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You can use this arrangement to adjust the speed of a motor with a small potentiometer, or perform other non-critical tasks of this nature; but for anything that requires precision and repeatability, this is just poor engineering: people should be able to substitute transistors used in your circuit with comparable alternatives, or use a 5% accuracy input resistor, and still have it work. 100, and the load requires a current of 500 mA, at least 5 mA of base current should be supplied (supplying more will be wasteful and may lower switching speed, but the margins here are pretty wide). This ensures that both transistors are operating in common emitter / common source mode, suitable for switching. 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. 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 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. 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). 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). This has an interesting consequence: when the signal is a sine wave of a very low frequency, much lower than the capacitor charge time - the output voltage will, with a slight lag (phase shift), simply follow the input.

The circuit on the left is, essentially, a band-pass filter: the capacitor needs the signal to change slowly enough to charge it up to an appreciable level - and above this frequency, serves as a shunt; but when the current is not changing fast enough, the inductor will begin conducting and will discharge the capacitor. This circuit is, therefore, a simple, passive low-pass filter. This circuit passes higher frequency sine wave signals largely unaltered, but attenuates slow-changing sine waves - a high-pass filter. That said, this process has its limits: every stage needs to have an impedance much lower than the subsequent stage it is driving, which quickly leads to impractical or very inefficient arrangements; and every stage attenuates the desired signal to some extent, inevitably reducing signal-to-noise ratio. The most common use of an op-amp is probably building a "normal" single-input amplifier; the amplification is then equal to the ratio of the feedback resistor (R1) to the impedance of the other signal supplied the inverting input (R2, generally in the 10 - 100 kΩ, depending on the op-amp). Another important and more subtle use of these semiconductor devices is amplification, however - modulating output signals in relation to input voltage or current.

The previous section discusses the use of transistors as binary switches operated in their saturation region - that is, the point where the resistance is minimal, and the admitted current is at its peak. He is, perhaps, struggling with difficulties; but when he says he does not believe in a creative power, I am convinced he does not faithfully express what is in his own mind, He does not fully express his own ideas. Transistors differ from passive electronic circuits because of their ability to introduce gain - that is, turn low-amplitude voltage signals into high-amplitude ones, or high-impedance signals into low-impedance ones. In all cases, the driving voltage applied to the base (or gate) must be high enough to trigger the transistor; that is, at least 0.6V in BJT, and at least 1-2V for most MOSFETs. Because of the need to limit current, the NPN arrangement shown in column B is universally a bad idea: it will likely destroy the transistor; its PNP equivalent is equally disastrous. A brief review of some of the European systems that have been constructed will convince us of this. By delving into the inner workings of electrical cables, we can appreciate their significance and make informed decisions when it comes to electrical systems in our everyday lives.

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