Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2003-02-10
2004-08-31
Vu, Bao Q. (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
C323S316000, C323S314000
Reexamination Certificate
active
06784651
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a controllable assembly of current sources, and in particular to such a controllable current source assembly controlling an amplifier having a gain varying exponentially as a response to an analog control voltage.
Such an amplifier may be formed as an integrated circuit and is for example used as a gain scan amplifier in a mobile telephony equipment.
2. Description of the Related Art
FIG. 1
schematically shows a conventional amplifier, the gain of which exponentially varies as a response to an analog control voltage. Such an amplifier is described in patent U.S. Pat. No. 5,077,541. The amplifier includes an input terminal E receiving a variable positive input voltage Vin. The amplifier further includes two control terminals A and B receiving a control voltage VAB, and an output terminal generating an output voltage Vout. The amplifier includes an attenuator block
2
. Attenuator block
2
has an input terminal connected to terminal E, and n output terminals Oi (i ranging between 1 and n). Each output terminal Oi of block
2
generates a control voltage equal to input voltage Vin attenuated according to a predetermined ratio, for example 2
i
in the case of an R/2R attenuator. Each output terminal Oi of block
2
is associated with a transconductance element g
m
i controllable by a current. Each transconductance element g
m
i receives as an input the voltage generated by the terminal Oi of same rank i. An output block
4
, connected to a supply voltage VDD, generates voltage Vout as a response to the sum of the currents provided by transconductance elements g
m
i. Output block
4
is also connected to provide a feedback signal to transconductance elements g
m
i. A controllable set of current sources
6
includes outputs Si, each output Si being connected to control the transconductances of the transconductance element g
m
i of same rank i. Control terminals A and B are connected to controllable current source assembly
6
.
FIG. 2
schematically shows a conventional controllable current source assembly
6
, also described in patent U.S. Pat. No. 5,077,541. Each output terminal Si is connected to the collector of an NPN-type bipolar transistor T
1
i. The emitters of transistors T
1
i are interconnected and connected to ground (GND) via a constant current source CS
1
. The base of each transistor T
1
i is connected to a node Ni of a control means
8
. In control means
8
:
the first node Nl is coupled to terminal A;
the last node Nn is coupled to terminal B;
each node Nj (j ranging between 1 and n−1) is connected to node Nj+1 via a resistor
10
; and
each node Nj (j ranging between 2 and n−1) is connected to a first terminal of a constant current source
12
specific to this node.
FIG. 3
illustrates in a simplified manner the variation of the gain of the amplifier of
FIG. 1
when the successive transconductance elements g
m
i are activated. In the example shown, it is assumed that n=6. If each of transconductance elements g
m
6
to g
m
1
is successively and separately activated, output block
4
receives a current successively proportional to an attenuation of ratio 2
6
, 2
5
, 2
4
, 2
3
, 2
2
, and 2
1
of voltage Vin. The amplifier gain thus varies exponentially. The all-or-nothing activation of transconductance elements g
m
i causes abrupt variations of the gain, according to a stepped curve such as the curve shown in full line. Current source assembly
6
is provided to suppress these abrupt gain variations and provide the gain variation illustrated in dotted lines in FIG.
3
.
FIG. 4
schematically illustrates the control currents Ii respectively generated by output terminals Si of current source assembly
6
. Control means
8
is provided, when control voltage VAB describes a predetermined voltage range, for successively progressively turning on each transistor T
1
i, then progressively turning it off, while having the sum of the currents Ii running through all transistors T
1
i remain substantially constant. It is considered hereafter that control means
8
is provided so that at most two transistors T
1
i are on at the same time. It is assumed that the preceding predetermined voltage range is included between a minimum voltage−Vmax (VA=0 and VB=Vmax), and a maximum voltage+Vmax (VA=Vmax and VB=0). When voltage VAB is minimum, the voltages of nodes Ni are such that only transistor T
16
is on. Transistor T
16
is then run through by a maximum current of value Imax. When voltage VAB linearly increases, the voltage of nodes N
6
, N
5
varies so that transistor T
16
is progressively turned off while transistor T
15
is progressively turned on. The other transistors T
1
i are off. When transistor T
15
is completely on, it is run through by a maximum current of value Imax and transistor T
16
is off. When voltage VAB keeps on increasing until value+Vmax, transistor T
15
is progressively turned off while transistor T
14
is progressively turned on, and so on until transistor T
11
is on and run through by current Imax.
However, current sources
12
of control means
8
permanently provide a current in the bases of transistors T
1
i of current source assembly
6
. The existence of these base currents makes the forming of control means
8
difficult, especially for the determination of resistances
10
and of the values of current sources
12
. Further, these base currents especially depend on the manufacturing process and on the operating temperature. This results in that a same control voltage VAB will not have the same effect upon current sources
6
for two different operating temperatures.
A completely differential variable-gain amplifier receiving a differential input signal and generating a differential output signal is conventionally formed by means of two identical amplifiers. A first amplifier receives a voltage Vin and generates as a response a voltage Vout, and the second amplifier receives a voltage Vin′ and generates as a response a voltage Vout′. The two amplifiers must be identical so that the output signal is not distorted. It is in particular important that the current sources
6
controlling each amplifier be identical and generate identical control currents for a same control voltage VAB. Now, it is difficult to form two matched current sources
6
. In particular, when the amplifier is formed as an integrated circuit, the two sets of transistors T
1
i may be remote from each other, and it is difficult to form two sets of transistors T
1
i having the same properties.
Further, the structure of current source assembly
6
is such that its output terminals Si inevitably have a high operating voltage (for example, greater than 2.2 V), due, in particular, to the fact that a high operating voltage (2.5 V) must be provided on control nodes A and B of means
8
. This constraint and the existence of the other circuits constitutive of the amplifier of
FIG. 1
results in that such an amplifier must be supplied under a high voltage, generally 5 V. Many electronic devices have a supply voltage smaller than 3 V, especially mobile telephony equipment, and it is desirable to have a variable-gain amplifier which can be supplied by a voltage smaller than 3 V.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a variable-gain amplifier having the same functions as the amplifier of FIG.
1
and that can be supplied by a voltage lower than 3 V.
Another object of the present invention is to provide such a current source assembly including a control means which is easy to form.
Another object of the present invention is to provide such a current source assembly, having a response to temperature which does not depend on temperature.
Another object of the present invention is to provide such a current source assembly which can provide matched control current pairs.
To achieve these objects, the present invention provides a controllable assembly of current sources including several first output termi
Mouret Michel
Sabut Marc
Van Zanten François
Bennett II Harold H.
Jorgenson Lisa K.
Seed IP Law Group PLLC
STMicroelectronics S.A.
Vu Bao Q.
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