Years ago the transistor was thought of as a voltage controlled resistor thus the name TRANsfer reSISTOR. Then someone decided it was a current controlled current source. The voltage controlled current source is popular today. The debate is clouded with the fact that ß is not a fixed factor it varies with current and voltage and temperature. In this series I'm not going to define the actions within the transistor but will run SPICE programs to demonstrate them. Hopefully the sims will speak for themselves. It would be a good time for you to run some too. You can vary the component values and probe the circuits to see what is happening.
EDIT: I assembled this one and it drives my earbud with the AF generator set to minimum. The signal drops out after insert a 20db attenuator.
|5 ua in the base produces 1ma collector current. I put the 1 ohm resistors in the circuit to make measurements and later they can be changed to see the effect.
|Stepping the load resistor from 100 to 5.2k ohm gives basically the same collector current. Injecting current into the base controls the collector current BUT there are other factors involved.
|Probing the collector voltage displays one of these factor. As we change the load resistor the collector voltage varies and this has some effect on the collector current.
|Probing the base current shows it is effected by collector voltage too.
|Fixing the bias and allowing the collector voltage to swing displays the current variation. (changing Vce)
|Adding a signal shows the current swing responding to it.
|adding R1 and probing its current shows the carriers in the collector will produce some current when given a path.
|I moved the ground reference to the emitter. In the previous shot it could appear the signal was driving the collector too.
|Applying 1 volt to the collector encourages collector current.
|With 2 volts it is stronger.
|With 6 volts the amp is looking much better.
|With a current source a transformer can be used to couple the signal. This transformer is 100mh primary and 50mh secondary. It produces good results as you can see.
|100uv in producing 15mv out with PNP germanium general purpose transistor. Time for a build!
|10 uv will drive an earbud.
|My old ears don't hear well below 800Hz so for me it is ok. You could adjust the bandpass by changing the capacitors.
|The gain drops well below my local AM station so that should not be a problem.