M02011

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20

CMOS Transimpedance Amplifier with AGC

for Fiber Optic Networks up to 622 Mbps

Rev V5

In the positive direction after about 700 mV, the impedance of the input is relatively high. After the TIA is turned on,

the DC servo and AGC circuits attempt to null any input currents (up to the absolute maximum stated in Table 1-1)

as shown by the I/V curve in Figure 4-6.

It can be seen that any negative voltage below 200 mV is nulled and that any positive going voltage above the

PINA standing voltage is nulled by the DC servo. The DC servo upper bandwidth varies from part to part, but is

generally at least 30 kHz.

When externally biasing a detector such as an APD where the supply voltage of the APD exceeds that for PINA

Table 1-1, care should be taken to power up the TIA first and to keep the TIA powered up until after the power

supply voltage of the APD is removed. Failure to do this with the TIA unpowered may result in damage to the input

FET gate at PINA. In some cases the damage may be very subtle, in that nearly normal operation may be

experienced with the damage causing slight reductions in bandwidth and corresponding reductions in input

sensitivity.

Figure 4-6.

TIA Use with Externally Biased Detectors, Powered On

PINA biased

-1000

-800

-600

-400

-200

0

200

400

600

800

1000

-300

-200

-100

0

100

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500

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700

mV