FET active load and current source

Details FET active load and current source FET active load and current source

2001-12-28

2003-12-16

Cunningham, Terry D. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Gating

Utilizing three or more electrode solid-state device

C327S089000, C327S434000

Reexamination Certificate

active

06664842

ABSTRACT:

BACKGROUND INFORMATION
1. Field of the Invention
The present invention relates to electronic circuits comprising field effect transistors (FET), and more specifically to composite transistors comprising a plurality of FET devices.
2. Description of Related Art
Field-effect transistors (FET) are today the fundamental building block for the majority of integrated electronic circuits. These circuits include, but are not restricted to, integrated circuits for fiber optic communications systems, such as amplifiers and high-speed analog and digital circuits. Various types of FETs are implemented in a number of semiconductor technologies including silicon (CMOS), Gallium Arsenide (GaAs) and Indium Phosphide (InP).
FETs are widely used as active loads and current sources in integrated circuits. These applications employ the saturation region of the nonlinear channel current-voltage relation in which FETs behave as voltage-controlled current sources. The current in the FET channel, between the drain and source nodes, is controlled by the voltage applied between the gate node and the source node. In the saturation region, the channel current is largely independent of the voltage applied across the channel—this results in high impedance presented to AC signals while allowing substantial DC current to pass. Applications include the insertion of DC bias current (a current source) as well as signal current-to-voltage conversion (an “active load”) in amplifier circuits.
Applications such as fiber optic communications require circuits that operate near the speed limits of transistor technology. Fast FET devices like high electron mobility transistors (HEMT and PHEMT) fabricated in GaAs and InP, as well as silicon CMOS devices with ever smaller geometries, are being used in integrated circuits to meet these demands on switching speed and bandwidth. Traditional FET active load and current source circuits using high performance semiconductors become inadequate as the frequency of operation approaches the limits of the device.
FIG. 1
a
shows a stacked FET active load configuration
110
. Two FETs W
1
101
and W
2
102
are coupled in series and each gate and source wired together. This circuit exhibits poor high frequency response due to the interaction between the impedances at the FET gate
103
and FET drain
104
. In a high-frequency FET implementation, it may also exhibit negative resistance.
FIG. 1
b
shows a self-bootstrapped active load configuration
110
. Two FETs
111
and
112
have their channels coupled in series, and both the gate
113
of FET
111
and the gate
114
of FET
112
are coupled to the source node
115
of FET
112
. This circuit is not well suited to high-perfornance FET technologies, in which there is a single threshold voltage that is not variable by the circuit designer. This forces one FET device to operate near cutoff and the other in the triode region, with the result that the device periphery must be roughly twice that of the present invention to obtain the same DC current.
FIG. 1
c
is a cascode current source
120
. Two FETs
121
and
122
are coupled with their channels in series, with bias voltages and AC ground presented to the gate terminals of both. This circuit is potentially unstable at high frequencies, and its overall stability is highly sensitive to the impedance presented to a gate
123
of FET
121
, e.g. a small unavoidable inductance may lead to significant negative conductance at the output of the current source. Additionally, this circuit cannot be used as an active load with HEMT devices due to the lack of a P-type device.
Accordingly, there is a need for reliable high-performance active loads and current sources at higher frequencies.
SUMMARY OF THE INVENTION
The present invention is a circuit comprising two series-coupled field effect transistor (FET) devices with a resistor network coupled in parallel forming a composite device (that can be substituted directly for a single FET device). In applications such as active loads or current sources, the composite device exhibits a greater breakdown voltage and superior high-frequency characteristics. The resistor network provides optimum direct current (DC) bias for depletion mode devices and superior high-frequency impedance control. Bandwidth and stability are both increased. Furthermore, this circuit is compatible with depletion mode FET processes having a single fixed threshold voltage. Advantageously, the composite FET active load and current source in the present invention operates in an unconditionally stable state at all frequencies.


REFERENCES:
patent: 4100438 (1978-07-01), Yokoyama
patent: 4317055 (1982-02-01), Yoshida et al.
patent: 4394590 (1983-07-01), Honda
patent: 4692643 (1987-09-01), Tokunaga et al.
patent: 4751408 (1988-06-01), Rambert
patent: 6380793 (2002-04-01), Bancal et al.

Affiliated with

Also associated with

Rating

FET active load and current source does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with FET active load and current source, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and FET active load and current source will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3148929