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(Vce = Vbe - Vthm) Voltage Reference Cell. Collector to emitter voltage (Vce) of the BJT is stable when the voltage drop across the collector resistance (Rc) is equal to the thermal voltage (Vthm). Rc should be equal to the reciprocal of the transconductance (Gm) of the BJT (common emitter configuration). The transconductance of a BJT is equal to Ic/Vt or qIc/kT, where Ic is the collector current of the BJT k is Boltzmann's constant (1.38xe-23J/K) T is temperature in Kelvin q is the charge of electron (1.6xe-19C) (Vthm = kT/q) at 300 K (≈ 27 °C) = 0.025875 V = 25.875 mV At 100 µA Rc = 258 Ω E192 series (tolerance 0.5%, 0.25% and 0.1%)