Datasheet

VCA820
x1
x2
R
200
G
W
R
G+
V
IN
x1
R
G-
-V
IN
I
RG
V
G
V
OUT
FB
R
F
1kW
V
REF
+5V -5V
0.1 Fm
X2Y Cap
@
2.2 Fm2.2 Fm
+
+
20W
20W
20W
V
OUT
+V
IN
VCA820
www.ti.com ............................................................................................................................................ SBOS395C –OCTOBER 2007–REVISED OCTOBER 2009
APPLICATION INFORMATION
For test purposes, the input impedance is set to 50Ω
with a resistor to ground and the output impedance is
WIDEBAND VARIABLE GAIN AMPLIFIER
set to 50Ω with a series output resistor. Voltage
OPERATION
swings reported in the Electrical Characteristics table
The VCA820 provides an exceptional combination of
are taken directly at the input and output pins, while
high output power capability with a wideband, greater
output power (dBm) is at the matched 50Ω load. For
than 40dB gain adjust range, linear in dB variable
the circuit in Figure 76, the total effective load is
gain amplifier. The VCA820 input stage places the
100Ω 1kΩ. Note that for the SO-14 package, there
transconductance element between two input buffers,
is a voltage reference pin, V
REF
(pin 9). For the
using the output currents as the forward signal. As
SO-14 package, this pin must be connected to
the differential input voltage rises, a signal current is
ground through a 20Ω resistor in order to avoid
generated through the gain element. This current is
possible oscillations of the output stage. In the
then mirrored and gained by a factor of two before
MSOP-10 package, this pin is internally connected
reaching the multiplier. The other input of the
and does not require such precaution. An X2Y™
multiplier is the voltage gain control pin, V
G
.
capacitor has been used for power-supply bypassing.
Depending on the voltage present on V
G
, up to two
The combination of low inductance, high resonance
times the gain current is provided to the
frequency, and integration of three capacitors in one
transimpedance output stage. The transimpedance
package (two capacitors to ground and one across
output stage is a current-feedback amplifier providing
the supplies) of this capacitor enables to achieve the
high output current capability and high slew rate,
low second-harmonic distortion reported in the
1700V/μs. This exceptional full-power performance
Electrical Characteristics table. More information on
comes at the price of a relatively high quiescent
how the VCA820 operates can be found in the
current (34mA), but a low input voltage noise for this
Operating Suggestions section.
type of architecture (8.2nV/√Hz).
Figure 76 shows the dc-coupled, gain of 20dB, dual
power-supply circuit used as the basis of the ±5V
Electrical Characteristics and Typical Characteristics.
Figure 76. DC-Coupled, A
VMAX
= 20dB, Bipolar Supply Specification and Test Circuit
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