![]() Two and vary the base current while trying to maintain collector current Thus, to ensure that the transistor is in saturation, the current has toīe significantly higher than the minimum gain which is spec'ed atĪ good lab exercise is to wire the transistor up to a power supply or That this represents a forced gain of only 10 now. Note also that on the data sheet saturation isnt specified untilĪbout 400ma of base current for 4 amps collector current, and note Note how the voltage characteristic changedįrom roughly parallel to the Y axis to roughly parallel to the X axis Voltage drops more and more, and eventually becomes roughly With increased baseĬurrent eventually the transistor starts to enter saturation, and this isĮvident by noting where the voltage starts to curve drastically.Īs the transistor enters farther into saturation, the collector emitter Though and this is evident by noting that the collector emitter voltageĬharacteristic is roughly parallel to the Y axis. Note that the blue point marks 4 volts and aboutĢ50ma. To help understand saturation, we'll look at the graph for a collectorĬurrent of 4 amps. Here is a graph of collector emitter voltage for four different collectorĬurrent levels (blue, 1, 2, 3, and 4 amps) for a sweeping base currentįrom 10ma to 500ma for the 2N3055 transistor. * SPICE deck TIP3055 model comes right out of the data sheet: Question 4: does it sound like I am on the right track here? What am I missing? I'm probably still a dope no matter what but. To the extent that I think the estimations for V CE and V BE must be "close enough". When I turn around and run all this through SPICE*, the numbers** for I C, I B, V CE, and V BE all come out pretty close, actually. Question 3: The datasheet "On" characteristics shows V BE = 1.8V What is this? Should I be using 1.8V instead of 0.7V for V BE for saturation? So.įinally we get to the point of selecting R B to ensure 48mA base current. I Bsat where the overdrive factor is 2-10, so sayeth Sedra and Smith. ![]() Now, the book says to select I B = Overdrive-Factor = 480mA ÷ 100 (again using h FE at 0.5A from the datasheet) Moving on, the next step is to figure out base saturation current: ![]() How does that relate to the above? Would I use these higher voltages to more accurately calculate for higher I C values? Using a TIP3055, the datasheet says something like V CE=1.1V at I C=4A and V CE=3V at I C=10A. Question 2: The datasheet specifies a V CE under the "on characteristics" for various currents. The book estimates V CE ≈ 0.3V for further calculation. The book talks about how, when driving more than the above I B into the transistor, the CB junction becomes forward biased at about 0.4-0.5V and as a result, V CE is around 0.2 to 0.3V. As you increase I B, β (well, the book calls it β forced) starts to fall. Question 1: β is the same as h FE for an I C of ≈.5A from the datasheet? (Looking at the ON Semi datasheet.Īs I understand it, the idea is that if we drive more than 4.6mA into the base, the transistor enters into saturation, with no significant (or, more to the point, no proportional) gain in I C. This collector current is achieved by driving Start by finding the maximum I B where the transistor is in active mode set V CB = 0 and you get: My book (Microelectronic Circuits, Sedra & Smith) goes through a pretty simple process to make calculations for driving the transistor into saturation. So, I need to find an R B to drive the transistor into saturation. Let's say that R M is given to be 18Ω and V CC is given to be 9V. M stands for motor even though it is a pretty poor approximation of a motor! I'm just telling you that because you may wonder why I am using R M for the collector resistor rather than R C. Eventually I will make this more complex and attempt to drive a small DC motor, but that's a long ways away. The goal is to understand BJT saturation and to design a simple circuit by selecting R B to ensure the BJT is in saturation with a decent overdrive factor. (With more of a software focus, as you will no doubt be able to tell) Also had a couple of questions about interpreting bits of a datasheet. I just wanted to run this by the gurus to 'check my work' and see if I am getting it or if I'm a dope. ![]() I've already Googled this topic and consulted my old EE text so I think I have the right info.
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