Datasheet

MAXIMUM GAIN OF OPERATION
I =
RG
V
OUT
A R´
V
MAX G
(2)
INPUT VOLTAGE DYNAMIC RANGE
V =R I´
IN(PP) RG(PP)G
(3)
OUTPUT CURRENT AND VOLTAGE
R =
GMIN
=615.4W
3.2V
PP
5.2mA
PP
(4)
VCA821
SBOS407B – DECEMBER 2007 – REVISED DECEMBER 2008 ....................................................................................................................................... www.ti.com
VCA821 can drive ± 2.5V into 25 Ω or ± 3.5V into 50 Ω
without exceeding the output capabilities or the 1W
This section describes the use of the VCA821 in a
dissipation limit. A 100 Ω load line (the standard test
fixed-gain application in which the V
G
control pin is
circuit load) shows the full ± 3.9V output swing
set at V
G
= +1V. The tradeoffs described here are
capability, as shown in the Typical Characteristics .
with bandwidth, gain, and output voltage range.
The minimum specified output voltage and current
In the case of an application that does not make use
over-temperature are set by worst-case simulations at
of the V
GAIN
, but requires some other characteristic of
the cold temperature extreme. Only at cold startup do
the VCA821, the R
G
resistor must be set such that
the output current and voltage decrease to the
the maximum current flowing through the resistance
numbers shown in the Electrical Characteristics
I
RG
is less than ± 2.6mA typical, or 5.2mA
PP
as
tables. As the output transistors deliver power, the
defined in the Electrical Characteristics table, and
respective junction temperatures increase, thereby
must follow Equation 2 .
increasing the available output voltage swing and
output current.
In steady-state operation, the available output voltage
and current are always greater than the temperature
As Equation 2 illustrates, once the output dynamic
shown in the over-temperature specifications
range and maximum gain are defined, the gain
because the output stage junction temperatures are
resistor is set. This gain setting in turn affects the
higher than the specified operating ambient.
bandwidth, because in order to achieve the gain (and
with a set gain element), the feedback element of the
output stage amplifier is set as well. Keeping in mind
that the output amplifier of the VCA821 is a
The VCA821 has a input dynamic range limited to
current-feedback amplifier, the larger the feedback
+1.6V and – 2.1V. Increasing the input voltage
element, the lower the bandwidth because the
dynamic range can be done by using an attenuator
feedback resistor is the compensation element.
network on the input. If the VCA821 is trying to
regulate the amplitude at the output, such as in an
Limiting the discussion to the input voltage only and
AGC application, the input voltage dynamic range is
ignoring the output voltage and gain, Figure 1
directly proportional to Equation 3 .
illustrates the tradeoff between the input voltage and
the current flowing through the gain resistor.
As such, for unity-gain or under-attenuated
conditions, the input voltage must be limited to the
The VCA821 provides output voltage and current
CMIR of ± 1.6V (3.2V
PP
) and the current (I
RQ
) must
capabilities that are unsurpassed in a low-cost
flow through the gain resistor, ± 2.6mA (5.2mA
PP
).
monolithic VCA. Under no-load conditions at +25 ° C,
This configuration sets a minimum value for R
E
such
the output voltage typically swings closer than 1V to
that the gain resistor must be greater than
either supply rails; the +25 ° C swing limit is within
Equation 4 .
1.2V of either rails. Into a 15 Ω load (the minimum
tested load), it is tested to deliver more than ± 90mA.
The specifications described above, though familiar in
the industry, consider voltage and current limits
Values lower than 615.4 Ω are gain elements that
separately. In many applications, it is the voltage ×
result in reduced input range, as the dynamic input
current, or V-I product, that is more relevant to circuit
range is limited by the current flowing through the
operation. Refer to the Output Voltage and Current
gain resistor R
G
(I
RG
). If the I
RG
current limits the
Limitations plot (Figure 48 ) in the Typical
performance of the circuit, the input stage of the
Characteristics. The X- and Y-axes of this graph
VCA821 goes into overdrive, resulting in limited
show the zero-voltage output current limit and the
output voltage range. Such I
RG
-limited overdrive
zero-current output voltage limit, respectively. The
conditions are shown in Figure 50 for the gain of
four quadrants give a more detailed view of the
20dB and Figure 70 for the 32dB gain.
VCA821 output drive capabilities, noting that the
graph is bounded by a Safe Operating Area of 1W
maximum internal power dissipation. Superimposing
resistor load lines onto the plot shows that the
24 Submit Documentation Feedback Copyright © 2007 – 2008, Texas Instruments Incorporated
Product Folder Link(s): VCA821