I'm reading through Practical Electronics for Inventors, 3rd edition, on bipolar transistors, and they provide this example of a transistor switch:

enter image description here

I'm a bit confused by the calculation at the bottom for the base current:

\$I_{B} = \frac{V_{E} + 0.6V}{R_{1}} = \frac{0V + 0.6V}{R_{1}}\$

Shouldn't the base current be calculated as follows?

\$+V_{CC} = V_{R_{1}} + V_{BE} + V_{E}\$

\$+V_{CC} = I_{B}*R_{1} + V_{BE} + V_{E}\$

\$+V_{CC} = I_{B}*R_{1} + 0.6V + 0V\$

\$I_{B} = \frac{+V_{CC} - 0.6V}{R_{1}}\$

  • 1
    \$\begingroup\$ You sound correct to me. \$\endgroup\$ Aug 3, 2014 at 21:38
  • \$\begingroup\$ FYI for those also reading PEFI, there is an extensive unofficial errata for the Third Edition can be found at eevblog.com/forum/beginners/… \$\endgroup\$
    – cdwilson
    Aug 3, 2014 at 22:39
  • \$\begingroup\$ @cdwilson I'm not too keen on the idea of having [relatively obscure and not very widely used] abbreviations for book titles as tags. You're welcome to contest this in meta. \$\endgroup\$ Aug 3, 2014 at 23:46
  • \$\begingroup\$ @NickAlexeev do I just start a new topic in the meta SE and link this question? I've never used meta before, just curious what the normal procedure is for these types of things \$\endgroup\$
    – cdwilson
    Aug 4, 2014 at 0:39
  • 1
    \$\begingroup\$ @NickAlexeev FYI, looks like this question has already been asked/answered meta.electronics.stackexchange.com/questions/2813/… \$\endgroup\$
    – cdwilson
    Aug 5, 2014 at 2:30

1 Answer 1


I'd agree with you. When the switch is on, all the charge flowing through the resistor can only flow through the base-emitter junction of the transistor. The voltage across \$R_1\$ is \$V_{CC}-V_{BE} = V_{CC} - 0.6\mathrm V\$, making the current \$\frac{V_{CC}-0.6\mathrm{V}}{R_1}\$.


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