Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2002-06-27
2004-06-01
Nguyen, Patricia (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S253000
Reexamination Certificate
active
06744319
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a semiconductor circuit technology and, more particularly, to an exponential function generator and a variable gain amplifier (VGA) employing the same.
DESCRIPTION OF RELATED ART
In a wireless communication system, a receiver may receive a signal that experiences wide variations in signal power. In receivers such as are used in a wideband digital code division multiple access (CDMA) mobile station, it is necessary to control the power of the demodulated signal for proper signal processing. Moreover, in transmitters such as are used in a CDMA mobile station, it is indispensable to control the transmit power in order to avoid excessive interference to other mobile stations. These same power control considerations apply to narrowband analog frequency modulation (FM) wireless communication system transmitters and receivers.
Dual-mode CDMA/FM wireless communications systems should provide power control of transmitted and received signals of both digital CDMA and analog FM modulation. In these dual-mode mobile stations, the control process is complicated by the differing dynamic ranges and industry regulation standards associated with the CDMA and FM signals. Therefore, the provision of separate automatic gain control (AGC) circuitry for both the CDMA and the FM signals increases the complexity and expense of such dual-mode mobile stations. Accordingly, it is desired to provide AGC circuitry capable of operating upon both the CDMA and FM signals.
A variable gain amplifier (VGA), which is one of the AGC circuits, provides a gain in proportion to a control voltage. The VGA provides an exponential voltage gain as a function of linear increases in the applied control voltage thereby providing an approximately linear power gain in decibels (dB) in direct proportion to linear increases in the applied control voltage. The VGA can be used in many applications including receivers and transmitters.
Referring to
FIG. 1
, there is shown a block diagram of a conventional variable gain amplifier (VGA)
100
included in a receiver of a dual-mode CDMA/FM mobile station, which is found in U.S. Pat. No. 5,880,631, entitled “HIGH DYNAMIC RANGE VARIABLE GAIN AMPLIFIER” issued Mar. 9, 1999, and briefly summarized herein as representative of the prior art.
As shown in
FIG. 1
, the VGA
100
comprises an input stage
120
and two cascaded current amplifiers
160
A and
160
B. The current amplifiers
160
A and
160
B are successively cascaded to increase the dynamic range of the VGA
100
and the number of current amplifiers can be adjusted, as necessity requires.
The input stage
120
includes a separate FM input stage
121
and CDMA input stage
122
with respective input ports
171
and
170
. The FM input stage
121
and the CDMA input stage
122
are alternately connected to the current amplifier
160
A through switches
123
, which are controlled by a CDMA/FM mode select signal.
The VGA
100
also employs bias ports
110
,
130
,
150
A and
150
B for control voltages to be applied to the VGA
100
. The gain of each stage is controlled by control voltages, which, for example, may be generated by receiver detection circuitry that determines the signal strength. Each stage is comprised of a variety of components, including an active device such as a transistor.
Since it operates with a low supply voltage, about 3.6 V, the input stage
120
converts an input voltage signal to a current signal to prevent the VGA active devices from operating in their non-linear region, and distorting the input signal.
Meanwhile,
FIG. 1
provides the bias port
130
coupled to a transconductance bias control circuit
140
, which will be described later.
Referring to
FIG. 2
, there is shown a diagram of the CDMA input stage
122
of
FIG. 1
, which includes a Gilbert cell attenuator
226
and a variable transconductance amplifier
227
and serves four functions.
First, the variable transconductance amplifier
227
converts the input voltage signal to a current signal. Second, the combination of the variable transconductance amplifier
227
and the Gilbert cell attenuator
226
permits variable amplification of the signal, which may be varied exponentially by linearly adjusting control voltages at the bias port
110
. Third, increased emitter degeneration in the variable transconductance amplifier
227
reduces the intermodulation distortion (IMD) of the VGA
100
when the input voltage signal is large and the IMD would be most prominent. That is, as the emitter degeneration in the variable transconductance amplifier
227
is increased, the transconductance, and thus the IMD, of the CDMA input stage
122
is decreased. Fourth, decreased emitter degeneration in the variable transconductance amplifier
227
improves the noise feature of the VGA
100
when the input voltage signal is small and noise performance is the most critical. Namely, as the emitter degeneration in the variable transconductance amplifier
227
is decreased, the transconductance of the CDMA input stage
122
is increased, improving the noise feature of the receiver.
The variable transconductance amplifier
227
is comprised of two bipolar junction transistors (BJTs)
235
and
236
, two current sources
238
and
239
, and a field effect transistor (FET)
237
. The current sources
238
and
239
are serially connected to the emitters of the BJTs
235
and
236
, respectively. The source connection
228
and drain connection
229
of the FET
237
are respectively connected to the emitters of the BJTs
235
and
236
. The balanced signal at the VGA input port
170
is applied to the bases of the BJTs
235
and
236
. The balanced current output of the variable transconductance amplifier
227
flows from the collectors of the BJTs
235
and
236
.
The transconductance of the variable transconductance amplifier
227
may be adjusted by varying the emitter degeneration of the BJTs
235
and
236
. As a result, the gain of the VGA
100
may be varied. The emitter degeneration of the BJTs
235
and
236
is created by varying the channel resistance of the FET
237
. The FET
237
is operated like a variable resistor in its ohmic region and provides variable emitter degeneration for both of the BJTs
235
and
236
. The drain-source bias voltage of the FET
237
must therefore be less than the knee voltage of the FET
237
. The channel resistance may be varied by adjusting the bias across the gate-source junction of the FET
237
by varying the voltage applied at a bias port
124
. The transconductance of the variable transconductance amplifier
227
can be increased by decreasing the channel resistance of the FET
237
.
The differential output currents of the variable transconductance amplifier
227
are coupled to the Gilbert cell attenuator
226
. The Gilbert cell attenuator
226
varies the current amplitude of a signal applied to its inputs. The Gilbert cell attenuator
226
contains a first pair of BJTs
231
and
234
, and a second pair of BJTs
232
and
233
. The attenuation level of the Gilbert cell attenuator
226
is established by a control voltage applied at the bias port
110
.
The Gilbert cell attenuator
226
attenuates the output current of the variable transconductance amplifier
227
when the first pair of BJTs
231
and
234
are biased by the control voltage applied to the bias port
110
so that a component of the variable transconductance amplifier's output current flows through the first pair of BJTs
231
and
234
rather than through the second pair of BJTs
232
and
233
. Hence the balanced currents at an output port
190
of the Gilbert cell attenuator
226
are diminished.
The configuration of the FM input stage
121
is similar to that of the CDMA input stage
122
described in
FIG. 2
except that the FET
237
is replaced by a fixed resistance. As previously mentioned, the fixed resistance of the FM input stage
121
provides a fixed transconductance because industry standards, such as IS-95, allow compression of the input signal at a much lower input level than
Hynix / Semiconductor Inc.
Jacobson & Holman PLLC
Nguyen Patricia
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