Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
2000-02-04
2001-08-14
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C327S110000, C327S065000, C327S067000
Reexamination Certificate
active
06275078
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a line driver having a self adjustable output impedance and, in particular, to a transformer line driver.
2. Description of Related Art
Line drivers having a controlled output impedance are well known in the art. See, B. Nauta, et al., “Analog Video Line Driver with Adaptive Impedance Matching,” ISSCC98, pp. 318-19, 1998. A simplified schematic of one such driver
10
is illustrated in FIG.
1
A. The driver
10
(also referred to as a “buffer”) comprises an operational amplifier
12
whose negative input terminal receives an input voltage Vin. The output terminal of the operational amplifier
12
is connected to the gates of two field effect transistors
14
and
16
, where the illustrated “N” value is equal to the ratio of their respective drain currents. The sources of the field effect transistors
14
and
16
are connected to a reference voltage Vdd. The drains of the field effect transistors
14
and
16
are connected to each other by a resistor (R
1
)
18
. The drain of the field effect transistor
14
is connected in a feedback fashion to the positive input terminal of the operational amplifier
12
, and is also connected to ground through a resistor (R
2
)
20
. An output voltage Vout is supplied from the drain of the field effect transistor
16
to drive a transmission line
22
having a characteristic resistance equal to the load resistance (RL)
24
. By properly selecting the values of the resistors R
1
and R
2
for the driver
10
in a well known manner (and as illustrated) with respect to the “N” value and the value of the load resistance RL, the value of the output impedance from the driver may be set (i.e., controlled) substantially equal to the load resistance RL. An advantage of this driver is its reduced power dissipation which makes it very attractive for implementation in an integrated circuit. However, with respect to an integrated circuit fabrication, the precise resistance values needed to achieve substantial matching of driver-line impedance are very difficult to consistently obtain.
It is recognized that it would be advantageous to be able to exercise some adjustment control over the output impedance of the driver following the setting of the resistance values. The driver of
FIG. 1A
may be modified, as shown in
FIG. 1B
, to provide for such an adjustment mechanism. Controllable source degeneration (through circuit
30
) is applied to the transistors
14
and
16
. The current ratio value “N” is electrically tunable (through circuit
30
) via application of the voltage Vtune. In this implementation, the driver adapts to match the load resistance RL using a control loop
28
that integrates the output current of the transconductance amplifier onto the capacitor connected to Vtune for application to circuit
30
resulting in an adjustment to the source current of transistor
14
and a change in the value of N. At low frequencies, the control loop
28
forces Vout to equal Vin, in which case the gain of the driver is one. By then setting the resistances R
1
and R
2
as discussed above, approximate matching of the output impedance to the load resistance RL is obtained, with the control loop
28
further refining the matching.
Most telecommunications devices utilize a transformer decoupling of the driver and the transmission line. Because transformer driver-line decoupling is utilized in the push-pull configuration, a direct current output signal related to the load resistance is not available to be integrated by the control loop
28
and produce the adjustment signal Vtune. Furthermore, if the transmission line is relatively long, its direct current resistance is substantially different from the characteristic impedance. In such situations, the precision of the impedance adjustment provided by the
FIG. 1B
circuit is not sufficient. Additionally, the
FIG. 1A
prior art driver has not, historically, been well suited for use in a push-pull B-class circuit as two such drivers are needed and they do not operate well together in push-pull. When one half of the push-pull circuit (i.e., one driver
10
) generates some voltage in one half of the primary coil of the transformer, a flyback voltage appears in the other half of the primary coil. This flyback voltage penetrates to the input of the operational amplifier
12
of the other driver
10
through the feedback circuit connections and corrupts driver operation.
There accordingly exists a need for a line driver having a self-adjustable output impedance with reduced power dissipation and improved power efficiency for implementation in an integrated circuit.
SUMMARY OF THE INVENTION
A line driver circuit is provided for connection to a signal transmission line. The circuit includes a controlled or synthesized impedance buffer. The line driver circuit further includes an adjustment circuit that outputs an adjustment signal for application to an adjustable controlled current source within the buffer. By manipulating the adjustable controlled current source with the adjustment signal, the output impedance of the buffer can be made to substantially match the characteristic impedance of a transmission line connected to the driver.
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B. Nauta, et al., “Analog Video Line Driver with Adaptive Impedance Matching”, ISSCC98, Session 20, SA 20.1, Feb. 7, 1998.
R. Mahadevan, et al., “A Differential 160MHz Self-Terminating Adaptive CMOS Line Driver”, ISSCC2000, Session 26, WP 26.6, Feb. 9, 2000.
D. Johns, et al., “Integrated Circuits for Data Transmission Over Twisted Pair Channels”, 1997 IEEE Journal of Solid-State Circuits, vol. 32, No. 3, Mar. 1997, pp. 398-406.
Cunningham Terry D.
Galanthay Theodore E.
Jorgenson Lisa K.
Nguyen Long
STMicroelectronics Inc.
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