Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2002-12-12
2004-05-25
Nguyen, Patricia (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S292000, C330S009000
Reexamination Certificate
active
06741132
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present disclosure generally relates to the field of amplifier circuits, and in particular but not exclusively to a low noise level amplifier circuit.
2. Description of the Related Art
Low noise amplifier circuits are frequently used in the field of telecommunications and in particular in designing telephone interface circuits.
With some applications, it can happen that a relatively large input impedance—about several kilo-ohms—must be provided for. Such an impedance value is likely to generate non-negligible noise on the amplifier's input since noise varies in increase ratio to the square root of the input impedance. To minimize the effects of noise, a Low Noise Amplifier structure is then used, that is based on amplifiers mounted as cascode circuits, as illustrated in
FIG. 1. A
first and a second differential amplifier
110
and
120
, receive a signal, respectively INP and INN, on their respective positive input via a bypass capacitor C, respectively
114
and
124
. Differential mode gain is set by a voltage divider bridge R
1
-R
2
, respectively
130
-
140
for amplifier OA
1
and
150
-
160
for amplifier OA
2
, that makes it possible to feed part of the output voltage (resp. OUTP and OUTN) back into the switch input of the amplifiers.
A resistor
100
having a value R is connected between the positive input of OA
1
and the positive input of OA
2
and makes it possible to set the circuit input impedance.
The noise generated by resistor
100
is filtered through network R-C resulting from the presence of bypass capacitor C (respectively
114
and
124
) before it reaches the inputs of the amplifiers. For this reason such an amplifier structure, based on stages mounted as a cascode circuit, proves to be particularly adapted to design amplifiers having a large input impedance.
Nevertheless, the known circuit of
FIG. 1
faces a stabilization problem for both amplifiers
110
and
120
. Indeed, to avoid them from starting to oscillate at high frequencies, the amplifier's gain is made to drop down when approaching a critical phase shift of 180 degrees. This gain drop is classically operated by means of a capacitor Cm, known as a Miller capacitor, respectively
111
and
121
in
FIG. 1
, and more detailed in FIG.
2
. In
FIG. 2
, a conventional differential amplifier structure comprising a first stage formed by a differential pair
112
-
113
, a power source
114
and a current mirroring circuit
115
-
116
is shown. A second stage comprises a transistor
117
, for example a MOS-type transistor, and a power source
118
. Generally, the Miller capacitor is connected between the input and the output of the last stage, i.e., in the circuit of
FIG. 2
, between the grid and the drain of transistor
117
. Gain can thus efficiently drop when approaching the critical zone where output and input signals are phase-shifted by 180 degrees. It is observed that connecting a capacitor Cmc
119
between grid and voltage Vdd also allows to obtain gain drop, but with quite less effectiveness than with a Miller capacitor. Because of the presence of gain K of the last stage, a capacitor Cmc equal to Cm×K would be necessary to obtain two equivalent effects and, for this reason, a Miller capacitor is rather preferred to obtain amplifier stabilization.
Generally, this capacitor Cm is dimensioned according to the gain of the stage to stabilize. The lower the gain, the larger the value of this capacitor must be. The circuit of
FIG. 1
however has a gain that is different according to whether it operates in differential mode or in common mode. Indeed, in differential mode gain is set by the ratio of resistors, while in common mode, gain is equal to 1.
Stabilizing the circuit for common mode thus means choosing a capacitor Cm having a large value, whereas a much lower value could be chosen in differential mode, in particular in order to preserve the amplifier's gain-band product. Thus a dilemma arises: either stabilizing the circuit of
FIG. 1
for both common and differential modes, and in this case the largest capacitor value is chosen, which results in performance degradation in differential mode, or stabilizing only the differential mode in order to maintain performance in this mode, and then facing stability problems for the common mode.
FIG. 3
shows a known way of solving this problem. An amplifier circuit is based on two amplifiers
310
(OA
1
) and
320
(OA
2
) that are mounted as cascode amplifiers by means of a network R
1
-R
2
made up of resistors
330
-
340
and
350
-
360
, respectively. Two inputs, respectively INP and INN, are connected to the positive input of OA
1
via a capacitor
314
and to the positive input of OA
2
via a capacitor
315
. The input impedance of the circuit is set by a resistor
300
. Contrary to the circuit of
FIG. 1
, voltage VCM of the divider bridge's midpoint is now set, at the junction between resistors
340
and
350
, by means of an amplifier
370
(OA
3
) that is mounted as a cascode circuit. This circuit has a positive input that is connected to the midpoint of a resistor bridge (Rs
391
and
392
), having a voltage stabilized at low frequency by a capacitor
393
. If a sufficiently large value C of capacitor
393
is chosen, the output of amplifier
370
is more or less stabilized and thus voltage VCM is set to virtual ground.
Thus, for both stages
310
and
320
, a common mode gain can be obtained that is identical to the differential mode gain, which makes it possible to stabilize amplifiers OA
1
and OA
2
in both modes and with an optimal value when considering the gain-band product. Indeed a single value Cm, when judiciously selected, makes it possible to obtain stabilization in differential mode and in common mode without any loss of performance.
This is the conventional way to stabilize both amplifiers OA
1
and OA
2
. However, it can be observed that the stabilization problem is just transferred to the third amplifier OA
3
, that must also be associated with a Miller capacitor
380
that will have to be particularly effective, and in particular when approaching the critical operation zone for stages
310
and
320
. Indeed, it will be in this zone that amplifier OA
3
will be particularly used and thus likely to output large currents to maintain voltage VCM to virtual ground. Besides, the existence of an offset will amplify currents, especially as resistor R
1
will have a low value. Designing amplifier OA
3
is thus particularly delicate to do.
It is therefore desired to design a new low noise amplifier structure allowing to obtain stabilization more easily, in common mode as well as in differential mode.
BRIEF SUMMARY OF THE INVENTION
One embodiment of the present invention provides a low noise amplifier structure that is easy to stabilize in common mode as well as in differential mode, and without loss of performance.
Another embodiment of the invention provides a low noise amplifier circuit that consumes less current and occupies less room.
An embodiment provides an amplifier structure including:
a first amplifier comprising at least one input stage and one output stage;
a first Miller capacitor having a first electrode and a second electrode, said first and second electrodes being connected to the input and the output of said first amplifier's output stage, respectively;
a second amplifier comprising at least one input stage and one output stage;
a second Miller capacitor having a first electrode and a second electrode, said first and second electrodes of the second Miller capacitor being connected to the input and the output of said second amplifier's output stage;
wherein the amplifier structure comprises:
at least a first trimming capacitor having a first electrode and a second electrode, said first electrode being connected to said first electrode of said first Miller capacitor;
at least a second trimming capacitor having a first electrode and a second electrode, said first electrode being connected to said first electrode of said s
Lenz Kuno
Renous Claude
de Guzman Dennis M.
Jorgenson Lisa K.
Nguyen Patricia
Seed IP Law Group PLLC
STMicroelectronics S.A.
LandOfFree
Low noise level differential amplifier does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Low noise level differential amplifier, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Low noise level differential amplifier will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3267159