1.25Gbps/2.5Gbps, +3V to +5.5V, Low-Noise

Transimpedance Preamplifiers for LANs

6

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Detailed Description

The MAX3266 is a transimpedance amplifier designed

for 1.25Gbps fiber optic applications. Figure 2 is a func-

tional diagram of the MAX3266, which comprises a trans-

impedance amplifier, a voltage amplifier, an output

buffer, an output filter, and a DC cancellation circuit.

The MAX3267, a transimpedance amplifier designed

for 2.5Gbps fiber optic applications, shares similar

architecture with the MAX3266.

Transimpedance Amplifier

The signal current at the input flows into the summing

node of a high-gain amplifier. Shunt feedback through

imately 2.2k

Ω (1.0kΩ for MAX3267). Schottky diodes

clamp the output voltage for large input currents, as

shown in Figure 3.

Voltage Amplifier

The voltage amplifier converts single-ended signals to

differential signals and introduces a voltage gain.

Output Buffer

The output buffer provides a reverse-terminated volt-

age output. The buffer is designed to drive a 100

Ω dif-

ferential load between OUT+ and OUT-. The output

current is divided between internal 50

Ω load resistors

and the external load resistor. In the typical operating

circuit, this creates a voltage-divider with gain of 1/2.

The MAX3266 can also be terminated with higher out-

put impedances, which increases gain and output volt-

age swing.

For optimum supply-noise rejection, the MAX3266

should be terminated with a differential load. If a single-

ended output is required, the unused output should be

similarly terminated. The MAX3266 will not drive a DC-

coupled, 50

Ω grounded load.

VOLTAGE

AMPLIFIER

TRANSIMPEDANCE

AMPLIFIER

OUTPUT

BUFFER

OUTPUT

FILTER

OUT+

50

Ω

50

Ω

OUT-

LOWPASS FILTER

1.5k

FILTER

IN

GND

DC CANCELLATION

CIRCUIT

DISABLE

MAX3266

Figure 2. MAX3266 Functional Diagram

0.50

0.95

0.80

0.65

1.10

1.25

1.40

1.55

1.70

1.85

2.00

02

1

345

REVERSE BIAS (V)

Figure 1. Typical Photodiode Capacitance vs. Bias Voltage