CMOS variable gain amplifier and control method therefor

Details CMOS variable gain amplifier and control method therefor CMOS variable gain amplifier and control method therefor

2000-01-25

2001-07-10

Pascal, Robert (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C330S310000

Reexamination Certificate

active

06259321

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a high frequency variable gain amplifier, and more specifically, to a CMOS variable gain amplifier in which maximum high frequency is achieved and the variable gain characteristic is wide as well, and a control method therefor.
BACKGROUND OF THE INVENTION
Recently, with the rapid development of the mobile communication service, radio portable devices are becoming widespread, and correspondingly low-priced and compact devices have been actively researched.
Radio signals have varying magnitude at any given time due to the distance from the base station and various kinds of obstacles. Therefore, a variable gain amplifier is required to correct this.
There are several variable gain amplifiers, but two typical variable gain amplifiers are shown in
FIGS. 1 and 2
, respectively.
When the variable gain amplifier shown in
FIG. 1
is applied with input voltage through Vip and Vin, the input voltage is converted into the current by transistors M
51
and M
52
, and this current is applied to the load consisting of the current sources I
11
, I
12
, and transistor M
53
, making the output voltage appear at Von and Vop. At this time, if the control voltage Vcon is applied to the gate of the transistor M
53
, the resistance of the drain-source of the transistor M
53
is caused to vary according the control voltage Vcon. That is, the gain can be varied with the variation of the output resistance.
The advantage of the variable gain amplifier shown in
FIG. 1
is that the variable gain characteristic at high frequency is excellent, this is because the source connection point C of differential input transistors M
51
, M
52
is virtual ground, and parasitic capacitance C
11
of any magnitude generated in the source connection C has non influence on the variable operation.
However, the variable gain amplifier of
FIG. 1
has the disadvantage in that case of more than hundreds of mV voltage being applied to the input, a considerable distortion is generated while the input transistor M
51
or M
52
departs from the conductive state. That is, the variable gain amplifier described above is not able to be used for large input.
In
FIG. 2
another type of a variable gain amplifier is shown with the advantage in view of the operational voltage range. The structure of
FIG. 2
is similar to that of
FIG. 1
except that the transistor M
63
, which plays a role of active resistor varying with the control voltage Vcon, is connected between the sources of the transistors M
61
and M
62
forming a pair of inputs.
The variable gain amplifier shown in
FIG. 2
operates to raise the control voltage Von and reduce the resistance of transistor M
63
when the input voltage is small, thereby to increase the gain. At this time, the distortion is also small since the input is small. Conversely, the variable gain amplifier of
FIG. 2
operates to decrease the control voltage Vcon, thereby to increase the source resistance between the input transistors M
61
, M
62
when the input voltage is large. Then, since the input is provided with the negative-feedback due to the large resistance, the gain is reduced and the output is reduced. At this time, the negative-feedback to the large input is caused to reduce the distortion relatively.
However, the variable gain amplifier of
FIG. 2
has a problem of gain reduction characteristics at high operating frequency.
Specifically, there is little effect from parasitic elements at low frequency, but a greater effect is generated at high frequency.
In
FIG. 2
, parasitic capacitive elements C
21
, C
22
exist between the drains and sources of the transistors M
63
, M
64
, and M
65
and the ground. If the control voltage is reduced due to the large input voltage at the high frequency and the drain-source resistance of the transistor M
63
is increased, the source currents of the transistors M
61
and M
62
gradually flow through the parasitic capacitance elements C
21
, C
22
. This has an effect of reducing the impedance in view of the input transistor in spite of no variation of Vcon.
Due to this, the desired gain reduction cannot be obtained. Namely,
FIG. 2
shows the variable gain amplifier in which excellent characteristics can be obtained at a low frequency while the desired broad gain reduction characteristics cannot be obtained only at a high frequency. Therefore, the operating frequency region becomes narrowed.
SUMMARY OF THE INVENTION
The disclosed embodiments of the present invention provide a CMOS variable gain amplifier that has a broad input range and desirable high-frequency operational characteristics by overcoming the restrictive characteristics encountered in the magnitude of input and the operating frequency in the conventional CMOS variable gain amplifiers described above, and a control method is provided therefor.
The disclosed embodiments of the present invention provides a new high-frequency variable gain amplifier that operates within possible maximum high frequency range and has small distortion in spite of wide-range input voltage by using CMOS elements in which the operational characteristics are lower than bipolar elements but the manufacturing cost is low and integration with a digital circuit is easy, the output gain of the amplifier being in the form of an exponential function according to a control input voltage.
A CMOS variable gain amplifier of the present invention includes an amplifier and a control voltage generator.
The amplifier has a plurality of variable gain amplifier cells with substantially the same gains connected in series to continuously have a broad gain variation. The control voltage generator generates a control voltage of the variable gain amplifier cells to vary a gain of the amplifier according to an external control voltage.
Preferably, the variable gain amplifier cells of the amplifier include an input differential amplifier, a bias current source, an operating point controller and load resistors. The input differential amplifier includes the first and the second input differential transistors having voltage inputs. The bias current source includes a first transistor of which a drain is commonly connected to sources of the first and the second input differential transistors for supplying a bias current. The operating point controller includes the second and the third transistors, of which each source is connected to the drains of the first and the second input differential transistors, and a common gate is connected to the control voltage terminal for controlling the operating point to be in a linear region and a saturation region of the first and the second input differential transistors. The load resistors are respectively connected to drains of the second and the third transistors for loading output voltages at a connection point.
Preferably, the control voltage generator includes an exponential function voltage generator, a variable gain amplifier cell, a current source, and an operational amplifier. The exponential function voltage generator generates a voltage in the form of an exponential function according to a control input voltage. The variable gain amplifier cell amplifies an output voltage of the exponential function voltage generator. The current source is connected to an output node of the variable gain amplifier cell to provide a current in the direction of offsetting an output voltage of the variable gain amplifier cell. The operational amplifier receives the offset voltage from the current source and generates the control voltage for the variable gain amplifier cells.
Preferably, the exponential function voltage generator includes a control voltage converter, an exponential function voltage generator, and a base voltage generator. The control voltage converter converts the external control voltage to a magnitude suitable for an internal signal processing. The exponential function voltage generator generates a exponential function voltage with an emitter of an internal bipolar element being supplied with an output of the control voltage converter. The base voltage g

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