This paper presents an Operational Transconductance Amplifier realized using Floating Gate MOSFETs only. This can be used as Neuron Activation Function with wide linear range and adaptable threshold level and slope. This circuit is suitable for analog signal processing and neural network applications. This circuit generates log sigmoid and tan sigmoid NAF functions simultaneously. In this circuit the parameters of NAF function can be easily changed by changing voltage at the control gate of FGMOSFET. The linear range of the proposed FGMOSFET based OTA circuit is large and is equal to 2V with a power supply of +0.75V. Power analysis and noise analysis of the proposed circuit is performed. Mismatch study is performed using Monte Carlo simulation. TSPICE simulation results using 2 mu m N-well process are presented. Layout design of the circuit is done using L-Edit in Tanner Tool.