VCA821_12 Datasheet, PDF (24/37 Pages) Texas Instruments – Ultra-Wideband, > 40dB Gain Adjust Range, Linear in dB VARIABLE GAIN AMPLIFIER

VCA821
SBOS407B – DECEMBER 2007 – REVISED DECEMBER 2008 ....................................................................................................................................... www.ti.com
MAXIMUM GAIN OF OPERATION
This section describes the use of the VCA821 in a
fixed-gain application in which the V_G control pin is
set at V_G = +1V. The tradeoffs described here are
with bandwidth, gain, and output voltage range.
In the case of an application that does not make use
of the V_GAIN, but requires some other characteristic of
the VCA821, the R_G resistor must be set such that
the maximum current flowing through the resistance
I_RG is less than ±2.6mA typical, or 5.2mA_PP as
defined in the Electrical Characteristics table, and
must follow Equation 2.
I_RG =
V_OUT
A_VMAX ´ R_G
(2)
As Equation 2 illustrates, once the output dynamic
range and maximum gain are defined, the gain
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
that the output amplifier of the VCA821 is a
current-feedback amplifier, the larger the feedback
element, the lower the bandwidth because the
feedback resistor is the compensation element.
Limiting the discussion to the input voltage only and
ignoring the output voltage and gain, Figure 1
illustrates the tradeoff between the input voltage and
the current flowing through the gain resistor.
OUTPUT CURRENT AND VOLTAGE
The VCA821 provides output voltage and current
capabilities that are unsurpassed in a low-cost
monolithic VCA. Under no-load conditions at +25°C,
the output voltage typically swings closer than 1V to
either supply rails; the +25°C swing limit is within
1.2V of either rails. Into a 15Ω 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
separately. In many applications, it is the voltage ×
current, or V-I product, that is more relevant to circuit
operation. Refer to the Output Voltage and Current
Limitations plot (Figure 48) in the Typical
Characteristics. The X- and Y-axes of this graph
show the zero-voltage output current limit and the
zero-current output voltage limit, respectively. The
four quadrants give a more detailed view of the
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
VCA821 can drive ±2.5V into 25Ω or ±3.5V into 50Ω
without exceeding the output capabilities or the 1W
dissipation limit. A 100Ω load line (the standard test
circuit load) shows the full ±3.9V output swing
capability, as shown in the Typical Characteristics.
The minimum specified output voltage and current
over-temperature are set by worst-case simulations at
the cold temperature extreme. Only at cold startup do
the output current and voltage decrease to the
numbers shown in the Electrical Characteristics
tables. As the output transistors deliver power, the
respective junction temperatures increase, thereby
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
shown in the over-temperature specifications
because the output stage junction temperatures are
higher than the specified operating ambient.
INPUT VOLTAGE DYNAMIC RANGE
The VCA821 has a input dynamic range limited to
+1.6V and –2.1V. Increasing the input voltage
dynamic range can be done by using an attenuator
network on the input. If the VCA821 is trying to
regulate the amplitude at the output, such as in an
AGC application, the input voltage dynamic range is
directly proportional to Equation 3.
V_IN(PP) = R_G ´ I_RG(PP)
(3)
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
flow through the gain resistor, ±2.6mA (5.2mA_PP).
This configuration sets a minimum value for R_E such
that the gain resistor must be greater than
Equation 4.
R_GMIN =
3.2V_PP
5.2mA_PP
= 615.4W
(4)
Values lower than 615.4Ω are gain elements that
result in reduced input range, as the dynamic input
range is limited by the current flowing through the
gain resistor R_G (I_RG). If the I_RG current limits the
performance of the circuit, the input stage of the
VCA821 goes into overdrive, resulting in limited
output voltage range. Such I_RG-limited overdrive
conditions are shown in Figure 50 for the gain of
20dB and Figure 70 for the 32dB gain.
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