Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2002-06-21
2003-09-23
Young, Brian (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S136000
Reexamination Certificate
active
06624775
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a current output circuit, and more particularly to a current output circuit for use in a digital-to-analog converter.
BACKGROUND OF THE INVENTION
Along with technological development, the request for high quality signal transmission progressively increases. Hence, how to design a digital-to-analog converter with high accuracy becomes an important issue.
Please refer to
FIG. 1A
which is a schematic circuit block diagram illustrating a conventional current-type digital-to-analog converter. The current-type digital-to-analog converter includes an encoder
10
and a current output circuit
11
including plural current output cells
111
. When digital signal data[
0
:N] are inputted into the digital-to-analog converter, the digital signal data[
0
:N] are encoded into switch control signals via the encoder
10
. The switch control signals control respective switches of the current output cells
111
to output analog voltages equivalent to the digital signal data.
Please refer to
FIG. 1B
which is a schematic circuit diagram illustrating one of the current output cells of FIG.
1
A. PMOS transistor MP
1
is considered as a constant current source under a bias VB
1
, and the current output path is determined by control signals C and CB wherein the signal CB has a reverse phase compared to the phase of the signal C.
As known, the design of the current output cell in the digital-to-analog converter will affect the linearity of output current. For example, the current output linearity of the current output cell can be increased by serially connecting thereto a transistor under a common voltage to enhance output impedance. The above way is generally feasible under a high operational voltage. However, with the increasing demand of reduced operational voltage, it is difficult to increase the output impedance and thus the current output linearity in the above-mentioned way.
Therefore, the purpose of the present invention is to develop a current output circuit for use in a digital-to-analog converter to deal with the above situations encountered in the prior art.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a current output circuit for use in a digital-to-analog converter, which has a satisfying output impedance and current output linearity under a relatively small operational voltage.
According to an aspect of the present invention, there is provided a current output circuit for use in a digital-to-analog converter. The current output circuit includes a current source for providing a driving current, and a first output circuit coupled with the current source. The first output circuit includes a first metal-oxide semiconductor (MOS) transistor device having a source electrode thereof connected to the current source in series, a first voltage amplifier coupled between the source electrode and a gate electrode of the first MOS transistor device for keeping a voltage of the source electrode substantially constant, and a first controlled switch coupled between an operational voltage and the gate electrode of the first MOS transistor device for being switched ON or OFF in response to a first digital control signal, and allowing the driving current to be outputted from a drain electrode of the MOS transistor device when the first controlled switch is switched ON.
Preferably, the first digital control signal is generated by encoding a digital signal via an encoder.
For example, the first MOS transistor device can be a P-channel MOS transistor device.
Preferably, the current output circuit further includes a second output circuit. The second output circuit coupled with the current source includes a second metal-oxide semiconductor (MOS) transistor device having a source electrode thereof connected to the current source in series, a second voltage amplifier coupled between the source electrode and a gate electrode of the second MOS transistor device for keeping a voltage of the source electrode substantially constant, and a second controlled switch coupled between an operational voltage and the gate electrode of the second MOS transistor device for being switched ON or OFF in response to a second digital control signal, and allowing the driving current to be outputted from a drain electrode of the MOS transistor device when the second controlled switch is switched ON. The second digital control signal is preferably generated by encoding a digital signal via an encoder. For example, the second MOS transistor device can be a P-channel MOS transistor device.
Preferably, the first digital control signal received by the first output circuit has a reverse phase compared to a phase of the second digital control signal received by the second output circuit.
According to another aspect of the present invention, there is provided a current output circuit for use in a digital-to-analog converter. The current output circuit includes a current source for providing a driving current, a first output circuit and a second output circuit. Both first and second output circuit are coupled with the current source. The first output circuit includes a first metal-oxide semiconductor (MOS) transistor device having a source electrode thereof connected to the current source in series, a first voltage amplifier coupled between the source electrode and a gate electrode of the first MOS transistor device for keeping a voltage of the source electrode substantially constant, and a first controlled switch coupled between an operational voltage and the gate electrode of the first MOS transistor device for being switched ON or OFF in response to a first digital control signal, and allowing the driving current to be outputted from a drain electrode of the MOS transistor device when the first controlled switch is switched ON. The second output circuit includes a second metal-oxide semiconductor (MOS) transistor device having a source electrode thereof connected to the current source in series, a second voltage amplifier coupled between the source electrode and a gate electrode of the second MOS transistor device for keeping a voltage of the source electrode substantially constant, and a second controlled switch coupled between an operational voltage and the gate electrode of the second MOS transistor device for being switched ON or OFF in response to a second digital control signal, and allowing the driving current to be outputted from a drain electrode of the MOS transistor device when the second controlled switch is switched ON. The first digital control signal received by the first output circuit has a reverse phase compared to a phase of the second digital control signal received by the second output circuit.
According to a further aspect of the present invention, there is provided a current output circuit for use in a digital-to-analog converter. The current output circuit includes a current source for providing a driving current, and at least one output circuit coupled with the current source. The at least one output circuit includes a metal-oxide semiconductor (MOS) transistor device having a source electrode thereof connected to the current source in series, a voltage amplifier coupled between the source electrode and a gate electrode of the MOS transistor device for keeping a voltage of the source electrode substantially constant, and a controlled switch coupled between an operational voltage and the gate electrode of the MOS transistor device for being switched ON or OFF in response to a digital control signal, and allowing the driving current to be outputted from a drain electrode of the MOS transistor device when the controlled switch is switched ON.
REFERENCES:
patent: 6218871 (2001-04-01), Chiou
patent: 6545619 (2003-04-01), Segura
Lai Sheng-Yeh
Liu Hung-Chih
Lowe Hauptman & Gilman & Berner LLP
Silicon Integrated System Corp.
Young Brian
LandOfFree
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