Bulk linearisation of the MOS resistor

Abstract

A bulk-linearised composite MOSFET is presented, with an extended linear range and an equivalent saturation voltage of up to several hundred mV even in weak inversion. Some preliminary measurements are presented as well as a 300 MΩ equivalent resistor, with almost a 1 V quasi-linear range. A low frequency, 28 dB gain, biomedical bandpass filter/amplifier is also shown, taking advantage of the linearised MOSFETs to consume only 16 nA of supply current.

A Self-biased Current Source, using an Asymmetric Bulk-modified MOS Composite Transistor

“…1 and also in Fig. 2, showing the extended linear range and 'saturation' effects as described in [1], [2]. In Fig.…”

“…1 two transistors are connected as a MOS divider to bias the bulk of the selftransistors, defining the equivalent transistor Meq with its own Source (S), Gate (G), Drain (D) terminals; the equivalent bulk terminal is not defined for Meq. As pointed in [1], [2], Meq shows an enhanced linear region, where the best results are obtained when the aspect ratios (W/L)a of Ma and (W/L)b of Mb are very different (W/L)a >> (W/L)b. Depending on the transistors' sizes, the resulting composite structure is linear up to an equivalent saturation VSat_eq voltage of a few hundred mV like in Fig.…”

“…A bulk-linearization technique was presented in [1] to enhance the linearity of the MOS transistor, using two series MOSFETs like in Fig. 1; later in [2] this composite structure was examined as a new composite transistor.…”

“…1. Because (W/L)a ¹ (W/L)b the new composite transistor will be denoted as an asymmetrical bulk-modified transistor or ABM-MOS, and allows to build basic structures like differential pairs or current mirrors [2], to emulate large resistors [1], or exploiting its behavior to implement basic building blocks with a transfer function otherwise difficult to obtain, enabling a new family of circuits. In this work, a simple self-biased current reference in CMOS technology is presented, based in the characteristics of an ABM composite transistor.…”

“…1 always operate with the bulk connected to its source. First the saturation current of a single Ma is simulated (or calculated) for a fixed VG while varying VSa; then assuming that for the ABM in the linear region VSa is close to VD of Meq [1], VSat_eq can be calculated intersecting this curve with the linear approach ID = VDS/Req, where Req is the equivalent resistance of the ABM in the linear region ( [1], see Fig. 1).…”