It's bad design to make the circuit unnecessarily expensive and complex when something simpler and cheaper will meet all the specifications. You can reduce the bias point variation with temperature to almost any desired amount with the addition of extra circuitry, up to and including using op-amps to force a bias point. Whether it's good design or not really depends on the design parameters. Maybe you don't care much if that radio starts to distort a bit if someone is operating it at -40°, only that it works well from 10☌ to 35☌, and costs the minimum amount. You're also going to have to account for beta variation with temperature (and aging, especially at very high temperature, and radiation exposure), which depends on the required operating range, quite significant for military temperature range, relatively modest for consumer products. If you're amplifying an AC signal up to (say) 1V you don't much care about the DC voltage at the emitter. So your maximum swing is reduced a bit for those units at the extremes, but if your required signal levels are not pushing the limits of the supply you're fine. You build a batch of 10,000 pieces and you happen to get some transistors at the extremes. So you design the circuit for a beta of 100. Suppose beta can vary from part to part from 75 to 150 (2:1), which is practical for normal inexpensive binned Asian transistors. Now the reason is given as because it is dependent on beta.īut here what is meant by beta dependence? Does that mean two same models have different beta or does that mean beta is changing by time for the same unit? Now above assumes beta is known equal to 100 and is fixed and never changes. Using the KVL equation with this becomes: I neglect Vbe since it is small relative to the 7.5V I also take Ie=Ic=100*Ib. I will use beta and write KVL as follows: So the aim is to obtain around 7.5V(half-rail) at output when there is no small signal input. But I will still solve this by assuming we have a constant unchanging beta: Now I read that this is not a good design. This link is the main feature of transistor action.What I understand that, in a fixed bias BJT scheme the problem is beta dependence. Therefore if a transistor has a Beta value of 50, then for every 50 electrons flowing between the emitter-collector terminals one electron will flow from the base terminal.īy combining the expressions for both Alpha, α and Beta, β the current gain of the transistor can be given as:Īs seen from the equations above, electron mobility between the Collector and Emitter circuits is the only link between these two circuits. Beta values normally range between 20 and 200 for most general purpose transistors. NPN transistors are good amplifying devices when the Beta value is large. The current gain of the transistor from the Collector terminal to the Base terminal is signified by Beta, ( β ). The current gain of the transistor from the Collector terminal to the Emitter terminal, Ic/Ie, is a function of the electrons diffusing across the junction. The ratio of the collector current to the emitter current is called Alpha (α). Since the physical construction of the transistor determines the electrical relationship between these three currents, (Ib), (Ic) and (Ie), any small change in the base current ( Ib ), will result in a much larger change in the collector current ( Ic ). Note: “Ic” is the current flowing into the collector terminal, “Ib” is the current flowing into the base terminal and “Ie” is the current flowing out of the emitter terminal. The current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as A nother way to display a NPN Transistor is shown in Figure 2 below. The Base terminal is always positive with respect to the Emitter. The voltage between the Base and Emitter ( V BE ), is positive at the Base and negative at the Emitter. \): NPN Transistor schematic.įor a bipolar NPN transistor to conduct the Collector is always more positive with respect to both the Base and the Emitter.
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