• An optical receiver system converts optical energy into electrical signal, amplify the signal and process Therefore the important blocks of optical receiver are :sdsfd
    • Photodetector / Front-end
    • Amplifier / Liner channel
    • Signal processing circuitry / Data

 

  • Noise generated in receiver must be controlled precisely as it decides the lowest signal level that can be detected and processed. Hence noise consideration is an important factor in receiver design. Another important performance criteria of optical receiver is average error

Receiver Configuration

 

  • Configuration of typical optical receiver is shown in 5.1.2.

  • Photodetector parameters –
    • PIN or APD type
  • Gain M = 1
  • Quantum efficiency η
  • Capacitance Cd
  • Dias resistance Rb
  • Thermal noise current ib (t) generated by Rb.
  • Amplifier parameters –
    • Input impendence Ra
    • Shunt input capacitance Co
    • Transconductance gm (Amp/volts)
    • Input noise current ia (t) because of thermal noise of Ra
    • Input noise voltage source ea (t)
  • Equalizer is frequency shapping filter used to mollify the effects of signal distortion and

Expression for Mean Output Current from Photodiode Assumptions :

  1. All noise sources are Gaussian in
  2. All nose sources are flat in
  3. All noise sources are uncorrelated (statistically independent).
    • Binary digital pulse train incident on photodector is fiven by –

                                     … (5.1.1)

Where,

 

P(t) is received optical power. Tb is bit period.

bn is amplitude parameter representing nth message bit. hp (t) is received pulse shape.

  • At time t, the mean output current due to pulse train P(t) is –

 

 

 

                                                            … (5.1.2)

 

Where, M is gain of photodectector

 

 

 

 

is responsivity of photodiode

 

 

  • Neglecting dark current, the mean output current is given as –

 

… (5.1.3)

 

 

  • Then mean output current is amplified, filtered to give mean voltage at the

Preamplifier Types

 

  • The bandwidth, BER, noise and sensitivity of optical receiver are determined by preamplifier stage. Preamplifier circuit must be designed with the aim of optimizing these
  • Commonly used preamplifier in optical communication receiver are –
  1. Low – impedance preamplifier (LZ)
  2. High – impedance preamplifier (HZ)
  3. Transimpedance preamplifier (TZ)

1.      Low – impedance preamplifier (LZ)

  • In low-impedance preamplifier, the photodiode is configured in low – impedance The bias resister Rb is used to match the amplifier impedance. Rb along with the input capacitance of amplifier decides the bandwidth of amplifier.
  • Low – impedance preamplifier can operate over a wide bandwidth but they have poor receiver sensitivity. Therefore the low – impedance amplifier are used where sensitivity is of not prime

2.      High – impedance preamplifier (HZ)

  • In high – impedance preamplifier the objective is to minimize the noise from all This can be achieved by –
    • Reducing input capacitance by selecting proper devices.
    • Selecting detectors with low dark
    • Minimizing thermal noise of baising
    • Using high impedance amplifier with large Rb.
  • The high impedance amplifier uses FET or a BJT. As the high impedance circuit has large RC time constant, the bandwidth is reduced. Fig. 5.1.3 shows equivalent circuit of high input impedance pre-amplifier.

 

  • High-input impedance preamplifier are most sensitive and finds application in long – wavelength, long haul routes. The high sensitivity is due to the use of a high input resistance (typically > 1 MΩ), which results in exceptionally low thermal noise. The combination of high resistance and receiver input capacitance, results in very low BW, typically < 30 kHz, and this causes integration of the received A differentiating, equalizing or compensation network at the receiver output corrects for this integration.

3.      Transimpedance preamplifier (TZ)

  • The drawbacks of ghigh input impedance are eliminated in transimpedance A negative feedback is introduced by a feedback resistor Rf to increase the bandwidth of open loop preamplifier with an equivalent thermal nose current if (t) shunting the input. An equivalent circuit of transimpedance preamplifier is shown in Fig. 5.1.4.

ea (t) = Equivalent series voltage noise source ia(t) = Equivalent shunt current noise.

Rin = Ra ║Ca.

Rf = Feedback resistor.

if (t) = Equivalent thermal noise current.

 

  • High input impedance preamplifier using FET is shown in Fig. 1.5.

