# Common base transistor biasing

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?

• A couple of series resistors from power to ground with midpoint to the base. Decoupler cap on base to ground too. – Andy aka Jun 7 '15 at 15:32
• @Andyaka why do we need decoupling capacitor? – zxcv91 Jun 7 '15 at 15:39
• 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). – LvW Jun 7 '15 at 16:00
• @LvW thank you for your feedback. Now i get how biasing is done. – zxcv91 Jun 7 '15 at 22:52

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:

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.

• Please forgive the awkward voltage rails layout. Is there a better way to do this with circuitlab, and still have a simulating circuit? – corecode Jun 7 '15 at 16:17
• Just single-click the component, then Right-Click on the component to bring up the properties dialog box. Then select "Flip Vertically". – Dwayne Reid Jun 7 '15 at 16:21
• @corecode and dwayne : thanks guys, appreciate the explanation. – zxcv91 Jun 7 '15 at 22:50

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