High-Voltage Bc547

0I It is sometimes desired to use a BC547 at rather higher voltages than permitted according to the data book. Yet, it can be done by connecting a number of them in series as shown in the ac- companying diagram. The set-up has a few, small, disadvan- tages: there is a constant leakage cur- rent through the series resistors and the saturation voltage is rather higher. Where these disadvantages are of little or no consequence, the circuit shown here can be used with voltages up to about 100 V. Assume that a voltage of 100 V has to be switched and that the maximum current is 2 mA. If the current amplification is 200, the base current will be 10 pA. Transistor T3 will then switch on as soon as the p.d. across R4 is 0.68 V. The base current of T2 also flows through R4, so that the drop across this resistor rises to 1.36 V. The current that switches Ti flows through Ri, so that it does not cause an additional p.d. across the potential div- ider. There is, of course, the usual saturation...