Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2000-06-08
2001-10-16
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S260000
Reexamination Certificate
active
06304142
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a variable transconductance amplifier. Such an amplifier may be used, for example, in radio frequency circuitry as part of an automatic gain control (AGC) circuit. For example, such an amplifier may be used within a mixer or front end low noise amplifier (LNA).
2. Description of the Prior Art
FIG. 1
illustrates a known type of voltage controlled variable gain amplifier comprising NPN bipolar transistors. The amplifier comprises a transconductance stage in the form of transistors T
1
and
12
, resistors R
1
and R
2
and a constant current source CC
1
, one of whose terminals is connected to ground gnd. The bases of the transistors T
1
and T
2
are connected to differential signal input terminals IN+ and IN−, respectively.
The collectors of the transistors T
1
and T
2
are connected to a current steering circuit comprising transistors
1
to
4
. The emitters of the transistors
1
and
2
are connected to the collector of the transistor T
1
whereas the emitters of the transistors
3
and
4
are connected to the collector of the transistor T
2
. The collectors of the transistors
2
and
4
are connected to a positive supply input or line vcc. The collectors of the transistors
1
and
3
are connected to differential output terminals OUT− and OUT+ and via load resistors
5
and
6
, respectively, to the supply line vcc. The bases of the transistors
1
and
3
are connected to a first gain control input “agc adjust+” whereas the bases of the transistors
2
and
4
are connected to a second gain control input “agc adjust”. In use, a differential gain control voltage is supplied to the gain control terminals so that, when maximum gain is required, all of the signal current from each of the transistors T
1
and T
2
is steered through the respective load resistor
5
and
6
. Conversely, when minimum gain is required, the signal current is steered via the transistors
2
and
4
to the supply line vcc.
Although a voltage controlled variable gain amplifier of the type shown in
FIG. 1
may be used in an AGC arrangement of a radio frequency (RF) tuner, it has certain disadvantages when used in such an application. For example, as the gain of the amplifier decreases, the noise figure (NF) increase. Also, using NPN technology (or NMOS technology for field effect transistors) increases the difficulty of connecting the outputs to, for example, a mixer of a frequency changer.
Further, in order to handle large amplitude input signals, the transconductance input stage requires sufficient power to maintain linearity. This may be achieved by providing the resistors R
1
and R
2
with relatively high values and using a low quiescent current, in which case the noise figure of the transconductance stage is compromised. Alternatively, higher quiescent current and lower values of the resistors R
1
and R
2
may be used but this results in increased power dissipation.
SUMMARY OF THE INVENTION
According to the invention, there is provided a variable transconductance amplifier comprising first and second amplifying devices whose common terminals are connected via first and second resistors, respectively, to a first current source, and a feedback amplifier of variable transconductance having outputs connected to the common terminals of the first and second devices for supplying a differential current corresponding to the product of a differential output signal of the first and second devices and the variable transconductance.
The first current source may comprise a first constant current source.
The first and second devices may be of a first conduction type and the feedback amplifier may comprise amplifying devices of a second conduction type opposite the first type.
The feedback amplifier may have an input stage comprising third and fourth devices whose control terminals are connected to output terminals of the first and second devices, respectively, and whose common terminals are connected via third and fourth resistors, respectively, to a second current source.
The second current source may comprise a second constant current source.
The output terminal of the third device may be connected to the common terminals of fifth and sixth devices whose output terminals are connected to the common terminal of the first device and a first power supply input, respectively, the output terminal of the fourth device may be connected to the common terminals of seventh and eighth devices whose output terminals are connected to the common terminal of the second device and the first power supply input, respectively, the control terminals of the sixth and eighth devices may be connected to a first gain control input and the control terminals of the fifth and seventh devices may be connected to a second gain control input.
The output terminals of the first and second devices may be connected via fifth and sixth resistors to the common terminals of ninth and tenth devices, respectively, whose, control terminals are arranged to receive a substantially constant bias voltage.
The output terminals of the first and second devices may be connected via seventh and eighth resistors to first and second signal inputs, respectively, of a mixer comprising: eleventh and twelfth devices, whose common terminals are connected to the first signal input, whose control terminals are connected to first and second local oscillator inputs and whose output terminals are connected to first and second outputs, respectively; and thirteenth and fourteenth devices whose common terminals are connected to the second signal input, whose control terminals are connected to the first and second local oscillator inputs and whose output terminals are connected to the first and second outputs, respectively.
Each of the devices may comprise a transistor. Each of the devices may comprise a bipolar transistor and the common, control and output terminals may comprise emitter, base and collector electrodes, respectively. Each of the first and second devices may comprise an NPN transistor.
The common, control and output terminals of an amplifying device refer to the normal use of such a device in an amplifying and inverting configuration. Thus, as mentioned hereinbefore, in the case of bipolar transistors the common, control and output terminals are the emitter, base and collector electrodes. In the case of field effect transistors, the common, control and output terminals are the source, gate and drain electrodes, respectively.
It is thus possible to provide a low noise variable transconductance amplifier whose pain may be varied without unacceptably comprising the noise figure. Also, the use of complementary technology means that large supply voltages are not required. This arrangement also facilitates the use of the amplifier as part of a mixer, such as a Gilbert mixer, which may use the amplifier as its input stage and at the same time provide automatic gain control.
REFERENCES:
patent: 3769605 (1973-10-01), Long
patent: 4287476 (1981-09-01), Cense
patent: 4437070 (1984-03-01), Horl
patent: 4514702 (1985-04-01), Zogg
patent: 5528197 (1996-06-01), Frey
Kirschstein et al.
Mitel Semiconductor Limited
Mottola Steven J.
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