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I have a basic question regarding bipolar junction transistors.As far as i know, the correct biasing of the transistor is achieved using a process known commonly as base bias.Correct me if i am wrong. Base bias networks can be used for Common-base (CB), common-collector (CC) or common-emitter (CE) transistor configurations.My question is, how does biasing of a common base configuration is done? or in other words, how to base bias a common base configuration transistor?

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  • \$\begingroup\$ A couple of series resistors from power to ground with midpoint to the base. Decoupler cap on base to ground too. \$\endgroup\$ – Andy aka Jun 7 '15 at 15:32
  • \$\begingroup\$ @Andyaka why do we need decoupling capacitor? \$\endgroup\$ – zxcv91 Jun 7 '15 at 15:39
  • \$\begingroup\$ Realize a common-emitter stage (with Re-feedback). If you feed the signal into the emitter instead of the base node. Then, you have a common-base amplifier (DC operatinmg point unchangend) - if you ground the base for ac signals (capacitor). \$\endgroup\$ – LvW Jun 7 '15 at 16:00
  • \$\begingroup\$ @LvW thank you for your feedback. Now i get how biasing is done. \$\endgroup\$ – zxcv91 Jun 7 '15 at 22:52
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For the transistor to work in conduction, its \$V_{BE} \ge 0.6V\$. For a common base circuit, you can achieve this for example by grounding the base and biasing the emitter input negatively.

The following circuit illustrates the principle:

schematic

simulate this circuit – Schematic created using CircuitLab

Or if you do not have a negative rail, you can bias the base with a resistor divider, as Andy suggested.

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  • \$\begingroup\$ Please forgive the awkward voltage rails layout. Is there a better way to do this with circuitlab, and still have a simulating circuit? \$\endgroup\$ – corecode Jun 7 '15 at 16:17
  • \$\begingroup\$ Just single-click the component, then Right-Click on the component to bring up the properties dialog box. Then select "Flip Vertically". \$\endgroup\$ – Dwayne Reid Jun 7 '15 at 16:21
  • \$\begingroup\$ @corecode and dwayne : thanks guys, appreciate the explanation. \$\endgroup\$ – zxcv91 Jun 7 '15 at 22:50
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you have same circuit as voltage divider CE amplifier with degeneration resistor; no bypass cap on it; simply connect a large capacitor like 10 micro to the base where its connected to divider resistors and the other side to ground; this will form an AC ground point while blocking DC bias from getting drained to ground

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