raywl

I'm taking a basic EP class and on our pre-test there is a question about a common emmiter circuit. The question is as follows; If the circuit is operating above optimum gain, what must be done to the resistor which aids in detemining the correct amount of fwd bias to increase gain. The choices are increase resistance, decrease resistance or nothing. No where in the book does it speak of optimum gain of anything. Is this question a miss type or am I missing something.

Thanks

k7elp60

I can tell you this, the gain of a common emitter transistor amplifier is determined by the ratio of the collector load resistor divided by the unbypassed emitter resistor.

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audioguru

The question talks about increasing the forward bias to increase gain.
I can sim a transistor without any negative feedback and a high output level. If its bias is low then its output is so distorted that its gain looks low. The transistor isn't clipping, it is just very badly distorting.
With more bias then the transistor isn't as distorted and its gain looks a little higher. Maybe that is what they are talking about.
Here is a pic of a transistor distorting with low forward bias:

Attached Images

transistor distortion.PNG (21.0 KB, 12 views)

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kchriste

If the there is no unbypassed emitter resistor then remember that for a small signal transistor rE = 25mV/I and that AV=rC/rE
Where;
AV = gain
rC = collector resistor
rE = AC resistance of emitter for a given emitter current
So it can be seen that decreasing the base resistor results in increasing the emitter current which lowers rE which increases the gain. All to a point in theory, because transistors in the real world have lower Beta at higher currents.

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