Biasing

Why biasing is important? I've laid out 3 examples. 1. In the first case the input signal is 300mV. The minimum base voltage is 700mV. So using 300mV will ensure that the transistor never conducts because the input never reaches the 700mV threshold. The first amp is inoperable. (yellow) 2. The second one is somewhat different. Same component layout but this time the input voltage is 2V. This time it reaches the 700mV minimum threshold. But the input voltage falls beneath that threshold every now and then leading to hard cutoff distortion. See every time the signal passes through the base threshold it activates the transistor (shortly) after which it goes back to zero (and then the megative cycle comes) which disables the transistor from further amplifying the wave. So this amplifies only a small part of it. Sometimes tgis can be used in a class C amp where an output filter restores the signal but that's only for RF applications. (blue) 3.In the 3rd case I've chosen a safe operating point at the middle of the voltage supply. This way the transistor is always on <more or less> which enables it to amplify the whole wave without bringing it to cut off distortion. This way the input wave does not fall beneath the 700mV nor does it go above the maximum operating voltage effectively ensuring it wont be brought to cut off distortion. This operation mode is called class A. As i said the transistor is always conducting more or less so Efficiency levels are around 25% but fidelity is way higher compared to other classes. (green)

published 11 years ago