# NPN transistor

Discussion in 'General Electronics Chat' started by aamirali, Mar 10, 2012.

1. ### aamirali Thread Starter Member

Feb 2, 2012
415
2
1. can gain of npn transistor be less than 1.

2. When ac signAL is applid to base, & with it bias is applid (>0.7V) for forward bias, then what's max amplitude of ac signal that an be applied

2. ### jimkeith Well-Known Member

Oct 26, 2011
539
100
1. Possibly? Try inverting a transistor so that the collector is the emitter and the emitter is the collector--apply a low "collector" voltage and measure the gain--try a few devices, both NPN and PNP--observe if there is a slope to the hFE vs Ic curve.

2. Quite low for common emitter config--in the order of 10 to 100mV P-P --depends upon other circuit parameters too

Even with hFE = 1, significant power gain is possible--although hFE is involved in power gain, it is not as important as one might think

Last edited: Mar 10, 2012
3. ### wmodavis Distinguished Member

Oct 23, 2010
739
151
1. It depends on the circuit configuration it is used in and whether you are talking about current or voltage gain.

2. It depends on the circuit configuration it is used in and whether you are talking about current or voltage.

It would sure help to give a more definitive answer if you supplied a schematic.

4. ### aamirali Thread Starter Member

Feb 2, 2012
415
2
1.Its an normal CE amplifier (NPN). Can voltage gain go less than 1.
2. I don't think current gain go below 1. Its very high in CE configuration.
3. I asked about range of ac voltage applied in CE amp, because in it dc bias is provided at base so that vbe>0.7. So what range of ac signal can be applied

5. ### studiot AAC Fanatic!

Nov 9, 2007
5,003
522
1)The voltage gain of a CE stage is approximately

Beta x Load Resistance / Input resistance

If you require a gain of less than unity how would you arrange that equation to suit?

2) Yes, Vbe is around 0.7 volts. But 0.7 volts above what? gound? (grounded emitter) or, as others have said, some other biasing level? If the emitter is grounded then the max undistorted sinusoidal input signal is 0.7 volts peak to peak. However if the signal consists of a pulse train then this is limited to the breakdown voltage of the base - emitter junction, which is a property suplied by manufacturer's data sheets and given by two parameters Vcbo and Vebo.

Last edited: Mar 12, 2012