Datasheet AD811 (Analog Devices) - 15

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Data Sheet AD811 AN 80 MHZ VOLTAGE-CONTROLLED AMPLIFIER
The gain can be increased to 20 dB (×10) by raising R8 and R9
CIRCUIT
to 1.27 kΩ, with a corresponding decrease in −3 dB bandwidth to approximately 25 MHz. The maximum output voltage under The voltage-controlled amplifier (VCA) circuit of Figure 46 shows these conditions is increased to ±9 V using ±12 V supplies. the AD811 being used with the AD834, a 500 MHz, 4-quadrant multiplier. The AD834 multiplies the signal input by the dc control The gain-control input voltage, VG, may be a positive or negative voltage, VG. The AD834 outputs are in the form of differential ground-referenced voltage, or fully differential, depending on currents from a pair of open collectors, ensuring that the full the choice of connections at Pin 7 and Pin 8. A positive value of bandwidth of the multiplier (which exceeds 500 MHz) is VG results in an overall noninverting response. Reversing the sign available for certain applications. Here, the AD811 op amp of VG simply causes the sign of the overall response to invert. In provides a buffered, single-ended, ground-referenced output. fact, although this circuit has been classified as a voltage-controlled Using feedback resistors R8 and R9 of 511 Ω, the overall gain amplifier, it is also quite useful as a general-purpose, four-quadrant ranges from −70 dB for VG = 0 V to +12 V (a numerical gain of multiplier, with good load driving capabilities and fully +4) when VG = 1 V. The overall transfer function of the VCA is symmetrical responses from the X and Y inputs. VOUT = 4 (X1 − X2)(Y1 − Y2), which reduces to VOUT = 4 VG VIN The AD811 and AD834 can both be operated from power supply using the labeling conventions shown in Figure 46. The circuit’s voltages of ±5 V. While it is not necessary to power them from −3 dB bandwidth of 80 MHz is maintained essentially constant— the same supplies, the common-mode voltage at W1 and W2 that is, independent of gain. The response can be maintained must be biased within the common-mode range of the input flat to within ±0.1 dB from dc to 40 MHz at full gain with the stage of the AD811. To achieve the lowest differential gain and addition of an optional capacitor of about 0.3 pF across the phase errors, it is recommended that the AD811 be operated feedback resistor R8. The circuit produces a full-scale output of from power supply voltages of ±10 V or greater. This VCA ±4 V for a ±1 V input and can drive a reverse-terminated load circuit operates from a ±12 V dual power supply. of 50 Ω or 75 Ω to ±2 V.
FB +12V C1 0.1

F + R1 100 R8* VG – R2 100 8 7 6 5 R4 R6 182 294 X2 X1 +VS W1 7 2 – U1 U3 AD834 6 VOUT AD811 3 + 4 Y1 Y2 –VS W2 R5 R7 R 1 2 3 4 182 294 L VIN R9* R3 249 C2 0.1

F FB –12V
-047
*R8 = R9 = 511 FOR

4 GAIN R8 = R9 = 1.27k FOR

10 GAIN
00866-E Figure 46. An 80 MHz Voltage-Controlled Amplifier Rev. G | Page 15 of 20 Document Outline FEATURES APPLICATIONS CONNECTION DIAGRAMS GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS MAXIMUM POWER DISSIPATION METALIZATION PHOTOGRAPH ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS APPLICATIONS INFORMATION GENERAL DESIGN CONSIDERATIONS ACHIEVING THE FLATTEST GAIN RESPONSE AT HIGH FREQUENCY Choice of Feedback and Gain Resistors Printed Circuit Board Layout Considerations Quality of Coaxial Cable Power Supply Bypassing Driving Capacitive Loads OPERATION AS A VIDEO LINE DRIVER AN 80 MHZ VOLTAGE-CONTROLLED AMPLIFIER CIRCUIT A VIDEO KEYER CIRCUIT OUTLINE DIMENSIONS ORDERING GUIDE