Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2000-07-06
2001-09-18
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S252000, C330S261000
Reexamination Certificate
active
06292056
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to differential amplifiers and in particular to a differential amplifier having an adjustable common mode output voltage.
2. Description of Related Art
FIG. 1
illustrates in schematic diagram form a typical prior art differential amplifier
10
including a pair of transistors Q
1
and Q
2
, a pair of matching load resistors R
1
and R
2
linking collectors of transistors Q
1
and Q
2
to a voltage source V
D
, a pair of matching current sources I
1
and
12
supplying emitter current to transistors Q
1
and Q
2
, and a resistor R
3
connected between the emitters of transistors Q
1
and Q
2
. Amplifier
10
amplifies an input signal V
IN
applied across the bases of transistors Q
1
and Q
2
to produce an output signal V
OUT
across the collectors of transistors Q
1
and Q
2
.
The common mode output voltage of a differential amplifier is defined as the average voltage at its differential output terminals. For the amplifier
10
the common mode voltage V
CM
is a function of a load current and a resistance as follows:
V
CM
=V
D
−I
L
R
L
where load current I
L
=I
1
=I
2
and load resistance R
L
=R
1
=R
2
.
In many applications it is desirable to precisely set the common mode output voltage of a DC-coupled differential amplifier. From the above equation we can see that we can choose the common mode voltage by appropriately adjusting the values of load resistors R
1
and R
2
, the magnitude of load currents I
1
and I
2
and the magnitude of supply voltage V
D
. Thus when we design a differential amplifier we choose the values of load resistors R
1
and R
2
and load currents I
1
and I
2
in part to set the desired common mode voltage. However since process variations cause the actual resistance of resistors or current output of current sources to vary somewhat from their nominal values, the actual common mode voltage will vary somewhat from the design value. Load resistors R
1
and R
2
have been implemented using adjustable resistors, such as for example potentiometers or laser-trimmable resistors, when a more precise control over common mode voltage has been needed. The resistance of the load resistors can then be iteratively adjusted while measuring the actual common mode output voltage so as to more precisely set the common mode voltage to a desired level. similarly when current sources I
1
and I
2
are adjustable or when voltage source V
D
is adjustable, we can iteratively adjust the current produced by current sources I
1
and I
2
or iteratively adjust the voltage source until we obtain the desired common mode voltage.
However it is inconvenient and time-consuming to measure the common mode output voltage of a differential amplifier while iteratively adjusting its load current, load resistance or supply voltage. And even though we can precisely adjust an amplifier's common mode output voltage by such methods, we cannot be certain that the amplifier's common mode output voltage will stay where we set it. The resistance of the load resistors R
1
and R
2
, and the current produced by current sources I
1
and I
2
, and the source voltage V
D
produced by many power supplies are temperature-dependent. Thus when the temperature of amplifier
10
changes, so too does its common mode output voltage.
What is needed is a differential amplifier having a common mode output voltage that can be conveniently and accurately adjusted to a desired level that is stable irrespective of process and temperature variations.
SUMMARY OF THE INVENTION
In accordance with the invention, a control unit automatically adjusts the supply voltage to a differential amplifier so that the amplifier has a common mode output voltage matching a control voltage supplied as input to the control unit.
The control unit includes a reference circuit for generating a reference voltage in response to the supply voltage, wherein the reference voltage substantially matches the common mode output voltage of the differential amplifier. The control unit also includes an operational amplifier for receiving the control voltage and the reference voltage as inputs and for adjusting the supply voltage so that the reference voltage matches the control voltage, thereby ensuring that the amplifier's common mode output voltage substantially matches the control voltage.
It is accordingly an object of the invention to provide a means for setting the common mode output voltage of a differential amplifier to a desired level.
It is a further object of the invention to provide a means for maintaining the common mode output voltage of a differential amplifier to that desired level despite variations in temperature of the differential amplifier.
The concluding portion of this specification particularly points out and distinctly claims the subject matter of the present invention. However those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements.
REFERENCES:
patent: 4560947 (1985-12-01), Frey
patent: 4904953 (1990-02-01), McCormack
patent: 5283535 (1994-02-01), Sevenhans et al.
patent: 5412343 (1995-05-01), Rijns
patent: 9-153747-A (1997-06-01), None
Bedell Daniel J.
Credence Systems Corporation
Nguyen Patricia T.
Pascal Robert
Smith-Hill and Bedell
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