![]() ![]() This renders the value V CE = 7.5 V to remain as a constant throughout this vertical line. The rule that restricts V CE = constant demands the vertical line to be drawn in a way that it cuts through the operating point at V CE = 7.5 V. Now let's see how we can determine β ac for a region of the characteristics identified by an operating point having values I B = 25 μa and V CE = 7.5 V as shown in Fig 3.17. Here too the letter h is used for identifying the h as in the phrase hybrid equivalent circuit, and fe is derived from the phrases forward current gain and common- emitter configuration.įig 3.14a shows the best method of implementing the Eq.3.11 through a numerical example, with a set of characteristics, and this is produced again in Fig. ![]() Here we can see that the difference is only the in the lettering of the fe, which are in lowercase compared to the uppercase as used for β dc. In BJT datasheets, β ac is normally shown as hfe. However, for the equation 3.11 we find it possible to explain it with some clarity, and furthermore it also allows us to find the value of α ac from a derivation. In this section we avoided the procedure of determining the value of α ac from the characteristics curves due to the involved complexity of measuring the genuine changes between the I C and I E over the curve. The format of equation 3.11 quite resembles the format of α ac as discussed in our earlier section 3.4. When alternating current or an ac is involved, the beta magnitude is expressed as shown below: The subscripts F in ( hFE) is extracted from the phrase forward-current amplification and the term E is taken from the phrase common- emitter in a BJT common-emitter configuration, respectively. ![]() In this term the letter h is inspired from the word hybrid as in transistor hybrid equivalent ac circuit, we will discuss more on this in our upcoming articles. When you check datasheets you will find that the β dc of a transistor being represented as the hFE. To be more precise, if a BJT is specified with a beta value of 200, signifies that the capacity of its collector current I C is 200 times more the base current I B. These values provide us an idea regarding the magnitude of the currents between the the collector and base of the BJT. In real transistor circuits, the value of beta for a given BJT may typically vary within a range of 50 to 400, where the approximate mid-range being the most common value. Where the quantities are established over a specific operating point on the characteristic graph. ![]() We will find the similarity between ac and dc betas, and also prove through formulas why the factor beta is so important in BJT circuits.Ī BJT circuit in the dc bias mode forms a relationship across its collector and base currents I C and I B through a quantity called beta, and it is identified with the following expression: In this article we will discuss regarding beta ( β) and what is hFE in BJT configurations. That way the large variation in \$\beta\$ is not an issue.In other words, if the BJT uses relatively higher current to switch its collector load optimally then it has low β (beta), conversely if it is able to switch the rated collector current optimally using lower base current, then its beta is considered high. In most circuits, having a \$\beta\$ that is larger is OK. Because the base voltage is kept close to ground by the NPN BJT (about a diode drop above ground), the current developed by \$R_\text\$. Here, \$V_1\$ is a variable voltage source or else a ramp or sawtooth voltage. Simulate this circuit – Schematic created using CircuitLab Here's a simplified diagram using an NPN under test: You'll want a method for varying the base current in a "smooth" fashion over some desired range. Enough to be annoying.ĭesigning and using your own system for estimating \$\beta\$ can be fun, though. It's more likely that if you buy two different multimeters they will read somewhat different values for the same BJT. And their value usually isn't particularly accurate, either. There is no international standard used by multimeter manufacturers. Your question pretty much sums up the problem, doesn't it? You write, ![]()
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