February 2004 Charles A. DiMarzio, Northeastern University 10464-9-1

ECEG287 Optical Detection Course Notes

Part 9: Detector as a Circuit Element

Profs. Charles A. DiMarzio

and

Stephen W. McKnight

Northeastern University, Spring 2004

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-2

Some of the Issues

• Dark Current

• Photocurrent

• DC Current (Mean Power in Signal)

• Operating Point

• Capacitance (Maintain Reverse Bias)

• DC and AC Impedance

• Power Dissipation

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-3

The Basic Model

t

pIncident Light:

Detectorand Bias Circuit

t

VOutput Signal:

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-4

Bias Circuits

• The Bias Tee

• Transimpedance Amplifier

• Balanced Bridge

• Photovoltaic Mode

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-5

The Photodiode• Current Responsivity

pph

e

dt

dE

h

e

dt

dQi iqqP

h

eqi

V_V1

-10V -5V 0V 5V-I(R1)

-5.0mA

0A

5.0mA

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-6

Photodiode Model

C

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-7

Photodiode I-V Curves

V_V1

-10V -8V -6V -4V -2V 0V 2V

-I(R1)

-5.0mA

0A

5.0mA

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-8

Conventional Bias Circuit

DCCurrent(Dark andBias)

From ACSignalPower

t

pIncident Light:

C

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-9

Response to Varying Power

V_V1

-10V -8V -6V -4V -2V 0V 2V

-I(R1)

-5.0mA

0A

5.0mA

tt

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-10

DC Analysis Set Operating Point

V_V1

-10V -5V 0V 5V-I(R1)

-5.0mA

0A

5.0mAX

X

X

Choose Load Line toMaximize Voltage SwingAvoid SaturationAllow for DC Variations

3ma

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-11

AC Analysis

V_V1

-10V -5V 0V 5V-I(R1)

-5.0mA

0A

5.0mA

XX

AC Load Line May be Differentfrom DC

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-12

Transient Analysis of AC Signal

I_I10A 2mA 4mA 6mA 8mA 10mA

V(R1:2)

-10V

0V

10V

Voltage Across Diode

Time

0s 0.2us 0.4us 0.6us 0.8us 1.0usV(R2:2)

-40mV

0V

40mV

iP(DC)=6.0mA

6.5mA

7mA

Load Voltage

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-13

Saturation at High Power

• Voltage Goes to Zero• Solutions

– Lower Resistor

– Higher Supply Voltage

Time

0s 0.2us 0.4us 0.6us 0.8us 1.0usV(R2:2)

-40mV

0V

40mV

iP(DC)=6.0mA

6.5mA

7mA

Load Voltage

V_V1-I(R1)

-10V -5V 0V 5V

-5.0mA

0A

5.0mA

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-14

Transimpedance Amplifier

-10V -5V 0V 5V

-5.0mA

0A

5.0mA

Virtual Ground acts asa 0-Ohm Load Resistor(Prevents Saturation)

Time

0s 0.2us 0.4us 0.6us 0.8us 1.0usV(D1:1) V(U1:OUT)

-4.0V

-2.0V

0V

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-15

Balanced Bridge Circuit

Time

0s 0.2us0.4us0.6us0.8us1.0usV(RL:2)

-500mV

0V

500mV

R1, R2 Set Diode Bias VoltageRNull Nulls out Backgroundand Current through RF

-10V -5V 0V 5V

-5.0mA

0A

5.0mA

February 2004 Charles A. DiMarzio, Northeastern University 10464-9-16

Photovoltaic Mode

V_V1

-1.0V -0.5V 0V 0.5V1.0V-I(R1)

-5.0mA

0A

5.0mA

200 Ohms

P=IV<0