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Phot od iode Am pl i f iers Changing Light to Electricity
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Paul Rak o
St ra t eg ic App l ic a t ions Eng ineer Am pl i f ie r Group
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© 2004 National Semiconductor Corporation
The Phot odiode:
Simple? 2 Volts
Light
Big Resistor (1 Meg)
Tiny current
flows here (10
nanoAmperes)
Makes about a
10 millivolts here
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The Phot odiode:
No, not really
simple:2 Volts
Light
Big
resistors
make noise
Dark Current
(diode leakage)flows too and is
worse with temp.
10 millivolts is
not very useful.
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The Phot odiode:
Worse yet: 2 Volts
Light
Diodes are
capacitors too,so fast signals
are difficult.
High impedance
point difficult tointerface with.
And the capacitance
changes with voltageacross the diode.
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The Phot odiode:
Still Worse: 2 Volts
Light
To make the
diode more
sensitive to lightyou make the P-N
junction big.
And that big junction
has even morecapacitance.
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Ins ide t he Phot od iode:
A cap and a current source:
The bigger the
voltage across the
diode the further the junction boundaries
are pushed apart andthe lower the
capacitance.
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Ins ide t he Phot od iode:
(And a really big resistor)
There is also a bulk
resisistivity to the
diode but it is usually
very high (100 MΩ).
This represents the“Dark Current”.
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Phot od iode Am pl i f ier Types:
Two ways to use the diode:
1) The Photovoltaic
Mode:
Note ground– no voltageacross diode.
Light +
-
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Phot od iode Am pl i f ier Types:
The Photovoltaic Mode:
No voltage across diode means no
current though the big resistor ~
• No dark current.
Also:
• Linear output• Low Noise
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Phot od iode Am pl i f ier Types:
Use Photovoltaic Mode:
• Where precision is more important
then speed.
The lack of dark current removes an
entire error term. The lower noise makessmaller measurements possible. The
linear output makes calculations trivial.
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Phot od iode Am pl i f ier Types:
The Photoconductive Mode:
- 10V, there is voltage
across the diode.
+
-
Light
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Phot od iode Am pl i f ier Types:
Use Photoconductive Mode:
• Where speed is more important then
precision.
The voltage across the diode lowers it’s
capacitance. This allows faster amplifiers:
• Less capacitance allows a faster
amplifier while maintaining stability.
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Bias ing t he Phot od iode:
• Apply a big voltage
(that doesn’t change):We want a low
capacitance so put abig voltage across
the diode. We want
fast response sodon’t let the voltage
ever change. How?
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The Phot od iode Am pl i f ie r :
• Connect the diode to a virtual ground:
If this pin is at
ground so must this
pin be at ground.
-10 Volts
Light
+
-
As much reverse
voltage as the diode
can stand.
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The Phot od iode Am pl i f ie r :
• Oh yeah, add
some feedback:
-10 Volts
Light +
-
+-
This pin stays at ground
so output goes more
positive with more light.
This current makes
positive voltage here
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The Phot od iode Am pl i f ie r :
• So it oscillates
and/or clips,what is wrong?
-10 Volts
Light +
-
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Am pl i f ie r St ab i l i t y :
• Oscillations caused
by capacitive diodeon input.
-10 Volts
+
-Photodiodelooks like
cap to amp
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Am pl i f ie r St ab i l i t y :
• Input pole (freq domain) or feedback
lag (time domain) is bad.
-10 Volts
+
-Photodiode
currentsource
causes
output tochange. But, photodiode capacitor
means feedback signal
will lag the actual outputchange.
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Mec hanic a l Ana logy:
• A gear and rack mechanical servo.
This gear is the
amp output stage.
This gear is the
feedback.
Thisrack is
the
outputvoltage.
You are the amplifier
front-end trying to keep
the pointers the same.
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Mec hanic a l Ana logy:
• Input cap is like backlash in feedback
mechanism.Backlash here is a
lag in the feedback.
The lag in your feedback
pointer will cause you to
oscillate the rack.
