Four-Quadrant Analog Multipliers Using G4-FETs

NASA’s Jet Propulsion Laboratory, Pasadena, California

Friday, December 01 2006

Devices with independently biased multiple inputs are exploited to simplify multiplier circuits.

Theoretical analysis and some experiments have shown that the silicon-on-insulator (SOI) 4-gate transistors known as G⁴-FETs can be used as building blocks of four-quadrant analog voltage multiplier circuits. Whereas a typical prior analog voltage multiplier contains between six and 10 transistors, it is possible to construct a superior voltage multiplier using only four G⁴-FETs.

A G⁴-FET is a combination of a junction field-effect transistor (JFET) and a metal oxide/semiconductor field-effect transistor (MOSFET). It can be regarded as a single transistor having four gates, which are parts of a structure that affords high functionality by enabling the utilization of independently biased multiple inputs. The structure of a G⁴-FET of the type of interest here (see Figure 1) is that of a partially-depleted SOI MOSFET with two independent body contacts, one on each side of the channel. The drain current comprises of majority charge carriers flowing from one body contact to the other — that is, what would otherwise be the side body contacts of the SOI MOSFET are used here as the end contacts [the drain (D) and the source (S)] of the G⁴-FET. What would otherwise be the source and drain of the SOI MOSFET serve, in the G⁴-FET, as two junction-based extra gates (JG1 and JG2), which are used to squeeze the channel via reverse-biased junctions as in a JFET. The G⁴-FET also includes a polysilicon top gate (G1), which plays the same role as does the gate in an accumulation-mode MOSFET. The substrate emulates a fourth MOS gate (G2).

By making proper choices of G⁴-FET device parameters in conjunction with bias voltages and currents, one can design a circuit in which two input gate voltages (V_in1,V_in2) control the conduction characteristics of G⁴-FETs such that the output voltage (V_out) closely approximates a value proportional to the product of the input voltages. Figure 2 depicts two such analog multiplier circuits. In each circuit, there is the following:

• The input and output voltages are differential,
• The multiplier core consists of four G⁴- FETs (M1 through M4) biased by a constant current sink (I_bias), and
• The G⁴-FETs in two pairs are loaded by two identical resistors (R_L), which convert a differential output current to a differential output voltage.

The difference between the two circuits stems from their input and bias configurations. In each case, provided that the input voltages remain within their design ranges as determined by considerations of bias, saturation, and cutoff, then the output voltage is nominally given by V_out = kV_in1V_in2, where k is a constant gain factor that depends on the design parameters and is different for the two circuits.

In experimental versions of these circuits constructed using discrete G⁴-FETs and resistors, multiplication of voltages in all four quadrants (that is, in all four combinations of input polarities) was demonstrated, and deviations of the output voltages from linear dependence on the input voltages were found to amount to no more than a few percent. It is anticipated that in fully integrated versions of these circuits, the deviations from linearity will be made considerably smaller through better matching of devices.

This work was done by Mohammad Mojarradi, Benjamin Blalock, Sorin Christoloveanu, Suheng Chen, and Kerem Akarvardar of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs. com/tsp under the Semiconductors & ICs category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
JPL
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Refer to NPO-41586, volume and number of this NASA Tech Briefs issue, and the page number.