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Reversing the order of these transistors is useful in some settings, but may lead to less predictable switching for the reasons discussed above. 3V. This circuit is shown in column B above. This is why the MOSFET circuit shown in column B is always a bad idea, too. Often called "Romex" after one popular brand name, nonmetallic (NM) cable is a type of circuit wiring designed for interior use in dry locations. Using a single cable to connect to your monitor, power, and accessories is modern luxury. When the voltage is somewhere in between, though, both transistors may end up conducting, shorting the circuit - so caution must be exercised; this problem can be controlled by carefully biasing bases / gates using resistor-based voltage dividers, but it may affect switching performance. If you are not familiar with how they work or feel uncomfortable using one, it is best to leave it to the professionals.

The best thing to do is have a rewiring survey completed on your property. How can you tell if your house needs rewiring is a question we get asked a lot at Mr Electric. You will also probably stumble upon many other, more exotic transistor circuits not covered here - such as, for example, current mirrors or common base / common gate amplifiers - but you can probably figure them out by now. It can be alternatively fixed by adding a large pull-down resistor from the gate to the ground, to dissipate the deposited charge when the switch is opened, but at the expense of lowering input impedance. Obviously the knotted ends should be cut off, and then the twisted pair cable can be measured and installed in the equipment, or used where it is required, being cut to length as necessary. Its steel armoring helps it to defy any mechanical extremities that can harm the cable. It does, however, need bipolar switching: if you simply apply a positive voltage to the gate, and then disconnect it - the gate-source "capacitor" will stay charged, and the transistor will continue conducting for a longer while (dependent on humidity, handling, etc); even after this charge disappears, a new one can be easily accumulated due to further handling, parasitic coupling, and so forth.

Also note that the upper transistor, connected to the positive rail, is PNP or p-type MOSFET; and the bottom one, grounded, is NPN or n-type MOSFET. When the voltage is close to the positive rail, the situation will reverse; in effect, this arrangement is an inverting switch. Digital electronics are a class of easy to understand circuits that use discrete voltage ranges and square waveforms to transmit and process data - most commonly, representing binary numbers for use in Boolean algebra: a signal close to 0V is meant to signify "0", and a signal close to Vcc signifies "1". In NPN and PNP circuits, note the use of a resistor to limit the base-emitter current: the current flowing through this path must be controlled, because the corresponding junction is essentially a normal, forward-biased diode - and will conduct as much current as you supply, possibly destroying the transistor in the process (and certainly making it misbehave). X some time later, the resulting electromagnetic field propagates a 2X voltage swing across the capacitor; but this time, the reverse-biased diode will not conduct, and will prevent the already deposited charge from exiting the capacitor (except through any externally connected load).

The base-emitter voltage is kept near the diode conduction threshold, so the current flowing through this path is also very low and self-limiting (and naturally, avoided entirely in MOSFETs). The current supplied to the load remains in some clear relation to the input current (BJT) or voltage (MOSFET), for as long as the input remains in the "linear" range for the device. Column C shows another arrangement that is not universally problematic, but should be avoided in switching where possible - and is all-too-common in hobbyist work: loading the emitter (BJT) or drain (MOSFET) - a configuration known as "common collector" or "common source". The circuits in the A column show common, proper transistor switch arrangements for NPN, PNP, and MOSFET n-channel enhancement mode transistors - known as "common emitter" (BJT) or "common drain" (FET). The drawback of the transistor switches shown earlier is that they work in a manner similar to a single-pole switch: they can connect the load to the ground (NPN, n-channel MOSFET) or to the supply rail (PNP, p-channel MOSFET), or simply leave it in open circuit state. Voltage followers may sound boring, but they are immensely useful in electronic circuits; for example, they can be used to bridge a significant number of RLC band-pass or band-stop filters, or to convert very faint high-impedance currents from external sensors into outputs suitable for other circuitry.

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