spatial bandpass filter
Which is commonly used
in computer vision to find
visual "edges" to determine
the shape of objects.
Vout drops logarithmically with Iphoto
Vf
Vout
Iphoto
Vf
Vout
Iphoto
High gain inverting amp
C1
Vf
R
Vout
Vp
Iphoto
High gain inverting amp
By closing the loop, we can make this adaptive.
Vout
Vp
Iphoto
If the loop is closed, we have our familiar circuit which has Vout =
Vout 
VT
Ibias VT
Iphoto
ln(
)

ln(
)
2
K
Io
K
Io
Vout VT 1

Iphoto K Iphoto
Because the ac gain of the amplifier is very high, in practice, it
is better to limit the gain using a capacitor divider.
C1
Vf
R
Vout
Vp
C2
Iphoto
High gain inverting amp
Since Vout now does the same job of feedback through the capacitor divider,
Vout VT 1 C 2  C1

Iphoto K Iphoto C 2
Delbruck and Mead Photoreceptor