![]() ![]() For the same quiescent current Ic in both cases the voltage gain will be the same. One simple example makes it clear: In a common emitter stage replace a BJT with a "current gain" of 100 with another BJT having a current gain of 200. And for many calculations (circuit analysis) we are allowed to treat the BJT as if it would be current-controlled (because of this fixed relationship between Ib and Ic). On the other hand, of course it is true that there is a - more or less - fixed relation between Ic and Ib. Instead, the collector current is controlled and determined by the base-emitter voltage Vbe only! That is a proven fact. I know that some participants of this thread will heavily disagree with me - but I think one should mention the BJT - from the physical point of view - is NOT a current-controlled device. ![]() However, they are small, simple, and tend to work. Specifically crafted amplifier circuits tend to provide far higher quality amplification. Another limit is that they are not very linear. This constrains their high frequency use. One limit is that their frequency response is limited by two transistors, not just one. Betas of 10,000 or 1,000,000 are achievable in these situations (limited by noise, of course).ĭarlington pairs have some limits. In theory, this pattern can be stacked as far as you like, each layer multiplying the total beta by the beta of that transistor. If we use two signal transistors (rather than one signal transistor and one power BJT), we can't handle the high currents, but we can have extremely high gains. If we only consider amplifying, a second major use case appears. Indeed Darlington pairs are sometimes used to power motors in the switching mode. The above situation works well in both switching and amplifying modes for the transistor. The base of this BJT is fed by the entire combined current flowing through the smaller transistor, so the total current flowing from its Collector to its Emitter is even larger. The second transistor is a power BJT, which has a much lower gain, but better maximum currents. The first transistor is a normal high gain BJT, multiplying the Base-Emitter current and permitting a large multiple of that to flow from Collector to Emitter. A darlington pair allows you to combine the best of both worlds. They make "power BJTs" which are optimized to have a high saturation current, but it is very difficult to make such power BJTs with a high gain. The second state, at higher currents, is known as the "saturation" state, where the Collector-Emitter current is relatively constant with respect to the Base-Emitter current.ĭarlington pairs often appear in situations where high current amplification is needed. The constant of proportionality of this relationship is called the "beta" of the transistor (often on the order of 100). The first is the "active" state, where the Collector-Emitter current is proportional to the Base-Emitter current. The relationship between these currents is well approximated by considering two regions. The more current travels through the Base-Emitter path, the more current is permitted to pass through the Collector-Emitter path. There are a few voltages that matter, such as the voltage drop between the base and emitter, but as a general rule, it's the currents which matter for BJTs.īJTs are current amplifiers. The key here is understanding that transistors (specifically BJTs) do not operate on voltages, but rather on currents. ![]()
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