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VCA824IDG4 Datasheet(PDF) 20 Page - Texas Instruments

Part No. VCA824IDG4
Description  Ultra-Wideband, > 40-dB Gain Adjust Range, Linear in V/V Variable Gain Amplifier
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com

VCA824IDG4 Datasheet(HTML) 20 Page - Texas Instruments

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Device Functional Modes (continued)
In the case of an application that does not make use of the VGAIN, but requires some other characteristic of the
VCA824, the RG resistor must be set such that the maximum current flowing through the resistance IRG is less
than ±2.6 mA typical, or 5.2mAPP as defined in Electrical Characteristics: VS = ±5 V, and must follow Equation 1.
As Equation 1 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 VCA824 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, Equation 2 illustrates
the tradeoff between the input voltage and the current flowing through the gain resistor.
8.4.2 Output Current And Voltage
The VCA824 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 1 V to either supply rails; the
25°C swing limit is within 1.2 V of either rails. Into a 15-
Ω load (the minimum tested load), the VCA824 device is
tested to deliver more than ±160 mA.
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 (See Figure 38). 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 VCA824
output drive capabilities, noting that the graph is bounded by a Safe Operating Area of 1-W maximum internal
power dissipation. Superimposing resistor load lines onto the plot shows that the VCA824 can drive ±2.5 V into
Ω or ±3.5 V into 50-Ω without exceeding the output capabilities or the 1-W dissipation limit. A 100-Ω load line
(the standard test circuit load) shows the full ±3.9-V output swing capability, as shown in Typical Characteristics.
The minimum specified output voltage and current overtemperature 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
Electrical Characteristic. 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 overtemperature specifications because the output stage junction temperatures are higher than the
specified operating ambient.
8.4.3 Input Voltage Dynamic Range
The VCA824 has a input dynamic range limited to 1.6 V and –2.1 V. Increasing the input voltage dynamic range
can be done by using an attenuator network on the input. If the VCA824 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 2.
As such, for unity-gain or under-attenuated conditions, the input voltage must be limited to the CMIR of ±1.6 V
(3.2 VPP) and the current (IRQ) must flow through the gain resistor, ±2.6 mA (5.2 mAPP). This configuration sets a
minimum value for RE such that the gain resistor must be greater than Equation 3.
Values lower than 615.4
Ω are gain elements that result in reduced input range, as the dynamic input range is
limited by the current flowing through the gain resistor RG (IRG). If the IRG current limits the performance of the
circuit, the input stage of the VCA824 goes into overdrive, resulting in limited output voltage range. Such IRG-
limited overdrive conditions are shown in Figure 40 for the gain of 10V/V and Figure 60 for the gain of 40 V/V.
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Product Folder Links: VCA824

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