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VCA2618 Datasheet(PDF) 11 Page - Texas Instruments |
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VCA2618 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 30 page  +5V -5V V OUT V C 0 2V ® - - ® 40dB +40dB Gain VCA810 X1 2 3 V+ V- 1 6 7 5 8 Gain Adjust + VCA810 V C V I V O +5V 6 7 + + 2 R C R T 25W RL 500W 0.1 F m 6.8 F m 3 5 8 1 -5V 0.1 F m 6.8 F m R S 50W 50W Source G = (V/V) 10 -2 (V + 1) C VCA810 www.ti.com SBOS275F – JUNE 2003 – REVISED DECEMBER 2010 APPLICATION INFORMATION Thus, G(dB) varies linearly over the specified −40dB to CIRCUIT DESCRIPTION +40dB range as VC varies from 0V to −2V. Optionally, making VC slightly positive (≥ +0.15V) effectively The VCA810 is a high gain adjust range, wideband, disables the amplifier, giving greater than 80dB of voltage amplifier with a voltage-controlled gain, as signal path attenuation at low frequencies. shown in Figure 29. The circuit’s basic voltage amplifier responds to the control of an internal Internally, the gain-control circuit varies the amplifier gain-control amplifier. At its input, the voltage gain by varying the transconductance, gm, of a bipolar amplifier presents the high impedance of a differential transistor using the transistor bias current. Varying stage, permitting flexible input impedance matching. the bias currents of differential stages varies gm to To preserve termination options, no internal circuitry control the voltage gain of the VCA810. A gm-based connects to the input bases of this differential stage. gain adjust normally suffers poor thermal stability. For this reason, the user must provide dc paths for The VCA810 includes circuitry to minimize this effect. the input base currents from a signal source, either through a grounded termination resistor or by a direct VCA810 OPERATION connection to ground. The differential input stage also permits rejection of common-mode signals. At its Figure 30 shows the circuit configuration used as the output, the voltage amplifier presents a low basis of the Electrical Characteristics and Typical impedance, simplifying impedance matching. An Characteristics. Voltage swings reported in the open-loop design produces wide bandwidth at all gain specifications are taken directly at the input and settings. A ground-referenced differential to output pins. For test purposes, the input impedance is single-ended conversion at the output retains the low set to 50 Ω with a resistance to ground. A 25Ω output offset voltage. resistance (RT) is included on the V− input to get bias current cancellation. Proper supply bypassing is shown in Figure 30, and consists of two capacitors on each supply pin: one large electrolytic capacitor (2.2mF to 6.8mF), effective at lower frequencies, and one small ceramic capacitor (0.1mF) for high-frequency decoupling. For more information on decoupling, refer to the Board Layout section. Figure 29. Block Diagram of the VCA810 A gain control voltage, VC, controls the amplifier gain magnitude through a high-speed control circuit. Gain polarity can be either inverting or noninverting, depending upon the amplifier input driven by the input signal. The gain control circuit presents the high-input impedance of a noninverting op amp connection. The control voltage pin is referred to ground as shown in Figure 30. Variable Gain, Specification and Test Figure 29. The control voltage VC varies the amplifier Circuit gain according to the exponential relationship: Notice that both inverting and noninverting inputs are connected to ground with a resistor (RS and RT). This translates to the log gain relationship: Matching the dc source impedance looking out of G(dB) = –40 ● (VC + 1)dB. each input will minimize input offset voltage error. Copyright © 2003–2010, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Link(s): VCA810 |
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