The blue dashed line shows the curve where we transition from saturation to active mode. After a certain point, the line becomes horizontal, and we only have the base current to change it in forward active mode. We can increase the output current by either changing the voltage across that C-E terminal or the current into the base. With a very low voltage across the junction, we have a linear curve. With very low base current, we're in the cut off mode, and the device doesn't do much of anything. Each increasing line shows an increase in the base current Ib. The axes here are Vce - the voltage across the collector-emitter junction, and Ic, the current through the collector. Ignore the thick red dashed line for now. Let's visualize this graphically now (courtesy Wikipedia). By changing the amount of current through the first loop, we can change much how current goes through the outer loop. The other loop goes from Vc to Rc to the collector to the emitter back to Vc. One loop goes from Vb to Rb to the base to the emitter back to Vb. We can think of this circuit as two current loops. The output current now is only dependent on the input current to the base Here, the output current has a linear dependency on the output voltage, as well as the input current Not much! Transistor just sits there doing nothing, conducting no current In text form, it's a bit dense, so let's put it into a table (where Vc, Vb, Ve is the voltage at the collector, base, emitter, respectively). Lastly, if we raise them both but keep Vc greater than Vb, we go into forward active mode, where the output is only a function of the input current. Here the output is dependent on both the input voltage and current. If we raise both Vb and Vc, but keeping Vb larger, we move into saturation mode. If they're both very small, the transistor is just off, and doesn't do anything. Like I said before, we have three modes to choose from, all of which are dependent on where we put the voltages Vb, Vc. This is a bit weird to think about without a circuit, so let's make one. For an npn BJT, applying a current through the base-emitter means we'll allow current to flow through the collector-emitter. By changing the current to the base, we can can change the current through the collector. We have three terminals: base (perpendicular in the schematic), emitter (with the arrow), and collector (the other one). The easiest way to think of BJTs is to imagine them as electronic switch. We can think of it as two diodes placed back to back in series, so we have two types of BJTs: npn (np-pn diodes), and pnp (pn-np diodes) This is our first three terminal device, and accordingly, it has three states. These are significantly harder to understand from a solid state physics background, so I'm not going to talk about that here in too much detail. They're used for gain, filtering, buffers, converters, mixers, and so many other core blocks. Bipolar junction transistors (BJTs) are some of the most common elements in analog circuit design. When operating in a close-loop configuration, the amplifier works in a negative feedback loop and stabilizes the circuit.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |