![]() This allowed much better performance than the grown-junction transistors, but it was still an inherently slow process as each transistor had to be made individually. These were produced by taking a piece of doped semiconductor and fusing beads of dopant metals of the opposite type onto it, where the alloyed areas would act as the opposite-type semiconductor to form a PNP or NPN structure. Later, alloy-junction transistors would be made. This works, but the transistors produced have poor characteristics by modern standards and they're quite expensive. Circuit designers use PNP transistors to build current sources (whose output current has the algebraic sign "+"), and they use NPN transistors to build current sinks (whose output current has the algebraic sign "-").The first transistors that would be recognised as bipolar junction transistors are the grown-junction transistors.¹ These were made by growing a germanium (later silicon) crystal while changing the composition of the melt it's grown out of by adding dopants. (A source whose output current is constant, independent of its output voltage). Circuit designers use one polarity of bipolar junction transistor to build a high side switch, and the other polarity of BJT to build a low side switch.įinally, there are some situations in which a sensor or other piece of electronics, must be fed from a near-ideal current source. ![]() Other circumstances call for a "high side switch" (in which one terminal of the load is connected to the -supply, and the other terminal of the load is connected thru the switch to the +supply). Some circumstances call for a "low side switch" (in which one terminal of the load is connected to the +supply, and the other terminal of the load is connected thru the switch to the -supply). Circuit designers use one polarity of bipolar junction transistor for the first kind, and the opposite polarity of BJT for the second kind.įurthermore, sometimes it is desired to switch a load on and off such as a motor, a heater, an I/O peripheral. In "switching applications" (a class which includes digital circuits as a subclass), sometimes the input signal is referred to the most negative supply and other times the input signal is referred to the most positive supply. They're simpler to use and usually perform better. PS: I would recommend using a FET instead of a BJT for basically anything you might want to use a transistor for, unless there's a specific reason to go with the BJT. There's also potential issues with the naive low-side drive if you have high enough currents that the transistor emitter/source voltage increases significantly above 0, in which case you can wind up with your transistor not being turned on fully and dissipating too much power. Another power example is the buck converter, which requires a high-side switch.Įven if you're just switching a single load, there's advantages to switching on the high side: there's a greater degree of safety, since it's harder for the wire you're switching to short to positive than to ground (especially if you're operating in a grounded metal box or chassis), and your device is not live when it's turned off. This also happens in power amplifiers where you create push-pull circuits, and CMOS logic also uses a similar arrangement. In power circuitry this is most common in the H-bridge, where you need to be able to send current through the load in either direction. ![]() ![]() This is sometimes worth the trouble in power circuits because PNPs/P-channels are worse (higher resistance) than NPN/n-channels with the same rating/price.Īs to why switching on the high side versus the low side matters: In some cases it's unavoidable that you have to be able to switch on both sides or on a specific side. You can use either kind of transistor on either side, but that requires that you have a voltage which is above the positive side of what you're switching or below the negative side. This is a consequence of how they are driven. In general, PNPs or p-channel FETs are used on the 'high side', to 'push' current into the load, and NPNs or n-channel FETS are used on the 'low side', to 'pull' current from the load. ![]()
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