Datasheet

APPLICATION INFORMATION
WIDEBAND VARIABLE GAIN AMPLIFIER
0.1 Fm
X2Y
Capacitor
(seedetail)
â
+V
S
-V
S
G
1
G
2
A
B
X2Y CapacitorDetail
®
SO-14
VCA821
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
2.2 Fm2.2 Fm
+
+
20W
20W
20W
V
OUT
+V
IN
VCA821
www.ti.com ....................................................................................................................................... SBOS407B – DECEMBER 2007 – REVISED DECEMBER 2008
For test purposes, the input impedance is set to 50 Ω
OPERATION with a resistor to ground and the output impedance is
set to 50 Ω with a series output resistor. Voltage
The VCA821 provides an exceptional combination of
swings reported in the Electrical Characteristics table
high output power capability with a wideband, greater
are taken directly at the input and output pins, while
than 40dB gain adjust range, linear in dB variable
output power (dBm) is at the matched 50 Ω load. For
gain amplifier. The VCA821 input stage places the
the circuit in Figure 74 , the total effective load is
transconductance element between two input buffers,
100 Ω 1k Ω . Note that for the SO-14 package, there
using the output currents as the forward signal. As
is a voltage reference pin, V
REF
(pin 9). For the
the differential input voltage rises, a signal current is
SO-14 package, this pin must be connected to
generated through the gain element. This current is
ground through a 20 Ω resistor in order to avoid
then mirrored and gained by a factor of two before
possible oscillations of the output stage. In the
reaching the multiplier. The other input of the
MSOP-10 package, this pin is internally connected
multiplier is the voltage gain control pin, V
G
.
and does not require such precaution. An X2Y™
Depending on the voltage present on V
G
, up to two
capacitor has been used for power-supply bypassing.
times the gain current is provided to the
The combination of low inductance, high resonance
transimpedance output stage. The transimpedance
frequency, and integration of three capacitors in one
output stage is a current-feedback amplifier providing
package (two capacitors to ground and one across
high output current capability and high slew rate,
the supplies) enables the VCA821 to achieve the low
2500V/ µ s. This exceptional full-power performance
second-harmonic distortion reported in the Electrical
comes at the price of relatively high quiescent current
Characteristics table. More information on how the
(34mA), but low input voltage noise for this type of
VCA821 operates can be found in the Operating
architecture (6nV/ √ Hz).
Suggestions section.
Figure 74 shows the dc-coupled, gain of +10V/V, dual
power-supply circuit used as the basis of the ± 5V
Electrical Characteristics and Typical Characteristics .
Figure 74. DC-Coupled, A
VMAX
= 20dB, Bipolar Supply Specification and Test Circuit
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