Variable gain amplifier with gain linear with control voltage

A variable gain amplifier (VGA) having a control voltage source that provides high gain-to-control voltage linearity over at least an 80 dB gain range. Further, the gain curve for the VGA is essentially independent of temperature. In the preferred embodiment, the VGA includes a two-stage bipolar differential amplifier. Each stage is a transconductor followed by current steering. The first stage amplifier is coupled to an exponentially varying current source to change the transconductance of the stage. The second stage amplifier is coupled to an fixed current source to maintain a fixed transconductance for the stage. To obtain exponential current steering, the control signal for the current steering circuitry is pre-distorted by the following equation: EQU I/(1+exp(-f(V.sub.CTRL /V.sub.T))=I*A*exp(V.sub.CTRL /V.sub.REF), where A is a scaling factor, V.sub.T =kT/q, and T is temperature in Kelvin. The invention includes a fast, inexpensive control voltage source that provides such a signal.

RELATED APPLICATIONS 
This application is related to co-pending U.S. patent application Ser. No. 
09/163,892 entitled "VARIABLE GAIN AMPLIFIER WITH HIGH LINEARITY AND LOW 
NOISE", filed Sep. 30, 1998, and assigned to the assignee of the present 
invention, the teachings of which are hereby incorporated by reference. 
1. Technical Field 
This invention relates to electronic circuits, and more particularly to an 
electronic variable gain amplifier having a control voltage source that 
provides high gain-to-control voltage linearity over a wide dB range and a 
gain curve that is essentially independent of temperature. 
2. Background 
In a radio frequency (RF) transceiver, the received signal has a high 
dynamic range (&gt;80 dB). In order to supply a signal of constant amplitude 
to a baseband section of the transceiver, a variable gain amplifier (VGA) 
with equivalent or better dynamic range is required. While it is desirable 
that the VGA gain in decibels (dB) be linear with respect to a control 
voltage, there is a certain range of tolerable non-linearity in gain, 
usually specified by gain slope variation in dB/V over some gain segment. 
Known solutions fail to provide a VGA that has sufficient linearity in the 
high gain range (typically in the top 10 dB) that are all suitable for 
some applications (such as a code division multiple access (CDMA) 
transceiver). 
Attempts have been made in the past to provide such linearity by using 
bipolar VGA's with current steering. However, current steering circuitry 
implemented in bipolar technology has a hyperbolic tangent characteristic 
instead of the desired exponential characteristic. Attempts have been made 
to use feedback circuits and read-only memory based look-up tables to 
generate appropriate control signals for such a circuit, but such 
approaches are relatively slow and expensive to implement. 
Accordingly, the inventor has perceived that there is a need for a variable 
gain amplifier having a control voltage source that provides high 
gain-to-control voltage linearity over a wide dB range and a gain curve 
that is essentially independent of temperature. The present invention 
provides such an amplifier. 
SUMMARY 
The invention includes a variable gain amplifier (VGA) having a control 
voltage source that provides high gain-to-control voltage linearity over 
at least an 80 dB gain range. Further, the gain curve for the VGA is 
essentially independent of temperature. 
In the preferred embodiment, the VGA includes a two-stage bipolar 
differential amplifier. Each stage is a transconductor followed by current 
steering. The first stage amplifier is coupled to an exponentially varying 
current source to change the transconductance of the stage. The second 
stage amplifier is coupled to an fixed current source to maintain a fixed 
transconductance for the stage. 
Accordingly, to obtain exponential current steering, the control signal for 
the current steering circuitry is pre-distorted by the following equation: 
EQU I/(1+exp(-f(V.sub.CTRL /V.sub.T))=I*A*exp(V.sub.CTRL /V.sub.REF), 
where A is a scaling factor, V.sub.T =kT/q, and T is temperature in Kelvin. 
The invention includes a fast, inexpensive control voltage source that 
provides such a signal. 
In particular, in one aspect the invention includes a variable gain 
amplifier with high gain-to-control voltage linearity over a wide dB 
range, including: at least one differential amplifier stage having an 
amplifier section and a current steering section exhibiting a hyperbolic 
tangent characteristic; a voltage source, coupled to the current steering 
section of at least one differential amplifier stage, for generating a 
first control signal output proportional to I.sub.EXP, and a second 
control signal output proportional to I.sub.CONST -I.sub.EXP, with 
I.sub.CONST being a constant current, and I.sub.EXP being exponentially 
related to an input control voltage V.sub.CTRL and independent of 
temperature, where the first and second control signal outputs control 
correct steering for the current steering section so that the current 
steering section has an exponential characteristic. In another aspect, the 
invention includes such a voltage source alone. 
The details of one or more embodiments of the invention are set forth in 
the accompanying drawings and the description below. Other features, 
objects, and advantages of the invention will be apparent from the 
description and drawings, and from the claims.

