An application of the calculus of finite differences has been made in obtaining the complete expression for the voltage gain of ann‐stage amplifier having identical grid‐to‐plate impedances and plate loads when driven by a generator of any given internal impedance. An application of these results was made in determining the conditions of oscillation in a multi‐stage amplifier using single‐tuned circuits. It was found that there is a certain minimum grid‐to‐plate capacitance required to cause oscillation. If the actual grid‐to‐plate capacitances are below this value, the system will not oscillate. There also exists a region of stable gain, zero to 21/n, wherein it is impossible to make the amplifier oscillate even with grid‐to‐plate capacitance arbitrarily large.