Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2001-07-27
2004-02-17
Nguyen, Khanh Van (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S258000, C330S259000
Reexamination Certificate
active
06693489
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to level-shift circuits, and in particular, to level-shift circuits and related methods that perform common-mode level shifting of the common-mode voltage of a power-supply-referenced circuit to the common-mode voltage of a ground-referenced circuit, and for distributing the overall voltage shift among a plurality of level-shifting stages.
BACKGROUND OF THE INVENTION
In many applications, there is a need to level shift the complementary signals of a power-supply-referenced circuit where the signals are referenced from the power supply voltage Vcc to the complementary signals of a ground-referenced circuit where the signals are referenced from ground potential. For example, the power-supply-referenced circuit may be a data latch or multiplexer where the complementary output signals swing between 3.3 Volts and 3.1 Volts and the ground-referenced circuit may be a laser driver or low voltage differential signaling (LVDS) circuit where the complementary input signals swing between 1.5 Volts and 1.3 Volts. Generally, the level shifting is referred to with reference to the common-mode voltage, i.e. the average of the complementary signal levels. In the above example, the level shifting is from a common-mode voltage of 3.2 Volts (average of 3.3 Volts and 3.1 Volts) of the power-supply-referenced circuit to the common-mode voltage of 1.4 Volts (average of 1.5 Volts and 1.3 Volts) of the ground-referenced circuit. Typically, a level-shift circuit is used to perform the level shifting of the common-mode voltage of the power-supply-referenced circuit to the common-mode voltage of the ground-referenced circuit.
FIG. 1A
illustrates a schematic diagram of an exemplary representation of a level shift circuit
100
interfacing a power-supply-referenced circuit
102
and a ground-referenced circuit
104
. In this example, the level shift circuit
100
is represented as two variable batteries
106
and
108
respectively coupling the respective outputs of the power-supply-referenced circuit
102
to the inputs of the ground-referenced circuit
102
. The ground-referenced circuit
104
may include an input differential pair of bipolar transistors Q
15
and Q
16
having bases respectively coupled to the variable batteries
108
and
106
of the level shift circuit
100
and emitters connected in common to a current source represented as bipolar transistor Q
17
.
The power-supply-referenced circuit
102
may include a pair of differential bipolar transistors Q
11
and Q
12
having bases configured to receive complementary signals, emitters electrically connected in common to a tail current source I
11
, and collectors respectively connected to collector resistors R
11
and R
12
. The power-supply-referenced circuit
102
further consists of emitter-follower output bipolar transistors Q
13
and Q
14
having bases respectively connected to the collectors of transistors Q
12
and Q
11
, emitters respectively connected to current sources I
12
and I
13
, and collectors connected to the power supply rail, which is connected to the source resistors R
11
and R
12
as well. The emitters of the emitter-follower output transistors Q
13
and Q
14
serve to produce the complementary output signals of the power-supply-referenced circuit
102
, and are respectively coupled to the variable batteries
106
and
108
of the level shift circuit
100
.
The common-mode voltage of the power-supply-referenced circuit
102
may be at a different voltage level than the common-mode voltage of the ground-referenced circuit
104
. In addition, the common-mode voltage of the power-supply-referenced circuit
104
varies with changes in the power supply voltage Vcc, with changes in temperature, and with changes in the production process. The common-mode voltage of the ground-referenced circuit
104
should be substantially independent to variations of the power supply voltage Vcc, temperature and process. Thus, the level shift circuit
100
performs two functions: (1) to provide the necessary voltage shift of the common-mode voltage of the power-supply-referenced circuit
102
to the common-mode voltage of the ground-referenced circuit
104
and (2) to isolate variations in the power supply voltage Vcc, temperature, and process from the common-mode voltage of the ground-referenced circuit
104
. Thus, the level shift circuit is represented as variable batteries
106
and
108
to perform such functions.
FIG. 1B
illustrates a schematic diagram of a prior art level shift circuit
150
for shifting the common-mode voltage of the power-supply-referenced circuit
102
(as previously described) to the common-mode voltage of the ground-referenced circuit
104
(as previously described). The prior art level shift circuit
150
consists of resistors R
13
and R
14
respectively coupling the outputs of the power-supply-referenced circuit
102
to the inputs of the ground-referenced circuit
104
, and variable current sources I
14
and I
15
coupled respectively to the resistors R
13
and R
14
and to ground potential. The respective voltage drops across the resistors R
13
and R
14
are formed by the currents induced through them by the respective current sources I
14
and I
15
. The voltage drops across the resistors R
13
and R
14
provide the appropriate level shift between the power-supply-referenced circuit
102
and the ground-referenced circuit
104
. In addition, the current sources I
14
and I
15
are made variable to absorb any variations in the quiescent voltage difference between the two circuits
102
and
104
. In addition, capacitors C
11
and C
12
may be coupled across respective resistors R
13
and R
14
to reduce the adverse effects of the resistors R
13
and R
14
on the frequency response of the circuit.
A drawback of the prior art level shift circuit
150
stems from the level shifting of the complementary signals being performed independently of each other. In this case, the resistor R
13
and current source I
14
perform the level shift of one of the complementary signals independently of the level shift performed by resistor R
14
and current source I
15
on the other complementary signal. In order to avoid the formation of pulse-width distortion of the signals at the ground-referenced section, it is desirable for the level shift on each of the complementary signals to be substantially the same. However, it is difficult to provide accurately-matched equal resistors R
13
and R
14
and current sources I
14
and I
15
in order to achieve substantially the same level shift for each signal. Another drawback is the adverse effects of the resistors R
13
and R
14
on the frequency response of the circuit, even though capacitors C
11
and C
12
are employed to reduce these adverse effects; desirably R
13
and R
14
should be small. Yet another drawback of R
13
and R
14
and of I
14
and I
15
is that they consume a relatively large amount of power, which is undesirable; the is especially the case when R
13
and R
14
are small, as is desirable.
Thus, there is a need for level shift circuits and related methods that overcome the above-mentioned drawbacks of the prior art level shift circuit. Such a need and others are met with the level shift circuits and related methods in accordance with the invention.
SUMMARY OF THE INVENTION
A first aspect of the invention relates to a method of level shifting the common-mode output voltage of a power-supply-referenced circuit to the common-mode input voltage of a ground-referenced circuit. The method entails performing a common (as distinct from independent and separate) level shifting of complementary signals derived from the power-supply-referenced circuit to produce the complementary signals for the ground-referenced circuit. Because the level shifting is performed in a common manner (i.e. the level shift or voltage drop is common to both complementary signals), pulse-width distortion is substantially reduced if not eliminated. A specific example of common-mode level shifting entails applying input complementary signals
Cherry Edward Moore
Martinez Christopher G.
Blakely , Sokoloff, Taylor & Zafman LLP
Maxim Integrated Products Inc.
Nguyen Khanh Van
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