Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2002-12-04
2004-04-20
Nguyen, Khanh Van (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S253000
Reexamination Certificate
active
06724258
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to fully differential voltage controlled transconductors, and more particularly relates to such transconductors that operate at low voltage with highly linear performance and wide input and control ranges.
BACKGROUND OF THE INVENTION
Voltage controlled transconductors are used in a wide range of applications, for example in active filters circuits-such as Gm-C filters-multipliers and oscillators. Detailed descriptions of their construction and principles of operation are readily available. For example, see Analog Integrated Circuit Design, by David A. Johns and Ken Martin, John Wiley & Sons, New York© 1997, pages 605-607.
FIG. 1
shows a representative prior art voltage controlled transconductor (VCT) circuit that is based on the linear mode of operation of MOSFETs. As is known, linear mode based transconductors provide better performance than saturation mode transconductors. The circuit comprises transistors M
1
through M
9
and operational amplifiers
101
,
102
and
103
, interconnected as shown. The circuit provides differential output currents Io+ and Io− in response to a differential input voltage Vid which is superimposed on a constant common-mode voltage signal Vcm. The drain-to-source voltages (Vds) of input transistors M
1
and M
2
are held to a control voltage Vc to control the current flowing into transistors M
1
and M
2
, thereby controlling the transconductance of the entire circuit.
In applications where high precision is required, a VCT circuit like that of
FIG. 1
can provide less than adequate performance in certain key areas. For example, such a circuit may not be truly fully differential. In other words, the absolute values of gm+ and gm− may not be exactly the same. In addition, such a circuit suffers from nonlinearities in currents Io+ and Io−. That is, the values of gm+ and gm− are functions of Vid, instead of being constants. These nonlinearities and that the circuit is not truly differential can be understood because the theoretical circuit operation is based on the ideal behavior of MOSFET transistors in the linear mode, which does not take into account the second order effects and the non-idealities.
Another problem is that the range of the control voltage Vc may be insufficient to provide an adequately robust and controlled performance. This is because in the circuit of
FIG. 1
if a wide input range is needed then the control voltage range must decrease in order to keep transistors M
1
and M
2
in the linear mode of operation, and vice versa, which is a disadvantage at low supply voltages.
Finally, the VCT circuit of
FIG. 1
requires a third operational amplifier
103
, for the common mode input Vcm. The common mode itself must be extracted from the input signal, which needs an extra circuit, as well.
It is therefore desirable to have a VCT circuit that overcomes some or all of the above-described problems.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a voltage controlled transconductor (VCT) for receiving a differential input signal comprising a first voltage signal and a second voltage signal, and for providing a differential output signal comprising a first output current signal and a second output current signal. The VCT includes a first side transconductor circuit having two parts of the same construction, the first part being capable of conducting a first current signal and the second part being capable of conducting a second current signal, the first current signal and the second current signal being controlled by the first voltage signal. The VCT also includes a second side transconductor circuit having two parts of the same construction, the first part being capable of conducting a third current signal and the second part being capable of conducting a fourth current signal, the third current signal and the fourth current signal being controlled by the second voltage signal. A first control circuit is adapted to control the first and second current signals of the first side transconductor circuit, while a second control circuit is adapted to control the third and fourth current signals of the second side transconductor circuit. The first side transconductor circuit is connected to the second side transconductor circuit to provide the first output current signal comprising a difference of the second current signal and the third current signal and to provide the second output current signal comprising a difference of the first current signal and the fourth current signal.
REFERENCES:
patent: 5442318 (1995-08-01), Badyal et al.
patent: 6177838 (2001-01-01), Chiu
patent: 6549074 (2003-04-01), Ugajin et al.
patent: 6590452 (2003-07-01), van Rhijn
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