1GHz Transimpedance Preamplifier

with 25dB Dynamic Range

_______________________________________________________________________________________

3

EYE DIAGRAM

1Gbps

2.0905V

1.0905V

37.98ns

250ps/div

40.48ns

14

0

SUPPLY CURRENT vs. TEMPERATURE

(OUTPUT AC COUPLED)

TEMPERATURE (

°C)

16

18

20

22

24

26

28

30

20

40

60

80

100

5.25V

5.0V

4.75V

____________________________Typical Operating Characteristics (continued)

_______________Detailed Description

The MAX3260 is a high-speed transimpedance amplifi-

er, designed to accomodate input currents with a large

dynamic range. This circuit is optimized to operate in a

1062Mbps Fibre Channel reciever, and is also suitable

for use in 622Mbps SONET applications.

The MAX3260 employs shunt-shunt feedback around a

bipolar amplifier. The resulting circuit provides an

inverted current-to-voltage conversion. The conversion

gain is nominally -2000V/A.

Modern fiber-optic communications systems place

many requirements on transimpedance preamplifiers.

Power budgets are important when considering a pre-

amp, since the circuit will generally be placed into a

small module or header, which limits the amount of heat

dissipation. In addition, the signal presented to the pre-

amp may carry an 18dB to 20dB dynamic range, which

must be amplified linearly to prevent the addition of jit-

ter. Finally, preamp noise generally determines the

receiver sensitivity, and must be held to a minimum.

The MAX3260 employs several techniques to address

the needs of fiber-optic preamplifiers. The output of the

MAX3260 operates in the 1V to 2V range (depending

on the average input) to keep output standing current

at a minimum. The reduced output voltage helps keep

power consumption low, but also reduces the dynamic

range of the output stage. Fiber communications tran-

simpedance amplifiers commonly use dynamic control

of the shunt-shunt feedback loop to vary the gain. The

unwanted side-effect of this technique is a circuit band-

width that varies with input current. To prevent output-

stage saturation, the MAX3260 employs a DC-restore

circuit. As input signal power increases, DC current is

drawn away from the input node of the amplifier. This

reduces the DC gain of the amplifier without affecting

the small-signal performance. To prevent noise feed-

back at low signals, an integrated comparator senses

the power level and disables the DC-restore function.

The MAX3260’s filter output provides a convenient volt-

age source for a photodiode, especially when the circuit

will be placed into a TO-style header with the photodi-

on-chip 1k

Ω resistor. In combination with a bypass

capacitor, the filter function reduces the bandwidth at

the anode of the photodiode, therefore significantly

reducing the amount of noise at the cathode. The filter

connection can be left unconnected if not used.