the supply voltage "v" must be greater than the bulb breakdown voltage "v" or the bulb can never conduct. most small neon lamps have breakdown voltages between 80 and 150 volts. if the supply voltage is close to the breakdown voltage, the capacitor voltage will be in the "tail" of its exponential curve by the time it reaches "v", so the frequency will depend sensitively on the breakdown threshold and supply voltage levels, causing variations in frequency. therefore, the supply voltage is usually made significantly higher than the bulb firing voltage. this also makes the charging more linear, and the sawtooth wave more triangular.