Detailed Description

The MAX3760 is a transimpedance amplifier designed

for 622Mbps fiber optic applications. As shown in the

Functional Diagram (Figure 1), it comprises a transim-

pedance amplifier, a paraphase amplifier with emitter-

follower outputs, and a DC cancellation circuit.

Transimpedance Amplifier

The signal current at the input flows into a high-gain

amplifier’s summing node. Shunt feedback through RF

converts this current to a voltage with 6.5k

Ω gain.

Diode D1 clamps the output voltage for large input cur-

rents. INREF is a direct connection to the input transis-

tor’s emitter, and must be connected directly to the

photodetector AC ground return for best performance.

Paraphase Amplifier

The paraphase amplifier converts single-ended signals to

differential signals and introduces a voltage gain of 2.

This signal drives a pair of internally biased emitter follow-

ers, Q2 and Q3, which form the output stage. Resistors

R1 and R2 provide back-termination at the output, deliv-

ering a 100

Ω differential output impedance. The output

emitter followers are designed to drive a 100

Ω differential

load between OUT+ and OUT-. The MAX3760 can also

be terminated with higher output impedances for

increased gain and output voltage swing.

The MAX3760

will not drive a 50

Ω load to ground. For best noise rejec-

tion, terminate the MAX3760 with differential loads.

DC Cancellation Circuit

The DC cancellation circuit removes the input signal’s

DC component by employing low-frequency feedback.

This feature centers the input signal within the transim-

pedance amplifier’s dynamic range, thereby reducing

pulse-width distortion on large input signals.

The paraphase amplifier’s output is sensed through

resistors R3 and R4, then filtered, amplified, and fed

back to the base of transistor Q4. The transistor draws

the input signal’s DC component away from the trans-

impedance amplifier’s summing node.

The MAX3760 DC cancellation loop is internally com-

pensated and does not require external capacitors in

most 622Mbps applications. Add external capacitance

at the COMP pin to reduce the DC cancellation circuit’s

frequency response and improve data-dependent jitter.

Connecting the COMP pin directly to GND disables the

circuit. The DC cancellation circuit can sink up to 1mA

at the input.

622Mbps, Low-Noise Transimpedance

Preamplifier for LAN and WAN Optical Receivers

_______________________________________________________________________________________

5

MAX3760

IN

Q4

Q1

Q3

R2

R1

1k

Q2

RF

D1

R4

R3

(FILTER)

OUT-

OUT+

GND

COMP

DC

CANCELLATION

AMPLIFIER

TRANSIMPEDANCE

AMPLIFIER

PARAPHASE

AMP

GND

INREF

( ) INDICATE PINS AVAILABLE ON DIE ONLY.

Figure 1. Functional Diagram