7.20 The essence of transistor operation is that a change in vBE, ∆vBE, produces a change in iC, ∆iC. By keeping ∆vBE small, ∆iC is approximately linearly related to ∆vBE, ∆iC = gm∆vBE, where gm is known as the transistor transconductance. By passing ∆iC through RC, an output voltage signal ∆vO is obtained. Use the expression for the small-signal voltage gain in Eq. (7.20) to derive an expression for gm. Find the value of gm for a transistor biased at IC = 0.5 mA.