There is no easy to understand direct relation so as a beginner, don't worry about this too much. I never (need to) use \$\alpha\$ f, I always use \$\beta\$ (same as \$h_\$ increases and that is due to base with modulation. The bipolar transistor uses one more layer of semiconductor material to produce a device with properties and. Unlike semiconductor diodes which are made up from two pieces of semiconductor material to form one simple pn-junction. This can for example be seen in opamp circuits where overall gain is defined by resistor ratios (which can be quite precise), where the equation's accuracy is acceptable if the opamp gain is 'high'. The Bipolar Junction Transistor is a semiconductor device which can be used for switching or amplification. This often takes the form of 'sufficiently large' or 'close enough' calculations. Good circuits will setup components so that the performance remains acceptable (constant, or 'high enough') as component parameters may vary. Physically, however, a (BJT) transistor can be considered to be controlled by the base-emitter voltage but since the current is exponentially dependent on this value, it is not generally used this way in a circuit (except in a current mirror).Ī lot of circuit design involves making approximations or estimates of component performance because component parameters (especially transistors) are not precise, and may vary with temperature or current levels). It is quite constant over a range of current levels, and so is a useful parameter to use for calculations. Beta for a transistor depends on the ratio of emitter doping to base doping, as well as the width of the base diffusion. The physical significance is that these parameters relate the effects of (usually considered non-ideal) base currents to collector currents. A transistor’s Beta value, sometimes referred to as h FE on datasheets, defines the transistor’s forward current gain in the common emitter configuration. Transistor Basics The base is lightly doped and sandwiched between the collector and the emitter. The three layers are called the emitter, base and collector. There are two pn junctions in the transistor. The model parameter SUBS facilitates the modeling of both vertical and lateral geometrics. For BiCMOS devices, use the high current Beta degradation parameters, IKF and IKR, to modify high injection effects. However, in a common-emitter circuit, it may be more intuitive to consider β as the non-ideal factor, and in common-base circuits to use □. A Bipolar Junction Transistor is a three layer (npn or pnp) semiconductor device. The BJT model is used to develop BiCMOS, TTL, and ECL circuits. After all, it's not like we've found some transistor that is all encompassing and perfect in every scenario.□ and β are directly related - □ = β/(β+1) It doesn't matter what circuit configuration is used (or if any circuit is used at all - these parameters are properties of the transistor, not the circuit). Also, taking the time to calculate Rin and Rpi never hurt either.įinally, at the end of the day, if β is really important, Mosfets and Jfets have an almost infinite β (~10^9 -> ~10^12). (which isn't hard for most signal level transistors)Īs for factors and considerations for varying β, I've never personally bothered to take any other than "use the worse case β." If the β, is higher, then you get less current draw from the source: Win. An example of forcing parameters is using a current source to bias the transistor and then simply ensuring that there's enough current and voltage on the input to keep it linear. An example of negative feedback is using emitter degeneration resistors in Common Emitter amplifiers. You can use either negative feedback, or you can force a certain parameter set, and allow for variation. But of course, you can also solve the problem the other way round: Assume that the BJT is saturated. Find up to which value of the assumption of point 1 is met, that is, find the maximum value of for which VCB VC VBE > 0 V C B V C V B E > 0. What general trends and rules of thumb are used to make sense of the varying β value? What are the factors and considerations for the varying β when you are designing transistor circuits? Calculate the collector potential VC V C as a function of.
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