed.
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