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The resistor on diode side should be selected to limit the current through the diode to a reasonable value, so that it has an opportunity to develop a voltage across its terminals in the first place, but so that it does not waste too much power. Adjacent channel interference - When the station you want is not receivable because of a much more powerful station in the next channel above or below, you have adjacent channel interference. 3V. This circuit is shown in column B above. 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 capacitor will slowly discharge through R1 - but R1 can't be arbitrarily low (the quiescent current flowing at all times through the follower would be rather impractical if we go too far, and input loading would increase, too). 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.

Reversing the order of these transistors is useful in some settings, what are electric cables but may lead to less predictable switching for the reasons discussed above. An unexpected low-pass filter distortion seen in a digital signal is usually indicative of excessive capacitance of the signal path, perhaps because the connection is too long, or runs too close to others; while a high-pass pattern may indicate a broken trace or cut wire, forming an unwelcome capacitor in series with the source of the signal. As with RC and RL filters, the gotcha with RLC circuits is that in signal processing, the impedance of the driven load must be significantly higher than that of the signal source and the series resistor in the circuit - or else, distortion will appear. 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. It must have a very low noise figure, and enough gain to overcome the cable loss and the receiver’s noise figure. So, instead, let's have a look at a more useful, if still modest, type of a simple transistor circuit: a voltage follower, also known as a buffer.

The behavior of the first circuit - known as a half-wave rectifier - should be fairly clear: the diode conducts, and therefore creates a voltage across the resistor (a dummy load), only if the first input is more positive than the other; in this circuit, diode breakdown voltage is selected high enough not to interfere with this process. 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. NM cable usually contains one or more "hot" (current-carrying) wires, a neutral wire, and a ground wire. This is more of a deal here with voltage followers than it is with switches: consider driving a capacitor as a load, connected across the "out" node and the ground. The second circuit - a bridge or full-wave rectifier - is a bit more clever, but also easy to follow: opposing pairs of diodes are used to select the more positive or negative out of two input leads, and always produce a particular output polarity.

Still, you are probably wondering about a different task more commonly associated with amplifiers: increasing the amplitude of a weak signal. Note that in both cases, the high-pass filter peak output amplitude is nearly twice the input amplitude; only the average power of the signal (RMS) is affected when the input frequency changes. This switch, by itself, is a NOT gate: it pulls the output to Vcc if input is 0V, and vice versa. Until then, the follower will not be following the input voltage at all. In many cases, this is not a big deal - the capacitor and the resistors can be selected with the interesting range of frequencies in mind; but fundamentally, the follower is no longer maintaining direct relationship between input and output voltages - and merely between their rates of change. Low-pass and high-pass filters can be cascaded to form band-pass or band-stop filters; identical filters can also be stacked to achieve a steeper response curve (n-th order filters, with gn frequency transmission function).