THS7530 Datasheet, PDF (13/19 Pages) Texas Instruments – HIGH-SPEED, FULLY DIFFERENTIAL, CONTINUOUSLY VARIABLE GAIN AMPLIFIER

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THS7530
THEORY OF OPERATION
SLOS405A DECEMBER 2002– REVISED APRIL 2003
Figure 26 shows a simplified schematic of the THS7530.
The input architecture is a modified Gilbert Cell. The output from the Gilbert Cell is converted to a voltage and buffered
to the output as a fully-differential signal. A summing node between the outputs is used to compare the output
common-mode voltage to the V_OCM input. The V_OCM error amplifier then servos the output common-mode voltage
to maintain it equal to the V_OCM input. Left unterminated, V_OCM is set to midsupply by internal resistors.
The gain control input is conditioned to give linear in dB gain control (block H). The gain control input is a differential
signal from 0 V to 0.9 V which varies the gain from 11.6 dB to 46.5 dB.
V_CL+ and V_CL– provide inputs that limit the output voltage swing of the amplifier.
VCL+ VCL_–
VS+
x1
Output
Buffer
VOUT+
VIN+
VIN_–
PD
VS_–
VG+
VG_–
VOCM Error
Amplifier
Power
Control
H
THS7530
Figure 26. THS7530 Simplified Schematic
VOUT_–
VOCM
13