Curve fitting with two exponential functions

Kindly help me in fitting a function which is formed by the sum of two exponentials to a given data set using cvx.

Given data set (x_i,y_i) i=1,2,…100 ;
Function expression = a_1 e^{b_1}x + a_2 e^{b_2}x

Cost function : \epsilon (a_1,b_1,a_2,b_2) = \sum_{1}^{100} [ (y_i) - a_1 e^{b_1}x_i + a_2 e^{b_2}x_i]^2