import numpy as np
import matplotlib.pyplot as plt

amp = 1
freq = 2
time = np.linspace(0, 2, 1000)

signal1 = amp*np.sin(2*np.pi*freq*time)

signal2 = np.where(signal1<0, 0, signal1)

plt.figure()
plt.style.use('seaborn-whitegrid')
plt.plot(time, signal2)
plt.ylim(bottom=-1) # To avoid automatic resizing of the plot
plt.show()