How does the transistors amplifying function differ from when AC signal applied and just a DC voltage applied? Thanks in advance

  • \$\begingroup\$ You should explain why you think that AC is treated differently from DC. Because: it isn't. A transistor generally cannot "see" the difference between AC and DC (assuming the AC is relatively low in frequency). \$\endgroup\$ Sep 23, 2018 at 19:24
  • \$\begingroup\$ DC is the bias operating point with hFE and AC gain is the impedance ratios defined at each pin and the relationships for each configuration, CB,CE,CC, while feedback can improve stability of gain ratio or DC operating points less dependent on hFE. \$\endgroup\$ Sep 23, 2018 at 19:38
  • \$\begingroup\$ If you search here; there are so many related answers \$\endgroup\$ Sep 23, 2018 at 19:42
  • \$\begingroup\$ @Bimpelrekkie ..sir I generally mean to ask that.. What is the difference between amplification in case of DC voltage and AC signal? \$\endgroup\$ Sep 23, 2018 at 19:47
  • \$\begingroup\$ If you share the specific circuit you're interested in, we'll be able to give you better answers. \$\endgroup\$
    – The Photon
    Sep 23, 2018 at 19:49

1 Answer 1


There is a difference in how AC gain is expressed vs. DC gain. DC gain testing is done at a few micro-amps to a milliamp and is sometimes called the beta or hFE of a bjt transistor.

AC gain is expressed as hfe and is the current gain around a common point or device. AC gain is often taken at several major frequencies such as unity gain, gain bandwidth product and full power gain. Full power gain is just a few percent of unity gain, so often there are many charts in a transistors datasheet.

The designers want to make it clear where a given transistor is useful with gain vs. frequency vs. 1/f noise, etc. This answer here helps out with the hFE vs. hfe issue.


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