Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2000-03-03
2001-01-16
Wong, Peter S. (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
C323S316000, C330S257000
Reexamination Certificate
active
06175226
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to differential amplifiers, and more particularly, to a fully differential amplifier with a circuit for regulating an output reference voltage.
BACKGROUND OF THE INVENTION
As is known, differential amplifiers with a fully differential output (fully differential amplifiers) are symmetrical circuit structures with two inputs and two outputs. Two voltages are taken from the outputs, and each voltage is the sum of two components, namely a differential component arising from the difference between input voltages of opposite sign, and a component, called the common-mode component, arising from variations of input voltages having the same sign.
Clearly, the working signal is the differential component which is obtained by taking the voltage between the two output terminals. The common-mode components, however, cause a unilateral voltage to be superimposed on the working signal, this being equivalent to a shift of a reference value of the differential output voltage. In many cases, the central value of the supply voltage of the differential amplifier is selected as this reference voltage, particularly in the case in which the input signals are symmetrical to an input reference voltage. In this way, the maximum dynamic range of the output signal is obtained. In the case of asymmetric input signals, however, it may be convenient to select a different value of the reference voltage.
The common-mode component, when arising from a structural imbalance of the final stage, in other words from components which are not identical in the two branches of the differential circuit, causes an additional shift of the output reference voltage. To prevent these effects of the common-mode component from reducing the output dynamic range, there is a known method of using a feedback control system which is provided by a circuit which senses the output common-mode component and acts on an operating parameter of the differential amplifier in such a way as to make the output reference voltage independent of the common-mode component.
A circuit structure of this type is shown schematically in
FIG. 1. A
differential amplifier, indicated as a whole by the number
10
, comprises two pnp-type bipolar transistors T
1
and T
2
which are identical to each other and have their emitters connected together, through a first current generator G
1
, to the positive terminal of the supply voltage, indicated by Vdd. The collectors of the transistors T
1
and T
2
are connected, each through an active load comprising an npn-type bipolar transistor T
3
and T
4
, to the negative terminal of the supply voltage, represented by the ground symbol. The two transistors T
3
and T
4
have their bases connected together, through a second current generator G
2
, to the ground terminal, and their collectors connected to corresponding constant current generators G
3
and G
4
which are identical to each other. The differential amplifier has two input terminals INP and INM connected to the bases of the input transistors T
1
and T
2
, and two output terminals OUP and OUM connected to the collectors of the said transistors T
1
and T
2
.
A voltage divider consisting of two resistors R of equal resistance is connected between the output terminals OUP and OUM of the differential amplifier
10
. The intermediate connection of the divider is connected to the inverting terminal of an operational amplifier
11
. A voltage, indicated by V
cm
, which is the reference voltage selected for the differential output (for example the central value of the supply voltage of the differential circuit) is applied to the non-inverting terminal of the operational amplifier
11
. The voltage divider and the operational amplifier
11
together form a circuit, indicated as a whole by the number
15
, for regulating the output reference voltage.
In operation, an error voltage Ve, which is proportional to the deviation of the mean output voltage (V
OUP
−V
OUM
)/2 from the reference value V
cm
, is present at the output of the operational amplifier
11
. The error voltage Ve is applied to the differential amplifier
10
to be regulated. In this example, it acts on the generator G
1
in such a way as to modify in the same direction and by the same amount the currents in the two branches of the differential amplifier, and thus to modify the output voltages V
OUP
and V
OUM
with a sign such that the error is cancelled.
However, a solution such as that described above requires a power consumption which in many applications in not negligible, and which leads to the operational amplifier
11
and the resistors R taking up a certain amount of space when the amplifier and the corresponding regulator are formed in an integrated circuit.
Another known solution for the feedback circuit is shown in FIG.
2
. Two pairs of differential amplifiers having a common input terminal, to which the common-mode reference voltage V
cm
is applied, and having their other two input terminals connected to the outputs V
OUM
and V
OUP
of the differential amplifier
10
, are used to detect the mean value of the output of the differential amplifier to be regulated (not shown). Each amplifier is formed by two identical npn-type bipolar transistors, indicated by T
5
, T
6
and T
7
, T
8
, each having its emitter connected to one terminal of the supply voltage, indicated by the ground symbol, through a resistor R
1
and a constant current generator G
3
. The collectors of the transistors T
6
and T
7
are connected directly to the positive pole of the supply voltage Vdd, and the collectors of the transistors T
5
and T
8
are connected together at a node N which is connected, through a MOSFET transistor M
1
connected as a diode (with the drain and gate terminals in common), to the said positive pole Vdd. The gate terminal of the transistor M
1
is connected to one input terminal In
1
of an operational amplifier
12
. The other terminal In
2
of the operational amplifier
12
is connected to the junction between a constant current generator G
3
, identical to those of the differential amplifiers, whose other terminal is connected to ground, and another MOSFET transistor M
2
, which is identical to the transistor M
1
and is also connected as a diode, with its source terminal connected to the positive pole Vdd. The output of the operational amplifier
12
is connected to the differential amplifier (not shown) which requires the common-mode regulation.
In conditions of perfect balancing, a current I/
2
, where I is the current generated by each of the generators G
3
, flows in each branch of the coupled differential amplifiers. The current of two branches is added together at the node N, so that a current I flows through the transistor M
1
. There will be a voltage determined by the resistance between the source and drain of the transistor M
1
at the input In
1
of the operational amplifier
12
. A current I also flows through the transistor M
2
, and therefore the same voltage will be present at the terminal In
2
of the operational amplifier
12
. Consequently, no error voltage will be present at the output of the operational amplifier
12
.
As may easily be verified, any imbalance, in other words any variation in the same direction of the voltages V
OUP
and V
OUM
at the outputs OUP and OUM of the differential amplifier to be regulated, is immediately compensated by the feedback of the circuit described above, and therefore the mean value of the output voltages is maintained at the predetermined reference value V
cm
.
The circuit structure described above also has disadvantages, the main disadvantage being a reduced dynamic range; in other words, it operates correctly only with small variations of the output voltages of the differential amplifier to be regulated. Another disadvantage is that the circuit takes up a significant amount of space on an integrated circuit.
A feedback circuit operating with switched capacitors has also been proposed to reduce the power consumption. However, this circuit requires a clock generator at
Clerici Giancarlo
Tomasini Luciano
Galanthay Theodore E.
Iannucci Robert
Seed IP Law Group PLLC
STMicroelectronics S.r.l.
Vu Bao Q.
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