#### nathalie

Joined Jan 9, 2011
14
What is the relationship between the voltage output swing, in the class A dc biasing of a Bjt transistor, and the Voltage swing obtained when a an Ac signal is introduced? How does the dc bias voltage effect the ac Voltage gain?

Thanks

#### t_n_k

Joined Mar 6, 2009
5,447
I guess the key might be the relationship between the static bias collector current and the small signal current gain - hfe. I think hfe is fairly stable over a wide range of Ic but it possibly reduces as Ic decreases & nears cut-off.

The venerable Audioguru is probably the go-to person on this stuff.

#### t_n_k

Joined Mar 6, 2009
5,447
Look at the data for the small signal BJT 2N3904 for instance....

#### Attachments

Joined Dec 26, 2010
2,148
The current gain of a common-emitter stage is dependent on hfe. This has a strong effect on the input impedance of the stage, but not on the voltage gain as measured from the base to collector.

The input impedance does influence the insertion gain of an amplifier driven from a significant source impedance, so in this situation the effective gain does strongly depend on hfe. This is particularly important in multi-stage amplifiers.

One approach to finding voltage gain is given on pages here on the AAC site. http://www.allaboutcircuits.com/vol_3/chpt_4/5.html

Here you will see the equation Av = β*Rout/Rin. This is valid, but from this you might be tempted to assume that β has a strong influence on voltage gain. Herein lies a fallacy: the point is that Rin is actually a function of β, approximately equal to β*(RE+0.026/Ie) at room temp. This gives us Av ≈ Rout/(RE+0.026/Ie), where RE is the un-bypassed emitter resistance, if any.

The base to collector voltage gain depends principally on the emitter current, any un-bypassed external emitter resistance RE, and the collector load impedance, including the transistor's own output slope impedance. This article may help with understanding. Note that equations dealing with voltage gain appear on the last page. http://www.seas.gwu.edu/~ece20/fall08/labs/tutorials/Tutorial5_Designing_Common_Emitter_Amplifier.pdf

It may be also useful to consider the hybrid-pi model. http://en.wikipedia.org/wiki/Hybrid-pi_model