capacitance C
P H
at the corresponding reverse voltage. The
test setup is shown in Fig. 11.
Signal Generator
HP 8648A
R
PH
TIA
R
F
C
F
C
PH
Oscilloscope
Tektronix TDS2024B
CH A CH B
Photodiode
Thevenin model
Fig. 11. Test setup for wideband TIA characterization. R
F
and C
F
are
the feedback elements of the operational amplifier.
In Fig.12 we show the measured transfer function of two
implemented TIAs. One TIA uses the OPA657 operational
amplifier from Texas Instruments [19] and the other one
uses the LTC6268-10 from Linear Technology [25]. The
photodiode parameters are R
P H
= 22 kΩ and C
P H
= 2
pF in both cases.
10
6
10
7
10
8
79
79.5
80
80.5
81
81.5
82
Frequency [Hz]
Magnitude [dB]
Phase [rad]
Frequency [Hz]
Magnitude [dB]
Phase [rad]
Frequency [Hz] Frequency [Hz]
OPA657 - RF: 10KΩ - CF: 300 fF LTC6268-10 - RF: 22KΩ - CF: 100 fF
Fig. 12. Transfer function measurement of developed TIAs.
The measured −3 dB frequency in the first TIA is 140
MHz and has a linear phase response up to 20 MHz. In
the second case, the −3 dB frequency is 320 MHz approx-
imately and has a linear phase response up to 40 MHz.
The LTC6268-10 has a broader frequency response and has
less noise than the OPA657. These characteristics arise from
the fact that the LTC6268-10 has less input capacitance.
However, the OPA657 has a flatter magnitude response. The
peak observed around 160 MHz in the transfer function of
the LTC6268-10 is also seen in [25]. An implementation of
the photodiode TIA using the OPA657 is shown on Fig.13.
Fig. 13. Implementation of a photodiode transimpedance amplifier using
an OPA657 operational amplifier.
A metal enclosure isolates the circuit from external noise
sources. The photodiode is coupled to a multimode fiber by
an ST connector. The output connector of the TIA is a 50Ω
male SMA connector.
V. CONCLUSIONS
We described the design and performance of three tran-
simpedance amplifiers optimized for optical dynamic inter-
ferometry. In the first case, the most significant charace-
teristics of the detector arise from the photodiode param-
eters. In the second case, we present the typical topology
of the operational amplifier TIA, for medium frequency
applications. Moreover, we present an approach to balanced
detection to improve the quality of the measurement and
the resolution of the entire interferometric scheme. Finally,
we present a wideband TIA implementation for heterodyne
optoelectronics. The design considerations are similar to
the second case: the gain and bandwidth must be set to
maximize the SNR in the band of interest, but good practices
in high speed PCB layout must be applied to overcome
parasitics. As a prospective, we will analyze the design of
ultra high speed transimpedance amplifiers for optical pulse
measurements.
VI. ACKNOWLEDGEMENTS
This work is supported by four UBACYT
grants from Universidad de Buenos Aires (2014-
2017 UBACYT 20020130100346BA, UBACYT
2016-2017 20020150200143BA, UBACYT 2017-
2019 20020160100042BA, UBACYT 2017-2019
20020160100052BA). The corresponding author
acknowledges a doctoral scholarship from CONICET.
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