## 2 Design and Implementation
### CMOS Retina Neuron IC: Design-III  
This subsection demonstrates the empirical and experimental demonstration of CMOS retina circuit for retinal image acquisition. The analysis of CMOS retina circuit consists of current event generator, current mirror, and dynamic feedback stage that can be seen in Figure 8a. The CMOS based neuron retina circuit employs a photosensitive element as photodiode. Basically, an image address-event representation (AER) is employed for biological image acquisition. In AER, a sequence of pulses or spikes is formed at the output by information where analog value of the data being transmitted is encoded by spike frequency or intervals. The data is encoded in form of digital pulses using step-forward encoding method (SW), which is immune to noise. SW encoding is easy to optimize and effective method compared to other methods such as threshold-based and moving-window [26]. The requirement of retina neuron for high resolution of image acquisition, which could be accomplished by using a technique as employed here into retina circuit. A conceptual investigation after wireless power transfer is discussed here. In fact, the contact lens ICs can receive RF based signal information for image visualization. This image-based information could reach to the interior of human eye where CMOS neuron IC can read number of read-out spikes as image in the form of light. The theoretical approach of current mirror block neuron circuit provides an output as read-out spikes. It is observed that amplitude of read-out spikes depends on two parameters: (1) light intensity, which applies to the photodiode. Here it is produced by using a custom off-chip \(\mu\)-LED. The custom \(\mu\)-LED is a power hungry device, which receives the power from harvesting IC. The power is extracted from the RF harvesting circuit that varies from 4 mW to 2.7 W. This large variation of power when applied to the \(\mu\)-LED produces light. The light intensity of 0 W/cm\({}^{2}\) to 2.7 W/cm\({}^{2}\) given to the photodiode of neuron circuit, achieves reduction in the amplitude of read-out spikes that are about to be 60.9% as compared to the existing one, and (2) control voltage, which introduces a controlled pole for promoting the amplitude tuning mechanism. It is observed that variation in control voltage gives indirect impact on the threshold voltage of M9 transistor that can be expressed as in Equation (3). \[\mathrm{V_{T9}}=\mathrm{V_{T90}}+\gamma\big{(}\sqrt{2\mathcal{O_{F}}-\mathrm{ V0}}-\sqrt{2\mathcal{O_{F}}}\big{)} \tag{3}\]  
where \(\mathrm{V_{T90}}\) is the zero-bias threshold voltage of M9, \(\gamma\) is body coefficient whose expression \(\left(\gamma=\frac{\sqrt{2\mathrm{q}\mathrm{e_{substrate}}\mathrm{P_{substrate}}}}{\mathrm{C_{ox}}}\right)\), \(\mathcal{O_{F}}\) is the fermi potential and V0 is the inverter output, respectively. This variation of threshold voltage increases the threshold current of all transistors from M9 to  
\begin{table}
\begin{tabular}{c c} \hline
**Component** & **Dimension** \\ \hline M1 & 40/0.18 \(\mu\)m \\ M2 & 23.5/0.18 \(\mu\)m \\ M3 & 112/0.18 \(\mu\)m \\ M4 & 58/0.18 \(\mu\)m \\ M1\({}^{\prime}\) & 38/0.18 \(\mu\)m \\ M2\({}^{\prime}\) & 23.5/0.18 \(\mu\)m \\ M3\({}^{\prime}\) & 10/0.18 \(\mu\)m \\ M4\({}^{\prime}\) & 28/0.18 \(\mu\)m \\ C1M11. Due to this, amplitude of read-out spikes can be tuned. The tuned amplitude basically depends upon transfer function \(\mathrm{h(s)}\) of the closed loop M8 and M9, and which can be expressed in Equation (4). \[\mathrm{h(s)}=\frac{\mathrm{A(s)}}{1+\mathrm{A(s)}\beta(\mathrm{s})} \tag{4}\]  
where  
\[\beta(\mathrm{s})=\left(\frac{\mathrm{g_{m,M9}}}{\mathrm{g_{m,M11}}}\right) \left(\frac{\mathrm{Y_{A}}}{\mathrm{Y_{A}}+\mathrm{Y_{B}}}\right) \tag{5}\]  
\(\mathrm{g_{m,M9}}\) and \(\mathrm{g_{m,M11}}\) are the transconductance of M9 and M11 while \(\left(\frac{\mathrm{Y_{A}}}{\mathrm{Y_{A}}+\mathrm{Y_{B}}}\right)\) term is fractional admittance of dynamic feedback stage.