Controlled impedance transformer line driver

Details Controlled impedance transformer line driver Controlled impedance transformer line driver

2000-02-04

2001-05-08

Pascal, Robert (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including push-pull amplifier

C330S195000

Reexamination Certificate

active

06229396

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a line driver having a controlled 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
. 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.
Most telecommunications devices utilize a push-pull B-class circuit with transformer decoupling of the driver and the transmission line. The
FIG. 1
prior art driver has not, historically, been well suited for use in the 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 push-pull type transformer line driver having a controlled or synthesized 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 transformer having a center tapped primary coil. The circuit includes a pair of equivalent pre-drivers. Each pre-driver receives a driver input signal and outputs a buffer input signal and a flyback compensation signal proportional thereto. The circuit further includes a pair of equivalent controlled or synthesized impedance buffers for connection in a push-pull configuration to the primary coil. Each buffer receives the buffer input signal generated from one of the pre-drivers for buffered output as a line driver signal applied to the primary coil. Each buffer further receives the flyback compensation signal generated from the other one of the pre-drivers, with the buffer further operating to cancel a flyback voltage effect induced in that buffer by the line driver signal applied to the primary coil by the other buffer using the flyback compensation signal received from the other one of the pre-drivers.
The induced flyback voltage effect produces a flyback current experienced in each buffer. The pre-driver generates the flyback compensation signal proportional to the buffer input signal (which subsequently induces that flyback voltage effect) such that the current of the flyback compensation signal is substantially equal to the flyback current. The buffer then uses the input flyback compensation signal to cancel the flyback current through a subtraction operation.


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patent: 6055418 (2000-04-01), Harris et al.
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.

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