Electronic circuit for providing a desired common mode...

Details Electronic circuit for providing a desired common mode... Electronic circuit for providing a desired common mode...

2001-03-30

2002-08-13

Shingleton, Michael B (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C330S258000

Reexamination Certificate

active

06433634

ABSTRACT:

The invention relates to an electronic circuit for providing a desired common mode voltage to a differential output of an amplifier stage, which differential output comprises a first output terminal and a second output terminal, comprising a supply terminal; a first field effect transistor with a source coupled to the supply terminal; a second field effect transistor with a source coupled to the supply terminal; a third field effect transistor with a source coupled to the supply terminal and with a gate coupled so as to receive a reference voltage; a first cascode circuit with a first main current electrode coupled to a drain of the first field effect transistor, with a second main current electrode coupled to a gate of the first field effect transistor and to the first output terminal, and with a control electrode; a second cascode circuit with a first main current electrode coupled to a drain of the second field effect transistor and to the drain of the first field effect transistor, with a second main current electrode coupled to a gate of the second field effect transistor and to the second output terminal, and with a control electrode coupled to the control electrode of the first cascode circuit; a third cascode circuit with a first main current electrode coupled to a drain of the third field effect transistor, with a second main current electrode coupled to the control electrode of the first cascode circuit, and with a control electrode coupled to the control electrode of the first cascode circuit; and current means for providing a current through the third field effect transistor and the third cascode circuit.
Such an electronic circuit is known from the prior art. The known electronic circuit is shown in FIG.
1
and comprises: first to third field effect transistors (T
1
to T
3
), fifth to seventh cascode field effect transistors (T
5
to T
7
), and a current source I for supplying a current through the third and the seventh field effect transistor T
3
and T
7
. The electronic circuit receives a supply voltage between a first supply terminal V
SS
and a second supply terminal V
DD
.
FIG. 1
also shows a model of a differential output of an amplifier stage AMPSTG, which differential output comprises a first output terminal
1
and a second output terminal
2
. The sources of the first to third transistors T
1
to T
3
are connected to the first supply terminal V
SS
. The drains of the first and second transistors T
1
and T
2
and the sources of the fifth and sixth transistors T
5
and T
6
are interconnected. The gate of the first transistor T
1
and the drain of the fifth transistor T
5
are connected to the first output terminal
1
. The gate of the second transistor T
2
and the drain of the sixth transistor T
6
are connected to the second output terminal
2
. The gates of the fifth, sixth, and seventh transistors T
5
to T
7
and the drain of the seventh transistor T
7
are interconnected. The drain of the third transistor T
3
is connected to the source of the seventh transistor T
7
. The gate of the third transistor T
3
receives a reference voltage V
CM
. The voltages at the output terminals
1
and
2
are referenced V
out1
and V
out2
, respectively. The common mode voltage at the output terminals
1
and
2
per definition then is equal to the sum of V
out1
and V
out2
divided by 2. The electronic circuit is dimensioned such that the common mode voltage is equal to the reference voltage V
CM
. This is represented in equation 1
V
out1
+
V
out2
2
=
V
CM
(
1
)
It should be noted that the first and the second transistor must always be set for their linear operation ranges if the electronic circuit is to operate correctly. In the known electronic circuit, the reference voltage V
CM
, and accordingly also the common mode voltage at the first and the second output terminal
1
and
2
, is usually chosen to be approximately equal to half the supply voltage. The reference voltage V
CM
is often greater than necessary then, but a sufficient margin is present for accommodating, for example, spreads in transistor parameters.
It is a disadvantage of the known electronic circuit, accordingly, that it is less suitable for applications in which an operation at a minimum supply voltage is desired.
It is an object of the invention to provide an electronic circuit capable of supplying a lowest possible common mode voltage to a differential output of an amplifier stage.
According to the invention, the electronic circuit mentioned in the opening paragraph is for this purpose characterized in that the electronic circuit further comprises: a fourth field effect transistor with a source coupled to the first supply terminal; a fourth cascode circuit with a first main current electrode coupled to a drain of the fourth field effect transistor, with a second main current electrode coupled to a gate of the fourth field effect transistor, and with a control electrode coupled to the control electrode of the first cascode circuit; further current means for supplying a current through the fourth field effect transistor and the fourth cascode circuit; and voltage-generating means coupled in series between the further current means and the second main current electrode of the fourth cascode circuit, and in that the gate of the third field effect transistor is coupled to the voltage-generating means for receiving the reference voltage, and in that the first, the second, and the third field effect transistor are dimensioned such that they have approximately the same current densities, and in that the fourth field effect transistor is dimensioned such that it has a current density which is a factor N smaller than said current densities, said factor N being approximately equal to the ratio of the nominal current to the minimum current through the first field effect transistor.
It is achieved thereby that the reference voltage to be defined is substantially independent of transistor parameters.
An embodiment of an electronic circuit according to the invention is characterized in that the first cascode circuit comprises a fifth transistor with a first and a second main current electrode which form the first and the second main current electrode, respectively, of the first cascode circuit, and with a control electrode which forms the control electrode of the first cascode circuit; and in that the second cascode circuit comprises a sixth transistor with a first and a second main current electrode which form the first and the second main current electrode, respectively, of the second cascode circuit, and with a control electrode which forms the control electrode of the second cascode circuit; and in that the third cascode circuit comprises a seventh transistor with a first and a second main current electrode which form the first and the second main current electrode, respectively, of the third cascode circuit, and with a control electrode which forms the control electrode of the third cascode circuit; and in that the fourth cascode circuit comprises an eighth transistor with a first and a second main electrode which form the first and the second main current electrode, respectively, of the fourth cascode circuit, and with a control electrode which forms the control electrode of the fourth cascode circuit.
A very simple embodiment for the cascode circuits is obtained thereby. The fifth to seventh transistors are either all implemented with bipolar transistors of the same conductivity type or all implemented with field effect transistors of the same conductivity type.
An embodiment of an electronic circuit according to the invention is furthermore characterized in that the fifth, the sixth, the seventh, and the eighth transistor are dimensioned such that they have approximately the same current densities.
As a result, the potentials at the drains of the first to fourth field effect transistors are approximately equal. This achieves a better accuracy for defining the common mode voltage at the first and the second output terminal
1
and
2
.
Further advantageous embodiments of the electronic circu

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