M02006 Datasheet, PDF (8/16 Pages) M/A-COM Technology Solutions, Inc. – CMOS Pre-amplifier with AGC for Long-reach 155 Mbps Fiber-optics Based Transmitter

M02006
CMOS Pre-amplifier with AGC for Long-reach
155 Mbps Fiber-optics Based Transmitter
Rev V4
3.2
TZA
The transimpedance amplifier consists of a high gain single-ended CMOS amplifier, with a feedback resistor. The
feedback creates a virtual low impedance at the input and nearly all of the input current passes through the
feedback resistor, defining the voltage at the output. Advanced CMOS design techniques are employed to maintain
the stability of this stage across all input conditions.
Single-ended amplifiers have inherently poor power supply noise rejection. For this reason, an on-chip low dropout
linear regulator has been incorporated into the design to give excellent noise rejection up to several MHz. Higher
frequency power supply noise is removed by external decoupling.
The circuit is designed for PIN photodiodes in the grounded cathode configuration, with the anode connected to the
input of the TZA and the cathode connected to AC ground. Reverse dc bias is applied via PINK to reduce the
photodiode capacitance.
3.3
AGC
The M02006 has been designed to operate over the input range of +3 dBm to –39 dBm at long wavelengths.
This represents a ratio of 1:12500, whereas the acceptable dynamic range of the output is only 1:250 which implies
a compression of 50:1 in the transimpedance.
The design uses a MOS transistor configured as a voltage controlled resistor to achieve transimpedance variation,
which allows the device to maintain a relatively constant output amplitude over more than 30db of optical input
power.
3.4
Output Stage
The signal from the TZA enters a phase splitter and a pair of voltage follower outputs. These are designed to drive
a high impedance (>500 Ω) load. They are stable for driving capacitive loads, such as interstage filters.
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