Tobi Delbrück, C. A. Mead
We describe an electronic photoreceptor circuit that is sensitive to small changes in incident light intensity. The sensitivity to change8 in the intensity is achieved by feeding back to the input a filtered version of the output. The feedback loop includes a hysteretic el(cid:173) ement. The circuit behaves in a manner reminiscent of the gain control properties and temporal responses of a variety of retinal cells, particularly retinal bipolar cells. We compare the thresholds for detection of intensity increments by a human and by the cir(cid:173) cuit. Both obey Weber's law and for both the temporal contrast sensitivities are nearly identical.
We previously described an electronic photoreceptor that outputs a voltage that is logarithmic in the light intensity (Mead, 1985). This report describes an extension of this circuit which was based on a suggestion by Frank Werblin that biological retinas may achieve greater sensitivity to change8 in the illumination by feeding back a filtered version of the output.
OPERATION OF THE CIRCUIT
The circuit (Figure 1) consists of a phototransistor (P), exponential feedback to P (Ql, Q2, and Q3), a transconductance amplifier (A), and the hysteretic element (Q4 and Qs). In general terms the operation of the circuit consists of two stages of amplification with hysteresis in the feedback loop. The light falls on the parasitic bipolar transistor P. (The rest of the circuit is shielded by metal.) P's collector is the substrate and the base is an isolated well. P and Ql form the first stage of amplification. The light produces a base current Is for P. The emitter current IE is PIs, neglecting collector resistance for now. P is typically a few hundred. The feedback current IQl is set by the gate voltage on QdQ2' which is set by the current through Q3, which is set by the feedback voltage Vjb. In equilibrium Vjb will be such that IQl = IE and some voltage Vp will be the output of the first stage. The
An Electronic Photoreceptor Sensitive to Small Changes
negative feedback through the transconductance amplifier A will make Vp ~ V/ b• This voltage is logarithmic in the light intensity, since in subthreshold operation the currents through Q2 and Q3 are exponential in their gate to source voltages. The DC output of the circuit will be Vout ~ V/b = Vdd - (2kT /q) log IE, neglecting the back-gate effect for Q2. Figure Sa (DC output) shows that the assumption of subthreshold operation is valid over about 4 orders of magnitude.