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The chip is fairly dated and quirky, but it has a number of modern, power-efficient FET successors, such as ICM7555. 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. The MOSFET transistor shown in column A generally does not require a resistor, at least at low signal frequencies, because it does not allow any appreciably long-lived current to flow through the gate (at very high frequencies, the small but non-zero gate-source capacitance becomes a factor, though). Simple transistor LC circuits, such as Hartley, Armstrong, or Clapp oscillators make use of this operating principle.

Relying on this intrinsic gain is usually not a good idea for production-grade circuits, as the parameter can change significantly at a whim - but by looking at its general order of magnitude, you can see what input impedance would be appropriate. 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). Whether subtraction occurs depends on the length of the bent path. A variation of this circuit is called an envelope follower: in this case, the resistor and the capacitor are matched to form a low-pass filter that "masks out" a high-frequency carrier, but passes through a lower frequency envelope (in this use, no significant output loading is present). Naturally, to avoid surprises, the capacitor must be large enough not to form such a high-pass filter with the load (or the always weakly conductive diode) as to substantially attenuate the input AC signal. Its key flaw is evident once we recognize the capacitor and the resistors as a high-pass filter: this filter inevitably attenuates sufficiently low frequencies and DC voltage drifts.

Another simple (but imperfect) way to prevent this problem begins with AC coupling - the introduction of a series capacitor that prevents any steady DC currents or voltage levels from propagating through it, but allows time-varying signals to induce a voltage across it. The problem with this is that all real-world loads will develop some voltage across them in normal operation; this raises the emitter or source voltage accordingly - perhaps close to, or even above, the driving base / gate voltage. Ohm's law states that the current needed to develop a particular voltage across the resistor will be proportional to the desired voltage, and inversely proportional to resistance; if R1 is reasonable, so is the collector-emitter current. Diodes are also commonly used to build constant-current sources, such as this circuit: this arrangement will admit only as much current as needed to create a particular voltage across the constant "sense" resistor, R2, regardless of the potentially variable voltage drop seen across the connected load. There are uses where this arrangement makes sense - but switching is not one of them. When Vin2 is higher than Vin1, the right transistor will insist on getting the emitter voltage to a point where the left one no longer conducts - and so, the current flowing through the right R1 (and the associated voltage drop) will increase.

At this point, the left transistor will turn on, and create a negative voltage on the gate of the other MOSFET, turning it off for a while. While experimental computers not using clocks at all, or using individual non-synchronized clocks for various subsystems, were devised in the past, the extra complexity of doing so made them, at least so far, rather impractical in real-world applications. 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. Continuous-flex or flexible cables used in moving applications within cable carriers can be secured using strain relief devices or cable ties. Another important and more subtle use of these semiconductor devices is amplification, however - modulating output signals in relation to input voltage or current. As to the point of it all: differential amplifiers are extremely useful for subtracting RF interference or removing or adding bias voltages from input signals without the need to use AC coupling.

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