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VCA824IDG4 Datasheet(PDF) 27 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
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VCA824IDG4 Datasheet(HTML) 27 Page - Texas Instruments

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background image
V +1
G
2
R
F
R
G
G=2 ´
´
-
£
0.8V
V
0.8V
G
£
G
2 R C
p
2
f =
8
R
F
1kW
+V
IN
R
G+
R
G-
-VIN
FB
R
G
200W
20W
V
OUT
VCA824
Out
V
G
OPA690
24pF
C
V
IN
R
332W
2
R
332W
1
24pF
50W
VCA824
www.ti.com
SBOS394D – NOVEMBER 2007 – REVISED JANUARY 2016
Figure 74. Voltage-Control Low-Pass Filter
The response control results from amplification of the feedback voltage applied to R2. First, consider the case
where the VCA824 produces G = 1V/V. Then this circuit performs as if the amplifier were replaced by a short
circuit. Visually replacing the amplifier by a short leaves a simple voltage-feedback amplifier with a feedback
resistor bypassed by a capacitor. Replacing this gain with a variable gain, G, the pole can be written as shown in
Equation 7.
(7)
Because the VCA824 is most linear in the midrange, the median of the adjustable pole should be set at VG = 0V
(see Figure 13, Figure 33, Figure 54, and Equation 8). Selecting R1 = R2 = 332Ω, and targeting a median
frequency of 10MHz, the capacitance (C) is 24pF. Because the OPA690 was selected for the circuit of Figure 74,
and in order to limit peaking in the OPA690 frequency response, a capacitor equal to C was added on the
inverting mode to ground. This architecture has the effect of setting the high-frequency noise gain of the OPA690
to 2V/V, ensuring stability and providing flat frequency response.
(8)
Once the median frequency is set, the maximum and minimum frequencies can be determined by using VG =
–0.8 V and VG = 0.8 V in the gain equation of Equation 9. Note that this is a first-order analysis and does not
take into consideration the open-loop gain limitation of the OPA690.
(9)
With the components shown, the circuit provides a linear variation of the low-pass cutoff from 2MHz to 20MHz,
using –1V
≤ VG ≤ 1V.
9.1.5 Wideband Variable Gain Amplifier Operation
The VCA824 provides an exceptional combination of high output power capability with a wideband, greater than
40dB gain adjust range, linear in V/V variable gain amplifier. The VCA824 input stage places the
transconductance element between two input buffers, using the output currents as the forward signal. As the
differential input voltage rises, a signal current is generated through the gain element. This current is then
mirrored and gained by a factor of two before reaching the multiplier. The other input of the multiplier is the
voltage gain control pin, VG. Depending on the voltage present on VG, up to two times the gain current is
provided to the transimpedance output stage. The transimpedance output stage is a current-feedback amplifier
providing high output current capability and high slew rate, 2500 V/
μs. This exceptional full-power performance
comes at the price of relatively high quiescent current (36.5 mA), but low input voltage noise for this type of
architecture (6 nV/
√Hz).
Copyright © 2007–2016, Texas Instruments Incorporated
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