Datasheet

MAXIMUM GAIN OF OPERATION
I =
RG
V
OUT
A R´
V
MAX G
(8)
INPUT VOLTAGE DYNAMIC RANGE
V =R I´
IN(PP) RG(PP)G
(9)
OUTPUT CURRENT AND VOLTAGE
R =
GMIN
=615.4W
3.2V
PP
5.2mA
PP
(10)
VCA824
www.ti.com ....................................................................................................................................... SBOS394C – NOVEMBER 2007 – REVISED DECEMBER 2008
VCA824 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 VCA824 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 VCA824, 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 Characteristic tables.
I
RG
is less than ± 2.6mA typical, or 5.2mA
PP
as
As the output transistors deliver power, the respective
defined in the Electrical Characteristics table, and
junction temperatures increase, thereby increasing
must follow Equation 8 .
the available output voltage swing and output current.
In steady-state operation, the available output voltage
and current are always greater than the temperature
shown in the over-temperature specifications
As Equation 8 illustrates, once the output dynamic
because the output stage junction temperatures are
range and maximum gain are defined, the gain
higher than the specified operating ambient.
resistor is set. This gain setting in turn affects the
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
The VCA824 has a input dynamic range limited to
that the output amplifier of the VCA824 is a
+1.6V and – 2.1V. Increasing the input voltage
current-feedback amplifier, the larger the feedback
dynamic range can be done by using an attenuator
element, the lower the bandwidth because the
network on the input. If the VCA824 is trying to
feedback resistor is the compensation element.
regulate the amplitude at the output, such as in an
AGC application, the input voltage dynamic range is
Limiting the discussion to the input voltage only and
directly proportional to Equation 9 .
ignoring the output voltage and gain, Figure 1
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
CMIR of ± 1.6V (3.2V
PP
) and the current (I
RQ
) must
The VCA824 provides output voltage and current
flow through the gain resistor, ± 2.6mA (5.2mA
PP
).
capabilities that are unsurpassed in a low-cost
This configuration sets a minimum value for R
E
such
monolithic VCA. Under no-load conditions at +25 ° C,
that the gain resistor must be greater than
the output voltage typically swings closer than 1V to
Equation 10 .
either supply rails; the +25 ° C swing limit is within
1.2V of either rails. Into a 15 Ω load (the minimum
tested load), it is tested to deliver more than ± 160mA.
The specifications described above, though familiar in
Values lower than 615.4 Ω are gain elements that
the industry, consider voltage and current limits
result in reduced input range, as the dynamic input
separately. In many applications, it is the voltage ×
range is limited by the current flowing through the
current, or V-I product, that is more relevant to circuit
gain resistor R
G
(I
RG
). If the I
RG
current limits the
operation. Refer to the Output Voltage and Current
performance of the circuit, the input stage of the
Limitations plot (Figure 49 ) in the Typical
VCA824 goes into overdrive, resulting in limited
Characteristics. The X- and Y-axes of this graph
output voltage range. Such I
RG
-limited overdrive
show the zero-voltage output current limit and the
conditions are shown in Figure 51 for the gain of
zero-current output voltage limit, respectively. The
+10V/V and Figure 71 for the +40V/V gain.
four quadrants give a more detailed view of the
VCA824 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
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