Current source assembly controllable in response to a...

Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...

Reexamination Certificate

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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

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