  • Basic noise sources in the circuit are –
    • Thermal noise associated with FET
    • Thermal noise from
    • Thermal noise from feedback
    • Shot noise due to gate – leakage current (Igate)
    • FET 1/f

 

  • As the amplifier input resistance is very high, the input current noise spectral density S1 is expressed as –

                                                               … (5.1.4)

Thermal noise associated with FET channel

 

  • The voltage noise spectral density is –

 

                                                                                                                 … (5.1.5)

where,

 

gm is transconductance. Γ is channel noise factor.

  • Thermal noise characteristic equation is a very useful figure of merit for a receiver as it measures the noiseness of amplifier. The equation is reproduced here –

 

Substituting S1 and SE, the equalizer output is then

 

Where,    C = Cd + Cgs + Cgd + Cs                                                                    … (5.1.6)

  • If bias resistor Rb is very large, so that the gate leakage current is very low. For this the detector output signal is integrated amplifier input It is to be compensated by differentiation in the equalizer. The integration – differentiation is known as high input impedance epreamplifier design technique. However, the integration of receive signal at the front end restricts the dynamic range of receiver. It may disrupt the biasing levels and receiver may fail. To correct it the line coded data or AGC may be employed such receivers can have dynamic ranges in excess of 20 dB.
  • Of course, FET with high gm is For high data rates GaAs MESFET are suitable while at lower frequencies silicon MOSFETs or JFET are preferred

.

 

High Impedance Bipolar Transistor Amplifier

 

  • High input impedance preamplifier using BJT is shown in Fig. 1.6.

  • Input resistance of BJT is given as –

                                                   … (5.1.7)

 

Where,

 

IBB is base bias current.

  • Spectral density of input noise current source because shot noise of base current is –

                                                                                                                      … (5.1.8)

  • Spectral height of noise voltage source is given as –

 

                                                                                                                     … (5.1.9)

Where, gm is transconductance.

 

  • The performance of receiver is expressed by thermal noise characteristic equation (W)

 

 

 

Substituting Rin, SI and SE in characteristic equation.

                                  … (5.10.10)

 

Where,             

If Rb >> Rin, then R ≈ Rin, the expression reduces to

 

                       … (5.1.11)

 

 

Transimpedance Amplifier

 

  • An ideal transimpedance preamplifier provides an output voltage which is directly proportional to the input current and independent of course and load impedance.
  • A transimpednace amplifier is a high-gain high-impedance amplifier with feedback resistor Rf 5.1.7 shows a simple CE/CC. Shunt feedback transimpedance receiver.

Bandwidth (BW)

 

  • To find BW, the transfer function of non-feedback amplifier and feedback amplifier is The transfer function of non-feedback amplifier is

                                                                                     … (5.1.12)

 

Where,

 

A is frequency independent gain of amplifier.

 

  • Now the transfer function of feedback (transimpedance) amplifier is –

                                    … (5.1.13)

 

  • This yields the BW of transimpedance

                                  … (5.1.14)

 

i.e. BW of transimpedance amplifier is A times that of high-impedance amplifier. Because of this equalization becomes easy.

Characteristic equation

 

  • The thermal noise characteristic equation (W) is reduced to –

 

                                                                                     … (5.1.5)

Where,

 

WHZ is noise characteristic of high-impedance amplifier (non-feedback amplifier).

Thus thermal nose of transimpedance amplifier is sum of ooutput noise of non-feedback amplifier and noise associated with Rf.

Benefits of transimpedance amplifier

 

  1. Wide dynamic range : As the BW of transimpedance preamplifier is high enough so that no integration takes place and dynamic range can be set by maximum voltage swing at preamplifier
  2. No equalization required : Since combination of Rin and Rf is very small hence the time constant of detector is small.
  3. Less susceptibe to external noise : The output resistance is small hence the amplifier is less susceptible to pick up noise, crosstalk, RFI and
  4. Easy control : Transimpedance amplifiers have easy control over its operation and is
  5. Compensating network not required : Since integration of detected signal does not occur, compensating network is not required.

 

 

 

 

High Speed Circuit

 

  • Now fiber optic technology is widely employed for long-distance communication, LAN and in telephone networks also because of improvement in overall performance, reliable operation and cost
  • Fiber optic link offers wide bandwidth to support high speed analog and digital

Because of advancement in technology minimized transmitters and receivers and available in integrated circuits package.