DETAILED DESCRIPTION 
FIG. 1 shows a schematic diagram of the amplifier section of one embodiment 
of the present invention. The amplifier section is shown as being 
implemented in bipolar circuitry. In the preferred embodiment, the VGA 
includes a two-stage bipolar differential amplifier. Each stage is a 
transconductor followed by current steering. The first stage amplifier 10 
is coupled to an exponentially varying current source 12 to change the 
transconductance of the stage. More particularly, referring to the first 
stage amplifier 10, differential IF inputs IN+, IN- are respectively 
coupled to the bases of transistors Q1 and Q2. Transistors Q1 and Q2, 
along with exponentially varying current source 12, form a first 
differential amplifier. The differential amplifier section of the second 
stage 14 is similar in construction, but uses a fixed current source to 
maintain a fixed transconductance for the stage. The second stage 
amplifier 14 is shown capacitively coupled to the output of the first 
stage amplifier 10. 
For purposes of the present invention, the amplifier stages may be 
implemented in other configurations and have fewer or more stages. An 
example of another configuration for the amplifier stages is shown in the 
above-referenced co-pending U.S. patent application Ser. No. 09/163,892. 
To improve the dynamic range of the variable gain amplifier, each the 
amplifier section of each stage 10, 14 is followed by corresponding 
current steering circuitry. More particularly, referring to the current 
steering circuitry of the first stage amplifier 10, paired transistors 
Q5-Q6 and Q7-Q8 act to steer the signal current from transistors Q1 and 
Q2, respectively, either to a voltage source V.sub.cc (through transistors 
Q6 and Q7), or to loads coupled to outputs OUT+, OUT- (through transistors 
Q5 and Q8). The steering voltage signals V.sub.CTRLOUT + and V.sub.CTRLOUT 
- for transistors Q5, Q6, Q7, and Q8 are output by a voltage source V1. 
The current steering circuitry of the second stage 14 is similar in 
construction. 
As noted above, current steering circuitry implemented in bipolar 
technology has a hyperbolic tangent characteristic. To obtain exponential 
steering, the control signal can be pre-distorted by the following 
equation: 
EQU I/(1+exp(-f(V.sub.CTRL /V.sub.T))=I*A*exp(V.sub.CTRL /V.sub.REF), 
where A is a scaling factor, V.sub.T =kT/q, T is temperature in Kelvin, and 
.function. is a function of V.sub.CTRL. 
FIG. 2 is a block diagram of an exponential control signal generation 
circuit for generating the desired outputs for the voltage source V1. For 
ease of manufacture in conjunction with the amplifier stages 10, 14, this 
circuit is preferably implemented in bipolar circuitry. However, a 
comparable circuit can be implemented in other technologies, such as 
complimentary metal oxide semiconductor (CMOS) field effect transistors 
(FET). Accordingly, the embodiment shown in FIG. 2 should be taken as 
exemplary only. 
A control voltage V.sub.CTRL is applied to a voltage-to-current converter 
200 to generate a corresponding proportional current I.sub.1 equal to 
V.sub.CTRL /R.sub.1, where R.sub.1 is a resistance value selected 
empirically to scale the output current I.sub.1 for a particular 
application. The current I.sub.1 is applied to a translinear current 
multiplier/divider 202. The current multiplier/divider 202 has as 
additional inputs current I.sub.2 from a 
proportional-to-absolute-temperature (PTAT) current source, and current 
I.sub.3 from a constant (independent of temperature) current source. 
Examples of such current sources are shown in U.S. Pat. No. 5,774,013 to 
Groe, issued Jun. 30, 1998 and assigned to the assignee of the present 
invention. The current multiplier/divider 202 provides an output I.sub.OUT 
equal to I.sub.1 *I.sub.2 /I.sub.3, which can be expressed as V.sub.CTRL 
*I.sub.2 /(R.sub.1 *I.sub.3). The output current I.sub.OUT is applied to 
an exponential current generator 204, which has as an additional input a 
constant current I.sub.4. The exponential current generator 204 provides 
an output current I.sub.EXP that is exponentially related to I.sub.OUT and 
is independent of temperature. In particular, I.sub.EXP =I.sub.4 
*A*exp(I.sub.OUT *R.sub.2 /V.sub.T), which can be expressed as I.sub.EXP 
=I.sub.4 *A*exp(V.sub.CTRL /V.sub.REF), where 1/V.sub.REF =I.sub.2 
*R.sub.2 /(I.sub.3 *R.sub.1 *V.sub.T), with A being a selectable scaling 
factor, and R.sub.2 is a resistance value selected empirically to scale 
the output current I.sub.EXP for a particular application. 
The output current I.sub.EXP from the exponential current generator 204 is 
coupled to a current replicator 206, which generates two essentially 
identical output currents equal to I.sub.EXP. One output of the current 
replicator 206 provides current flow from a voltage source V.sub.CC 
through a first diode 208a, thereby generating a control voltage 
V.sub.CRTLOUT - through a first buffer 210a that is coupled to the gates 
of transistors Q6 and Q7 in FIG. 1. The other output of the current 
replicator 206 is subtracted from a constant current I.sub.CONST by a 
current subtractor 212 to generate an output current I.sub.CONST 
-I.sub.EXP. The output current I.sub.CONST -I.sub.EXP provides current 
flow from a voltage source V.sub.CC through a second diode 208b, thereby 
generating a V.sub.CTRLOUT + control voltage through a second buffer 210b 
that is coupled to the gates of transistors Q5 and Q8 in FIG. 1. When the 
two output voltages V.sub.CTRLOUT -, V.sub.CTRLOUT + from voltage source 
V1 are applied to the current steering pairs of transistors, gain in dB is 
proportional to V.sub.CTRL. 
Thus, the output of the voltage source V1 is a function of V.sub.CTRL, and 
provides the desired pre-distorted control signals necessary to obtain 
exponential steering of the current steering circuitry. In one embodiment 
of invention, a VGA was fabricated having a gain-to-control voltage 
linearity over at least an 80 dB gain range. Further, the gain curve for 
this VGA was essentially independent of temperature. 
A number of embodiments of the present invention have been described. 
Nevertheless, it will be understood that various modifications may be made 
without departing from the spirit and scope of the invention. Accordingly, 
other embodiments are within the scope of the following claims.