### Design FAQs - Variable Gain Amplifiers

```Variable Gain
Amplifiers
Don Tuite Analog/Power Editor
10
9
Two gain selections
Gain multiplication factor
50
Gain (dB)
40
30
20
10
0
–10
0
0.2
Linear in dB
0.4
a.
25
20
8
15
7
Gain (dB)
60
ED Online 18875
6
5
4
2
0
b.
10
5
0
3
–5
1
0.6
0.8
1.0 1.1
VGAIN (V)
Gain increases or decreases
with control voltage
46 MHz
70 MHz
140 MHz
200 MHz
0
1
Linear in magnitude
2
3
VMAG (V)
4
5
–10
–4
0
5
10
11000 10100
01111
01010
Gain code
Linear in dB
c.
Binary scale
15
20
00101 00000
1. Analog gain control offers multiple advantages. First, it is linear in terms of both dB (a) and magnitude (b). (Figure 1a also demonstrates consistence in
0522FAQs-FIGURE
gain control
regardless 1a
of whether gain is being adjusted up or down.) Digital control (c) offers linearity in terms of dB and consistency, but in discrete steps.
0522FAQs-FIGURE 1b
What are VGAs?
Variable gain amplifiers (VGAs) are
signal-conditioning amplifiers with electronically settable voltage gain. There
are analog VGAs and digital VGAs, or
DVGAs. An analog voltage controls the
gain in both, which differ in how it
is applied. A digital-to-analog converter
(DAC), a functional source, or a dc
source can provide the control. VGAs are
available from dc to gigahertz frequencies
and in a variety of I/O configurations.
How is VGA gain controlled?
In analog VGAs, gain in dB is a linear
function of input voltage (Fig. 1a and 1b).
1. Input is
converted
to current
IINH
VGAs help tame signals that exhibit
wide dynamic range. Consider cellphone receivers. Depending on the distance between the basestation and cell
4. Sum of the exponential
ICEs a function of
IIN × IGAIN
Output
stage
IINL
Gain-control
conditioning
What applications use VGAs?
VGA
core
V–
2. Control signal is converted
to another current
a. (Translinear)
VOUT =
A(VIN × VGAIN)
5. Taking Log–1 of the
sum produces output
voltage equal to
product of input
voltages
3. When both currents are
applied to transistor junctions,
each ICE is an exponential
function of the input voltage
Gain control
voltage
1. Gain-control
voltage is
applied to
interpolator
Interpolator
Fixed-gain
amplifier
A
R
2. Interpolator steers
input-signal current
across precision
phone, their input signal levels range
from a few microvolts
to 1cvolts. Similar
0522FAQs-FIGURE
situations with wide dynamic range
are found in scientific, industrial, and
medical applications—such as the “front
ends” of measurement equipment and
ultrasound imaging, both for biological
diagnostics and industrial fault analysis.
Broadly speaking, VGAs are used in two
circumstances. The first encompasses
all those situations where the circuit
designer must match an input signal
level to the full-scale input of a device
such as an analog-to-digital converter
(ADC) or an FM discriminator. The
second addresses situations in which the
2. Translinear VGAs (a) are used where low cost is the
primary consideration. X-amp VGAs have advantages in
terms of noise and distortion characteristics and can be
trimmed for high accuracy.
V+
VINH
VINL
VGAIN
The datasheet will provide an intercept
point for 0-V input and a slope in terms
of dB/V on the VGAIN pin. With DVGAs,
a binary code or digital word applied
to a digital port or register controls the
gain. The word can be serial or parallel,
and it can operate like a register with an
address somewhere in the digital portion
of the system. In either case, the binary
input steps are weighted in dB (Fig. 1c).
2R
b. (Exponential)
0522FAQs-FIGURE 2b
3. Fixed-gain amp
conditions output
signal
a
d
v
e
r
t
i
s
e
m
e
n
t
designer must scale a fixed input voltage
to compensate for variable losses, for
example, to adjust the voltage level to a
transmission line. In these applications,
VGAs reduce bill-of-materials cost and
save space, but they also offer better
performance in terms of noise, distortion, and power consumption.
What kinds of internal architectures
do VGAs use?
Two approaches are in wide use: translinear and exponential amplifiers. The
translinear core uses the principles of
the diode equation, which expresses the
exponential relationship between junction current and base voltage in bipolar
devices (Fig. 2a). Exponential-amplifier
VGAs combine a precision-matched R-xR
followed by a fixed-gain amplifier (Fig.
2b). There are six to eight “rungs” on
the ladder, and the interpolator circuit
sweeps across that ladder in response to
the VGA’s control voltage.
What are some common real-world
applications for VGAs?
VGAs can be found in communications, cable TV, medical equipment,
and industrial applications. In medical
and industrial scanner applications, the
VGA is used in specialized circuits called
time gain controls (TGCs), which compensate for attenuation in the medium
being probed. In medical ultrasound
systems, echoes from structures deep in
the body must be amplified more than
echoes close to the skin.
In communications, VGAs function as
automatic gain control (AGC) amplifiers or as “output” VGAs that adjust
the voltage input to a cable system. The
device’s gain maintains signal integrity
and amplitude despite the length of the
cable or number of receivers attached.
What kinds of applications favor
analog or digital VGAs?
Two applications that favor analog
VGAs utilize time gain control: ultrasound scanners and phased-array radars.
Both involve a massive number of parallel amplifiers that all require a common
gain control. That’s easier to achieve
with a single analog control voltage
routed to each VGA.
On the other hand, consider a cable
TV system in which the level of the
upstream signal from the consumer’s
set-top box is adjusted for cable attenuation by a downstream signal from the
be a case for a digital VGA.
Digitally Programmable VGA with Transmit Driver
The AD8260, ideal for industrial and automotive cabling applications, includes a high-current
driver, usable as a transmitter, and a low-noise digitally programmable VGA, which is usable
as a receiver. The receiver consists of a single-ended input preamplifier and a linear-in-dB,
differential-output VGA. It has a gain span of 30 dB in 3-dB steps and a –3-dB bandwidth of
230 MHz. The driver-amplifier delivers ±300 mA on a 3.3-V supply, well suited for driving
low impedance loads. It is available in a 32-lead LFCSP and operates over –40°C to 105°C.
Versatile, Wide-Bandwidth, DC-Coupled VGA
low-noise, single-ended VGA usable
over a large range of supply voltages. The device provides 115-MHz
bandwidth over a gain range of 60
dB, with a slew rate of 550 V/ms for
a 2-V step. It is available in a 16-lead
LFCSP and operates over the industrial
temperature range of –55°C to 125°C.
Ultralow Distortion Digitally
Controlled Dual VGA
The AD8376 is the industry’s first
dual-channel, digitally controlled,
wide-bandwidth VGA that provides
precise gain control, high IP3, and low
noise. It achieves 50-dBm output IP3 at
200 MHz and provides a broad 24-dB gain range with 1-dB resolution. This highly integrated
solution replaces discrete circuits comprising digital attenuators and IF amplifiers while
consuming only 130 mA per channel.
It is available in a 32-lead LFCSP.
41-dB Range, 1-dB Step Size,
Programmable Dual VGA