I've got a 3904 connected in a voltage divider bias. Schematic

Probing the input and output I notice that the output signal is inverted. Scope

Would someone explain the interworkings of how and why the output signal is inverted?

  • \$\begingroup\$ You need to include a schematic of your circuit. There's lots of ways to bias a transistor with voltage dividers. \$\endgroup\$ – The Photon Oct 19 '14 at 17:13
  • \$\begingroup\$ Is there a mind reader or telepath in the house? \$\endgroup\$ – Andy aka Oct 19 '14 at 17:13
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    \$\begingroup\$ But the real answer is probably that you're using a common emitter circuit. That circuit is inverting regardless if whether you bias it with voltage dividers or some other way. \$\endgroup\$ – The Photon Oct 19 '14 at 17:15
  • \$\begingroup\$ Schematic imgur.com/nAIFBW2 \$\endgroup\$ – glennlopez Oct 19 '14 at 17:17
  • \$\begingroup\$ Any description of a common emitter amplifier should answer this. Google or Wikipedia may have something useful. \$\endgroup\$ – Brian Drummond Oct 19 '14 at 18:08

Your schematic shows a typical common emitter single transistor amplifier. To understand why the input signal is inverted at the output consider the following: The input signal voltage appears across the emitter resistor. As it increases in voltage, it increases the emitter current. This also increases the collector current since, in an active transistor with a reasonable value of beta (current gain), the collector current is the same as the emitter current. However, note that the output voltage at the collector is equal to the supply voltage minus the product of the collector current and the collector resistor. Since the actual output voltage is AC coupled through a capacitor, the output voltage does not include the supply voltage but is equal to just the product of the collector current and collector resistor. However, it is inverted with respect to the input signal voltage because of that minus sign.


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