As you may have already suspected, the collector voltage should hang at half the supply voltage in the absence of a signal so the collector voltage can have room to vary equally to a higher or lower voltage when a signal is finally applied at the base. For example, both the load and the collector voltage should have a baseling voltage of 4.5 volts with a 9 volt power supply. But in order to accomplish this, we need to find a way to keep the base-emitter junction forward-biased at 0.7 volts so as to allow current flow through the transistor. Otherwise, the collector will be closed off from ground and pulled all the way up to the supply voltage. But more importantly, without this current to keep the transistor active, any input signal or portion of the signal that is below 0.7 volts will not be amplified by the transistor, causing either distortion or no output response at all. A resistor (called the base resistor) connected between the base and the supply voltage will provide more than enough voltage to keep the base-emitter junction forward-biased.
Now, an incoming signal at the base will cause the collector voltage to shift from the baseline voltage of 4.5 volts, faithfully reproducing the signal at the collector, but at a much larger proportion. Let's say we are able to freeze an audio signal applied at the base and at that instant, we measure the signal to be .005 volts. With the amplifier operating with a gain of 200, the collector voltage will be reduced 1 volt to 3.5 volts. Likewise, an input voltage of -.005 volts will increase the collector voltage to 5.5 volts.
With the collector voltage set at half the supply voltage of 9 volts, the output signal can vary equally about the 4.5 volt baseline - up to the maximum possible voltage of 9 volts, or all the way down to ground.
An input signal voltage cannot change the base-emitter voltage above or below 0.7 volts, but it can increase or decrease the base current no matter how small the signal may be.
© 2007 Dave Cline. All Rights Reserved.
Choosing the correct component values