The proposed work introduces a single-stage operational transconductance amplifier (OTA) operating in sub-threshold region providing ultra-low-voltage (ULV), low-power (ULP) structure suitable for low frequency applications such as biomedical systems. The usage of source degenerated differential pair offers improved input impedance and linearity for OTA which is useful for processing ECG, EMG and EEG signals, whereas the integration of DTMOS and partial positive feedback improves the gain of OTA. The proposed design operates at ± 0.25 V supply voltage for a load capacitor of 10 pF with 22.06 nW power dissipation. To authenticate the design results, all the simulations are carried out using Cadence Spectre tool using 0.18 µm technology node. The proposed design offers a single-stage gain of 50.38 dB, unity-gain bandwidth (UGB) of 11.71 kHz with a phase margin value of 85.08°. The design offers robust performance against process (P), voltage (V), temperature (T) variations along with mismatch and process analysis using Monte-Carlo (MC) simulations. As a design application of proposed ULP OTA, a fifth-order low-pass filter (LPF) is realized which can be used for ECG signal processing system. The implemented filter offers a cut-off frequency of 250.55 Hz with a power dissipation of 66.06 nW.