I really cannot figure out what I'm doing wrong here. in stationary mode, is infinite, which means that the drain current can be. I'm really confused.įor those old designs, the ones I initially thought should work, I duplicated the same circuit with my old 2N3904s, and the values were right on the money. 4.2.3.3.8 The Power MOS Transistor Transistors with an insulated gate are. It can be found using the slope of the output chracteristics Icf (Vce). Figure 9.1 Basic Amplifier Model The transistor, as we have seen in the previous chapter, is a three-terminal device. The EARLY voltage is a definition rather than a real existing and measurable quantity. What gives guys? I thought maybe the spec for the transistor drifts a lot when it's outside of the measured conditions (5V 2mA Ic), but I used a 5V supply as well and the results are the same. An ideal amplifier has infinite input impedance (R in ), zero output impedance (R out 0) and infinite gain (A vo ) and infinite bandwidth if desired. This time, the values agree! The emitter voltage is about 940 mV, and the base voltage is about 1.54 V. I worked out values of 18k for R1 and 2.8k for R2 (bias resistance of about 2.42k). So, with that, the expected resistance looking into the base is about 20k. Eventually, I tried to work out the beta value backwards, and it looks like the transistor has a real beta value of about 43-44, not even close to the claimed 420-800. I tried to reduce the total base resistance to reduce the loading effect, but that made it worse. I've built a few more circuits with different values. Needless to say, I'm definitely not getting 2mA from this source. I expected to be at least somewhat close to a volt, but this is less than half. The emitter voltage worked out to be 447 mV. The divider above should work out to a Thevenin Equivalent of about 28.7k, pretty dang close.Īfter the quick sketch, I built the thing and to my surprise my design did not work out at all. This means I'll have to aim for a base resistance of about 28k and a base voltage of about 1.5-1.6 To bias the transistor, I multiplied the emitter resistance by beta (assumed 600) and divided by 10 as a rule of thumb. I want to put the emitter at about 1V, so at 1V / 2 mA gives 500 Ohms, I went with 470 Ohms as the closest value I have. The actual range is actually like 420 - 800 or something.Īnyway, I was trying to build a current source for a differential pair and wanted to pump out about 2mA of current through the transistor, so I set it up like this for a quick test: Reading through the datasheet, (and on digikey's page), I am led to believe that the DC gain for this transistor is at minimum 420 2mA, 5V]. A BJT doesn’t have a channel, though, so we need a different name at some point people decided on the Early effect, after James Early, though you will see shortly that we could also call it effective-base-width. an IC.īut none explained what physical mechanism is responsible for the manufacturing variations, possibly because the question is out-of-scope of an applied electronics book.I bought these transistors on digikey: BC549CTA It turns out that an analogous phenomenon affects the operation of a bipolar junction transistor. Better matched parts are selected from the same wafer, and the best possible matching is possible when transistors are on the same silicon die, i.e. If one wants matched parts, one should purchase hFE-grouped transistors. Some variations also exist within the same batch. In most textbooks and tutorials, the common explanation is something similar to the following: The transistors are not coming from the same batch, so there will be variations in the manufacturing process. Beta varies from transistor to transistor, and changes with temperature. Beta (also called hfe) is the current gain. It's common sense that the current gain β (hFE) in a bipolar junction transistor is somewhat unpredictable, and one must design a discrete transistor circuit to be insensitive to the variation of β. He turns his knob until the current flowing from collector to emitter is equal to the current flowing from base to emitter times the beta of the transistor.
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