The document discusses various optical receiver configurations, including low-impedance, high-impedance, and transimpedance front-end amplifiers, highlighting their advantages and limitations. It emphasizes the role of equalization in compensating for signal distortion and improving receiver sensitivity while addressing the trade-offs between bandwidth, sensitivity, and dynamic range. Additionally, it covers the noise performance variations among different amplifier types and the application of these technologies in wideband optical fiber communication systems.

1. Optical Receivers
MEC

2. Contents
• Digital Optical Fiber Receiver.
• Equalizer.
• Amplifier Configurations
- Low-impedance Front-end.
- High-impedance Front-end.
- Transimpedance Front-end.
• Comparison

3. Digital Optical Fiber Receiver
• Optical detector represented as a current
source idet.
• Noise sources - it, iTS and iamp , amplifier
and equalizer follows.
• Equalization compensates for signal
distortion due to combined transmitter,
medium and receiver characteristics.

4. Digital Optical Fiber Receiver
Equivalent Circuit

5. Equalizer
• Equalizer a frequency-shaping filter,
frequency response inverse of overall
system frequency response.
• Boost high-frequency components, correct
overall amplitude of frequency response.
• To acquire desired spectral shape & to
minimize intersymbol interference, system
phase frequency response to be linear.

6. Equalizer
• Applies selective
phase shifts to
particular frequency
components.
• Minimize noise
contributions from the
sources, maximize
receiver sensitivity,
maintain a suitable
bandwidth.

7. Amplifier Configurations
• Basic amplifier
configurations:
- Low-impedance
front-end.
- High-impedance
(integrating) front-
end.
- Transimpedance
front-end.

8. Low-impedance Front-end Amplifier
• Voltage amplifier
with effective input
resistance Ra.
• Detector loaded with
a bias resistor Rb
and an amplifier.
• Bandwidth
determined by
passive impedance
across detector
terminals (RL).
Modified total load
resistance

9. Low-impedance Front-end Amplifier
• RL may be modified to incorporate the
parallel resistance of detector bias
resistor Rb and amplifier input resistance
Ra.
• To achieve optimum bandwidth, Rb and Ra
to be minimized - low-impedance front-end
design.
• Design allows thermal noise to dominate
within the receiver, limits sensitivity.

10. Low-impedance Front-end Amplifier
• Trade-off between bandwidth and
sensitivity.
• Impractical for long-haul, wideband optical
fiber communication systems.

11. High-impedance (integrating)
Front-end Amplifier
• High i/p impedance
amplifier together
with a large detector
bias resistor to
reduce the effect of
thermal noise.
• Degraded frequency
response, bandwidth
relationship not
maintained for
wideband operation.

12. High-impedance (integrating)
Front-end Amplifier
• Detector output effectively integrated over
a large time constant, must be restored by
differentiation.
• Correct equalization required.
• Improvement in sensitivity over low-
impedance front-end design, creates a
heavy demand for equalization, limited
dynamic range.

13. High-impedance (integrating)
Front-end Amplifier
• Limitations on dynamic range result from
attenuation of the low-frequency signal
components by equalization, causes
amplifier to saturate at high signal levels.
• Amplifier saturates before equalization,
signal is heavily distorted.
• Reduction in dynamic range depends on
the amount of integration and subsequent
equalization employed.

14. Transimpedance Front-end
Amplifier
• Overcomes drawbacks of high-impedance
front end.
• Low-noise, high-input-impedance amplifier
with negative feedback.
• Operates as current mode amplifier, high
input impedance reduced by negative
feedback.

15. Transimpedance Front-end
Amplifier

16. Transimpedance Front-end
Amplifier
• Open loop current to voltage transfer
function HOL (ω)
G - open loop voltage gain of the
amplifier, ω - angular frequency of input.

17. Transimpedance Front-end
Amplifier
• Closed loop current to voltage transfer
function HCL(ω) (Rf - feedback resistor):
• Permitted electrical bandwidth (without
equalization)

18. Transimpedance Front-end
Amplifier
• Greater bandwidth than amplifiers without
feedback.
• When Rf << RTL, major noise contribution
is from thermal noise generated in Rf.
• Noise performance improved when Rf is
large.
• When Rf = RTL, noise performance
approaches that of high-impedance front
end.

19. Transimpedance Front-end
Amplifier
• Rf cannot be increased indefinitely due to
problems of stability with closed loop
design.
• Increasing Rf reduces bandwidth.
• Make G as large as the stability of closed
loop permits.
• Noise in transimpedance amplifier exceed
that incurred by high-impedance front-end
structure.

20. Transimpedance Front-end
Amplifier
• Greater bandwidth without equalization
than high-impedance front-end, but offset
by 13 dB noise penalty incurred.
• Optimized for noise performance at the
expense of bandwidth.
• Improvement in noise performance over
low-impedance front-end structures.
• Greater dynamic range.

21. Transimpedance Front-end
Amplifier
• Different attenuation mechanism for low
frequency components of the signal.
• Attenuation through negative feedback, low
frequency components amplified by device
closed loop rather than open loop gain.
• Improvement in dynamic range equal to
ratio of open loop to closed loop gains.
• Used in wideband optical fiber
communication receivers.

22. Thank You