The author uses R2 to prevent that from happening, but actually that's an extra, unnecessary component. I've had fun making touch sensors this way. If the load being driven were less heavy, such as an LED, you would probably be able switch the LED on by just touching the base with your finger, injecting current into the base (thereby also raising its potential), current which is coupled capacitively to your body from the mains wiring around you. The base is a relatively high impedance, and it's easy to couple current into it capacitively (from your own body in its vicinity), or electromagnetically (like an antenna). However, while it's unlikely that leaving a bipolar transistor's base unconnected will cause any great current to flow (either into the base, or from collector to emitter), it is possible.
The book is wrong, strike one.Īgain, you make a good point - current can't flow out of the base (OK, it can, but it's picoamps, nanoamps at worst, just leakage current).
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