Adaptive mosfet resistor

a resistor and mosfet technology, applied in the field of bipolarjunction transistor (bjt)/mosfet integrated circuit, can solve the problems of limited resistance value range of resistors, occupying substantial areas, and rarely being used in practi

Inactive Publication Date: 2005-04-21
MARYLAND SEMICON
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  • Abstract
  • Description
  • Claims
  • Application Information

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Problems solved by technology

Such resistors are limited in range of resistance values and may occupy substantial areas.
Although it is well-known that a MOSFET can operate as a resistor in its ohmic or “triode” region of its drain V-I characteristics, it is seldom used in practice, because the resistance value varies with the gate voltage.
Besides, the base resistance value may be too low to shunt the input signal Vin coupled to the base.

Method used

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Embodiment Construction

[0020] Operation of the MOSFET Resistor:

[0021]FIG. 2a shows the use of a n-channel MOSFET Mn as a base resistor for biasing a pnp BJT Qp of a common emitter amplifier. The dc drain characteristic is shown in FIG. 2b. The current ID of a MOSFET is given as:

ID=kp(W / L)[(VGS−Vt)VDS−VDS2 / 2]  (1)

where kp is a transconductance parameter, W / L is the width-to-length ratio of the gate, VGS is the gate to source voltage, Vt is the threshold voltage and VDS is drain-to-source voltage. When VDS is less than (VGS−Vt), the MOSFET is operating in the ohmic region, and

IC≈kp(W / L)(VGS−Vt)VDS  (2)

The ohmic resistance is:

RDS=VDS / IC=1 / [kP(VGS−Vt)]  (3)

By adjusting VGS, a wide range of resistance values can be obtained, so long as VDS is less than (VGS−Vt).

[0022]FIG. 2c shows the use of Qp′ similar to the pnp amplifier shown in FIG. 2a as the master section of a current mirror fed from the MOSFET resistor Mn. The slave section Qp mirrors the current through Mn to the master section Qp′. The gat...

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Abstract

A MOSFET can operate as a resistor by operating in the linear or ohmic region of the drain V-I characteristics. This region can be obtained by floating the gate of the MOSFET, when the dc current and the voltage drop are given. Multiple resistors can be duplicated (or mirrored) by sharing the same source and floating gate. The floating gate voltage can be simulated using a closed loop equivalent circuit. Alternatively, the gate voltage can also be derived from the given drain-to-source voltage and the given current in a feedback loop. With this adaptive MOSFET resistor, the minimum supply voltage can be as low as the sum of the BJT threshold and the complementary BJT saturation voltage, e.g. VCC≧VBE+Vsat (e.g. 0.8+0.15<1.0V). The threshold voltage Vt should be less than VBE.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] This invention relates bipolar-junction transistor (BJT) / MOSFET integrated circuit, particularly to resistive means to bias a BJT with a MOSFET. [0003] 2. Brief Description of Related Art [0004] In integrated circuits, a polycrystalline film or diffusion is often used to fabricate resistors. Such resistors are limited in range of resistance values and may occupy substantial areas. Although it is well-known that a MOSFET can operate as a resistor in its ohmic or “triode” region of its drain V-I characteristics, it is seldom used in practice, because the resistance value varies with the gate voltage. [0005] In circuit applications, a resistor is used to determine the current flow across a voltage differential, for instance between the supply voltage and an electrode of a transistor. For a BJT common emitter amplifier as shown in FIG. 1a, once the dc collector current IC1 is chosen, the dc base current IB must be 1 / β t...

Claims

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Application Information

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Patent Type & Authority Applications(United States)
IPC IPC(8): H03H11/48
CPCH03H11/53H03H11/48
Inventor LIN, HUNG CHANG
Owner MARYLAND SEMICON
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