F0321818B
2 GHz Limiting Amplifier
♦ General Description
A post-amplifier is positioned between a preamplifier (an amplifier for a faint photocurrent
from PIN photo diode) and a decision circuit (a circuit to discriminate the logic level of the
received signal), enlarging the output signal from the preamplifier to a higher level to dis-
criminate the logic level. The input signal amplitude of the post-amplifier, meaning the output
signal of the preamplifier, varies widely, because the optical signal power received by the
PIN photo diode depends on the length of the transmission line. Therefore, the post-ampli-
fier should function to output an almost constant signal level under widely varying input volt-
age. This is called a limiting function, and the F0321818B provides excellent limiting charac-
teristics. As shown in the data sheet, the increase of only 200 mV (400 mV→600 mV) in the
output voltage can be observed even if the 2.5 Gps input signal varies widely from 50 mV to
500 mV
Wide use analog IC’s having satisfactory limiting functions as described above can not
be found except for the F0321818B. Customized IC’s for each application or a circuit de-
signed by discrete transistors are believed to have been developed.
♦ Post Matching
Input/output VSWR’s of the F0321818B are well-designed for 50 Ω, typically showing
excellent VSWR’s of 1.1:1. Therefore, the F0321818B can be applied for 50 Ω systems with
no external parts. Furthermore, the fine VSWR characteristics provide stable operation even
for cascade connections of two IC’s for a higher gain. The excellent VSWR characteristics of
the F0321818B gives full play in a clock recovery system using SAW filters, because the
impedance mismatch has a significant effect on the quality of the recovered clock signal.
♦ Gain Consideration
The F0321818B has a small signal gain of 20 dB. A too high gain can be harmful because of
parasitic oscillation. If a slightly higher gain is needed, a 6 dB higher gain of 26 dB can be
obtained by a high impedance termination instead of a 50 Ω load. A double gain can be
achieved by simple cascade connection if a still higher gain is required.
EUDYNA [ EUDYNA DEVICES INC ]
