10/2/2023 0 Comments Emitter base voltage![]() Breakdown occurs when the electric field becomes strong enough to pull electrons from the molecules of the material, ionizing them. If the voltage applied across a piece of insulator is increased, at a certain electric field strength the number of charge carriers in the material suddenly increases enormously and its resistivity drops, causing a strong current to flow through it. However, if a strong enough electric field is applied, all insulators become conductors. Applying a voltage causes only a very small current to flow, giving the material a very high resistivity, and these are classed as insulators. In contrast in materials like plastics and ceramics all the electrons are tightly bound to atoms, so under normal conditions there are very few mobile charge carriers in the material. An electric field causes a large current to flow, so metals have low resistivity, making them good conductors. For example, in metals one or more of the negatively charged electrons in each atom, called conduction electrons, are free to move about the crystal lattice. The force of the field causes the charge carriers within the material to move, creating an electric current from the positive contact to the negative contact. An electric field is created across a piece of the material by applying a voltage difference between electrical contacts on different sides of the material. A conductor is a substance which contains many mobile charged particles called charge carriers which are free to move about inside the material. They are making a current amplifier curcuit, but the transistor itself is still controlled by the voltage of the current source.Materials are often classified as conductors or insulators based on their resistivity. That is what folks are doing when they attach a current generator to the base of a BJT. You can make just about anlything with voltage amplifiers (op-amps), true current amplifiers (magnetic amplifiers), BJTs, FETS, vacuum tubes (transconductance amplifiers) by incorporating them into a circuit. So you don't have a simple transistor anymore. Now, when you hook up a current source to a BJt, you are in effect insereting a high voltage source in series with a big fat resistor value into the base circuit. The physics ofthe transistor determine what is controlling the collector. ![]() It is the voltage on the base terminal that is controlling the collector current by modulating the back-voltage of the emitter-base junction. Fortunately or not, this waste base current is proportional to the collector current and fools a lot of folks into thinking the base current is controlling the collector current when it is not. The higher the transistor beta, the less waste current. This causes a waste current to exist in the base circuit that does absolutely nothing to control lthe collector current. Unfortunately, some of the electrons that meke up the collector current get diverted into the base circuit by the positive base voltage. By applying a positive voltage to the base, the back-voltage is lowered and the charge carriers (electrons in this case) can whiz through the very thin base (human hair size) and be sucked up by the higher voltage of the collector. This creates an increasing back-voltage that stops the migration when a equilibrium is reached. When they do, they leave behind positive ions in the emitter and create negative ions in the base. The emitter is highly doped with N-material and has plenty electrons itching to diflfuse into the very thin P-base and fill up those holes. "That proves a BJT is a current controlled device, right?" Wrong! They are ignoring the physicsof the BJT. I have heard folks say to me many times, "You can look at it two ways, voltage or current." Then they try to show that when they drive the base with a current supply, the collector current shows a rather linear relationship within a reasonable range. Click to expand.That is absolutely wrong! A BJT by itself is a transcondance amplifier (voltage controls current), just like a vacuum tube and FET.
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