Thats a simple audio amplifier transistor/bjt circuit. I wonder why the capacitor C1 is used in the circuit when ac variations from the mic can be superimposed on dc directly in many ways. Secondly why voltage divider bias is used usually to make the audio amplifiers. In almost all books voltage divider bias configuration is used so what is the significance of using it ?


simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ The way you put in V2 as a DC voltage source it really makes no sense to have C1, R1, R2. But put in an AC voltage source and simulate it. \$\endgroup\$
    – Janka
    Jul 11, 2017 at 19:56
  • \$\begingroup\$ @Janka That was me, I must have grabbed the wrong component in the schematic. Alex, go ahead and edit the schematic to replace V2 with an AC source. \$\endgroup\$
    – Chris M.
    Jul 11, 2017 at 20:11
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    \$\begingroup\$ @ChrisM. you made the wrong edit, you do the correction and although the original diagram was poor and had an emitter resistor missing (corrected by words) there was absolutely no need to scaremonger about getting down votes for a schematic drawn in MS Paint. A lot of the diagrams I post have a final edit in paint and some of them are MS Paint originals (worthless of course) but not down-vote-worthy. \$\endgroup\$
    – Andy aka
    Jul 11, 2017 at 20:56
  • \$\begingroup\$ @Andyaka Didn't mean to scare-monger, and more than happy to fix my mistake, just that the OP doesn't need peer-review for his edits. I'm more than happy to work with MS Paint schematics, I just know that some regulars here are not. Just trying to be helpful, perhaps that didn't convey in the comment. \$\endgroup\$
    – Chris M.
    Jul 11, 2017 at 21:03

1 Answer 1


C1 protects the dc voltage at the base being altered by a signal source that has less-than-infinite dc resistance. It is vital that the bias conditions on the base is not compromised by the signal source impedance. C1 allows ac signals to superimpose on the bias voltage at the base.

A voltage divider bias sets a fairly guaranteed voltage at the base. The alternatives are trying to feed a precise current and relying on hFE OR using dc negative feedback to set the collector quiescent voltage at a mid point between the power rails.

Relying on hFE is going to end in failure because a stable value of hFE relies so much on temperature being kept constant and the transistor not ageing its hFE.

Using a feedback resistor is sometimes used especially when the emitter resistor is zero ohms because circuit gain can also be controlled BUT it is only really useful when input and output signals are not expected to be a large percentage of the power rail i.e. don't use it if you want a large peak to peak output voltage.


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