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Mec hanic a l Ana logy:
• Input cap is like backlash in feedback mechanism.
Backlash =
-10 Volts
+
-
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Com pensat ed Am pl i f ie r :
• Add a feedback cap
to compensate.
-10 Volts
Light +
-
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Bias ing t he Am pl i f ier :
• The output is stable
but there is a big DCoffset. Why?
-10 Volts
Light +
-
Output never goes
below here even
with no li ht.
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Bias ing t he Am pl i f ier :
• Or maybe there is no
output at all. Why?
-10 Volts
Light +
-
Output stuck at zero
even with maximumli ht.
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Bias ing t he Am pl i f ier :
• Answer: Input bias
current.
-10 Volts
Light +
-
Input pins will have
small currents in or out
of the pin (bias current).
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Bias ing t he Am pl i f ier :
• Bias current may
exceed photodiodecurrent.
-10 Volts
Light +
-
15uA out of this pin reacts
against 1M feedback to try
and put -15 volts on output.
1M
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Am pl i f ie r Input St age:
• Input transistors have base current.
Input bias
current may
be 15 µA, butwon’t vary
much over PNPInput
Stage
NPNInput
Stage
temperature.
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Am pl i f ie r Input St age:
• Input JFETs have large drift.
Input bias
current may
be 15 pA, butwill double
every 10°C.
JFETInput
Stage
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Am pl i f ie r Input St age:
• CMOS parts have ESD diodes.
MOSFET has no DC
bias current but
mis-match in ESDdiodes causes bias
current to flow in
(or out) of pin. CMOSInput
Stage
?
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Correc t ing DC Bias
• Use resistor.
-10 Volts
Light +
-
LMH6642 (fast)
LMV751 (low noise)
Add resistor to
compensate for bias
current.
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Correc t ing DC Bias
• Servo out the
error.
-10 Volts
Light +
-
(But this setup will
only pass AC signals)
+
-LMV2011
LMH6624
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Am pl i f ie r no ise:
• With stability and
bias solved, nextproblem is noise.
+
-
Current noise
important onthis pin.
Voltage noiseimportant on
this pin.
Light
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Am pl i f ie r no ise:
• Low current and low voltage noise in the
same part is hard.• JFET amplifiers have low current noise.
• Bipolar amplifiers have low voltage noise.• Choppers can cause problems.
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S P i l P
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Som e Pot ent ia l Par t s :
Device
Input Noise
Voltage
(nV/RtHz)
Input Noise
Current
(pA/RtHz)
Input
Capacitance
(pF)
I bias
(max)
GBWP
(MHz)
GBWP/Cin
(MHz/pF)
LMH6628 2 2 1.5 20µA 200 133
LMH6626* 1.0 1.8 0.9 20µA 500 556
LMH6624* 0.92 2.3 0.9 20µA 500 556
LMH6622 1.6 1.5 0.9 10µA 200 222
LMH6654 /6655 4.5 1.7 1.8 12µA 150 83
LMH6672 4.5 1.7 2 14µA 100 50
LF411A 25 0.01 4 200pA 4 1
LMV751 7 0.005 5 100pA 5 1
LMC662 22 0.0002 4 0.01pA(typical)
1.4 0.3
LMV771 8 0.001 4 100pA 4 1
C l i
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Conclus ions:
• Photodiode amplifiers are tricky.
• The design should be tailored for theapplication, DC, Data, etc.
• The design requires a lot of trial and error.• Be prepared to do a lot of study.
• National Applications Engineering is here to
help you.
R
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© 2004 National Semiconductor Corporation
Resources:
• AN-1244: Photo-Diode Current-to-Voltage
Converters.• Amplifier WEBENCH®– On-line simulation of
amplifier performance
• Photodiode Amplifiers: OP AMP Solutions by JeraldGraeme
• Photodetection and Measurement: Maximizing
Performance in Optical Systems by Mark Johnson• Photodetectors: Devices, Circuits and Applications
by Silvano Donati
Th k Y !
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