Note that Beta has no units as it is a ratio.Īlso, the current gain of the transistor from the Collector terminal to the Emitter terminal, Ic/Ie, is called Alpha, ( α ), and is a function of the transistor itself (electrons diffusing across the junction). The value of β can be large up to 200 for standard transistors, and it is this large ratio between Ic and Ib that makes the bipolar NPN transistor a useful amplifying device when used in its active region as Ib provides the input and Ic provides the output. The current in a bipolar NPN transistor is the ratio of these two currents ( Ic/Ib ), called the DC Current Gain of the device and is given the symbol of hfe or nowadays Beta, ( β ). However, this only happens when a small biasing current ( Ib ) is flowing into the base terminal of the transistor at the same time thus allowing the Base to act as a sort of current control input. Then we can see that the transistor is a current operated device (Beta model) and that a large current ( Ic ) flows freely through the device between the collector and the emitter terminals when the transistor is switched “fully-ON”. This link between the input and output circuits is the main feature of transistor action because the transistors amplifying properties come from the consequent control which the Base exerts upon the Collector to Emitter current. So in a NPN Transistor it is the movement of negative current carriers (electrons) through the Base region that constitutes transistor action, since these mobile electrons provide the link between the Collector and Emitter circuits. The Base supply voltage V B is connected to the Base resistor R B, which again is used to limit the maximum Base current. The Collector is connected to the supply voltage V CC via the load resistor, RL which also acts to limit the maximum current flowing through the device. But still - it is the input resistance (betwen E and common ground) and NOT the resistance between E and B.Ĭomment: A detailed analysis shows that the rather small input resistance at the E node can be explained as a result of negative feedback internal to the BJT (the relatively large E-B-resistance is reduced drastically to 1/gm due to negative feedback).Then the voltage sources are connected to an NPN transistor as shown. in the common base stage the input resistance at the emitter node is re=1/gm. There is only one case, where it makes sense to write re=1/gm. Hence, it is not a resistive element at all. The quantity gm is a "transconductance" and it does NOT describe the current-to-voltage relation between two nodes - and the same applies, of course, to the inverse 1/gm. The quantity 1/gm has "ohms" as unit (because it is the inverse of a conductance) but, in fact, it is not a resistive element at all. because they have read this somewhere) that the term re is something like a dynamic emitter-base resistance. My suggestion: Forget the term re and always use 1/gm instead of re. The term re does NOT represent any resistance between the nodes E and B. I don't quite understand how they represent the same thing aka the "dynamic resistance between the base and emitter terminals". Is it about where we look at the base from? They are both the dynamic resistance between the base and emitter terminals in my mind. I mean their definitions are same but yet they are different things. Even though the formulas yield this relation between r e and r π, I don't quite understand how they represent the same thing aka the "dynamic resistance between the base and emitter terminals". You might think what is my question here. Since there Ic = Ib × β this yields to r π = β × r e Secondly r π is the change in Vbe with respect to a change in Ib. So what I understand is that r e is the change in Vbe with respect to a change in Ic. It is the slope of the Ic Vbe plot at a fixed bias collector current i.e: g m=∂Ic/∂Vbe. I know the concept of transconductance g m. They both represent the dynamic resistor between the base and the emitter terminals.īut I read that they are different by a factor of β as: In BJT small-signal models there is both r e and r π parameters.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |