Mead's adaptive retina [Mead 89a] is an enhanced implementation of Mahowald's silicon retina described in Section 2.2. The chip uses floating gate MOSFETs (FGMOS) as a feedback element used for correcting the problem of offset and mismatch between transistors.
Figure 2.3 shows two circuits with and without the FGMOS transistor. The retina chip using the circuit in Figure 2.3-a had demonstrated a very sensitive operation in which the output voltage of many pixels were stuck to Vdd or Gnd supply voltages under uniform illumination. The reason is that a small offset in OTA1 is amplified by the inverting amplifier and the output will saturate to one of the supply rails.
In order to mitigate this problem a feedback loop has been constructed to compensate for the effect of mismatch by changing the effective threshold voltage of transistor M2. This has been realized by the UV activated coupler, which is a simple poly1-poly2 structure. When the poly1-poly2 structure is exposed to the UV light, the feedback loop is closed and the floating gate sits at a voltage which holds the output voltage, at a level which depends on the input current (if the coupler becomes short circuit, the pull up circuit will be the simple two stacked MOS diode).
Figure 2.3: a) the circuit without FGMOS, b) the circuit with FGMOS.