Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2001-07-27
2003-05-20
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S543000
Reexamination Certificate
active
06566938
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to current sources, and more particularly, to a constant current source using CMOS technology.
BACKGROUND OF THE INVENTION
Typically, the use of NMOS components made it relatively easy to realize a constant current source.
FIG. 1
shows an NMOS transistor
100
configured as a typical constant current source. The gate terminal
102
is connected to the source terminal
104
so that the gate to source voltage (Vgs) equals zero. Since the transistor
100
has a negative voltage threshold (Vt), constant drain current is available. This typical constant current source has been widely used to make an RC time constant that is independent of the power supply (Vcc).
FIG. 2
shows an RC circuit
200
that uses the typical constant current source circuit of FIG.
1
. The circuit
200
includes NMOS transistor
202
that forms the constant current source and transistor
204
that receives a control input
206
. The circuit
200
also includes a timing capacitor
208
couple to output
210
. During operation, the constant current source
202
provides constant current to charge the timing capacitor
208
and thereby form a consistent output signal.
FIG. 3
shows graph
300
of the output of the RC circuit
200
of
FIG. 2
plotted against time. As indicated at
302
, when the input signal is at a high level (in H), the output signal approaches zero. As indicated at
304
, when the input signal is at a low level (in L), the output signal rises at a constant rate as a result of the constant current source providing constant current. For example, dv/dt at the output is a constant, since a constant current is received by the capacitor
208
. Thus, the constant current source allows many useful circuits to be constructed, such as the constant current RC circuit.
However, as technology has migrated to utilizing CMOS processes and components, the negative threshold NMOS transistor has become generally unavailable. Therefore, it is desirable to have a way to obtain a constant current source utilizing CMOS technology.
SUMMARY OF THE INVENTION
The present invention includes a system for providing a constant current source utilizing CMOS technology. The system includes a CMOS circuit that replaces typical NMOS circuits to produce a constant current.
In one embodiment of the invention, a CMOS circuit that operates as a constant current source is provided. The circuit comprises a bias circuit that includes a bias output terminal. The bias circuit produces a bias signal that is output at the bias output terminal. The circuit also includes a switch circuit having a switch input terminal coupled to the bias output terminal to receive the bias signal. The switch circuit also includes a switch output terminal and a switch control terminal that is coupled to receive an input signal, and wherein the bias signal is switched to the switch output terminal to form a switched bias signal in response to the input signal. The circuit also includes an output circuit having a first input terminal coupled to the switch output terminal to receive the switched bias signal, a second input terminal coupled to receive the input signal and an output terminal. The output circuit operates to produce an output signal that has constant current at the output terminal.
In one embodiment of the invention, a method for generating constant current from a CMOS circuit is provided. The method comprises the steps of generating a bias signal, switching the bias signal in response to an input signal, wherein a switched bias signal is produced, and receiving the input signal and the switched bias signal to produced an output signal that provides constant current.
REFERENCES:
patent: 5568084 (1996-10-01), McClure et al.
patent: 5942809 (1999-08-01), Hashimoto
patent: 6348833 (2002-02-01), Tsujimoto et al.
Sheppard Mullin Richter & Hampton LLP
Wells Kenneth B.
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