Amplifiers – With semiconductor amplifying device – Including current mirror amplifier
Reexamination Certificate
2000-10-11
2003-04-08
Choe, Henry (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including current mirror amplifier
C330S302000, C330S306000
Reexamination Certificate
active
06545540
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to communication systems and components, and is particularly directed to new and improved low pass filter-incorporating current mirror circuit architecture, that is readily suited for low voltage, low noise applications, such as, but not limited to subscriber line interface circuits.
BACKGROUND OF THE INVENTION
Systems employed by telecommunication service providers contain what are known as subscriber line interface circuits or ‘SLIC's, which interface communication signals with tip and ring leads of a wireline pair serving a relatively remote piece of subscriber communication equipment. In order to be interfaced with a variety of telecommunication circuits, including those providing codec functionality, present day SLICs must conform with a very demanding set of performance requirements, including accuracy, linearity, insensitivity to common mode signals, low noise, low power consumption, filtering, and ease of impedance matching programmability.
One of circuits commonly employed in a variety of SLIC implementations is a current mirror, the basic configuration of a typical one of which is schematically illustrated in FIG.
1
. As shown therein, for a bipolar PNP transistor-based architecture, the current mirror comprises a current mirror input PNP transistor
10
and a current mirror output PNP transistor
20
. The input transistor
10
has its base
11
coupled to receive a voltage reference V
REF
through a bias resistor
15
, that is coupled to a (VCC) power supply rail
16
. The collector
13
of transistor
10
is coupled to an input terminal IN, to which an input current I
in
is supplied from an associated communication device interfaced therewith. The emitter
12
of the input transistor
10
is coupled through an emitter resistor
14
(having a resistance value R
14
) to the VCC supply rail
16
.
In order to mirror the input current supplied to the input terminal IN, the base
11
of the current mirror input transistor
10
is coupled in common with the base
21
of current mirror output PNP transistor
20
. It is also coupled to the emitter
32
of a base-offset PNP transistor
30
, the collector
33
of which is coupled to a voltage reference terminal, such as ground (GND). The base
31
of PNP transistor
30
is coupled in common to the collector
13
of the current mirror input transistor
10
. The emitter
22
of the current mirror output transistor
20
is coupled through an emitter resistor
24
(having a resistance value R
24
) to the (VCC) power supply rail
16
, and its collector
23
is coupled to an output terminal OUT, from which a mirrored output current I
out
is derived.
For the current mirror circuit architecture of
FIG. 1
the following loop equation may be written:
I
in
R
14
/&agr;
10
+Vbe
10
=I
out
R
24
/&agr;
20
+Vbe
20
. (1)
As the circuit of
FIG. 1
is normally implemented with the input transistor
10
being well matched with the current mirror output transistor
20
(&agr;
10
=&agr;
20
, and Vbe
10
=Vbe
20
), and with resistors
14
and
24
being of equal resistance values (R
14
=R
24
), it follows that:
I
in
=I
out
. (2)
As pointed out above, among the performance requirements of present day SLICs is the need to provide (low pass) filtering. While this can be accomplished by means of a discrete filter circuit to which signal interface path with the SLIC is interfaced, such an approach increases the hardware complexity of the circuit and typically involves the use of power supply rails that provide sufficient overhead voltage. However, as designers of telecommunication integrated circuits, such as codecs and the like, continue to ‘lower the voltage supply rail bar’ requirements for their devices (e.g., from five volts down to a voltage on the order of three volts or slightly higher), through the use of differential voltage-based implementations, the service provider is faced with the problem that such low voltages restrict voltage headroom.
SUMMARY OF THE INVENTION
With these restrictions in mind, the current mirror circuit of
FIG. 1
is modified in accordance with the present invention to incorporate a relatively simple resistor-capacitor (RC) filter circuit in the base-coupling path of the input and output transistors, so as to realize a highly integrated low pass filter current mirror architecture, that not only reduces implementation complexity, but readily complies with the reduced power supply parameters of the SLIC.
In particular, a series resistor is substituted in place of the direct or common connection of the bases of the input and output transistors. In addition, a filter capacitor is coupled between the base of the output transistor and a the VCC supply rail. The effect of this RC circuit is to modify the transfer function of the current mirror to include a low pass filter, such that the output current is equal to the frequency content of the input current below the cut-off frequency as defined by the time constant of the RC filter.
REFERENCES:
patent: 4645999 (1987-02-01), Szepesi
patent: 4769619 (1988-09-01), Taylor
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Intersil America's Inc.
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