Noise can cause errors on small signals. This problem is especially true for amplifying small signals coming over a transmission line or an antenna. The noise model for current-feedback (CFB) amplifiers is the same as for voltage-feedback (VFB) amplifiers. The only difference between CFB and VFB amplifiers is that CFB amplifiers generally specify different current-noise parameters for each input, whereas VFB amplifiers usually only specify one noise-current parameter. Figure 7-1 shows the noise model. This model includes all of the noise sources as follows:

• e_n = Amplifier internal voltage noise (nV/√Hz)
• IN+ = Noninverting current noise (pA/√Hz)
• IN– = Inverting current noise (pA/√Hz)
• e_Rx = Thermal voltage noise associated with each resistor (e_Rx = 4 kTR_x)

Figure 7-1 Noise Model

The total equivalent input noise density (e_ni) is calculated by using the following equation:

Equation 1.

To get the equivalent output noise of the amplifier, just multiply the equivalent input noise density (e_ni) by the overall amplifier gain (A_V).

Equation 2.

The previous equations show that to keep noise at a minimum, use small-value resistors. As the closed-loop gain is increased (by reducing R_G), the input noise is reduced considerably because of the parallel resistance term. This result leads to the general conclusion that the most dominant noise sources are the source resistor (R_S) and the internal amplifier noise voltage (e_n). Noise is summed in a root-mean-squares method; therefore, noise sources smaller than 25% of the largest noise source can be effectively ignored. This threshold can greatly simplify the formula and make noise calculations much easier.