Many transimpedance applications can have unpredicted, large input currents that can cause the amplifier's

output to saturate. It is often important to understand how the amplifier will behave when its output is saturated

at various levels of overdrive. A typical linear amplifier like the OPA856 can be expected to have an output

saturation recovery time that increases as the amount of output overdrive increases. When using a pulse based

input signal, the output saturation recovery time effectively extends the duration of the pulse. Figure 9-3 shows

the test setup configuration to measure a pulsed optical input to the OPA856. The optical modulator used in the

test setup had limited output amplitude capability to create a saturated signal. In order to prevent the modulator

from saturating, the OPA856 output was saturated by adjusting VOFFSET close to the the amplifier's output

swing limit on the negative rail.

-

- -

+

±

1 NŸ

5V

ON

OFF

1 pF

169

71.5

1330 nm

Optical

Modulator

Tunable Laser

Oscilloscope

OPA856

200-Ÿ ORDG PDWFKHG

to 50-Ÿ LQSXW

Fiber connected

photodiode

1V

Pulse Generator

+

±

VOFFSET

Figure 9-3. OPA856 Transimpedance Pulse Saturation Extention Test Setup

Figure 9-5 shows the resulting shifted output pulses labelled by the magnitude of voltage they are overdriving

level of the signal because of expected variances from the optical interface setup. Figure 9-6 shows a magnified

view of the rising edge of the measured pulse responses in order to better detail the pulse extension created by

the signal overdrive. These plots have been scaled and normalized to be easier to read and compare. As

expected, Figure 9-6 shows that the pulse duration is extended as the overdrive level increases. The data only

captured an overdrive voltage maximum of 640 mV, but it can be expected that larger voltages would extend the

pulse further.

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OPA856

SBOS623 – OCTOBER 2020

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