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VCA824IDG4 Datasheet(PDF) 27 Page  Texas Instruments 

VCA824IDG4 Datasheet(HTML) 27 Page  Texas Instruments 
27 / 41 page 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. VoltageControl LowPass 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 voltagefeedback 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 highfrequency 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 firstorder analysis and does not take into consideration the openloop gain limitation of the OPA690. (9) With the components shown, the circuit provides a linear variation of the lowpass 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 currentfeedback amplifier providing high output current capability and high slew rate, 2500 V/ μs. This exceptional fullpower 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 Submit Documentation Feedback 27 Product Folder Links: VCA824 
