Oscillators Questions and Answers

Answer: A

A relaxation oscillator charges a capacitor until a threshold is crossed, then discharges it, so its waveform is a ramp or an exponential rather than a sine and its output is naturally a square wave. A resonant circuit is what the sinusoidal family uses, an active device is essential, and the frequency depends directly on the timing components. The two families differ in whether energy oscillates between two storage elements or is repeatedly dumped, and every other difference follows from that.

Answer: B

Each half is held off while a capacitor discharges through a resistor towards the supply, and the time constant of that pair sets each half period. Supply voltage cancels out of the timing to first order, which is a real advantage, and the collector resistors set the amplitude and the edge speed rather than the period. A timing that depends on a resistor and a capacitor and not on the supply is what makes the circuit usable without regulation.

Answer: C

The capacitor charges towards the supply and the switching threshold is a junction drop, so a higher supply charges faster but has further to go, and the two effects largely cancel. No current source is involved, and saturation voltage is a small correction. That a timing circuit can be made insensitive to its supply by arranging the threshold to scale with it is a recurring idea, and it is why this simple circuit outlasted more elaborate alternatives.

Answer: D

The capacitor charges through both resistors and discharges through only the second, so the high time depends on their sum and the low time on the second alone, both scaled by the capacitance. Supply and load do not enter the timing, the comparators' thresholds are fixed fractions of the supply and so cancel, and reset and control are auxiliary functions. That the mark and space are set by different resistances is why a plain 555 astable cannot reach a symmetrical square wave without a diode across one resistor.

Answer: A

Applying the standard astable relation:

\[ \begin{aligned} f &= \frac{1.44}{(R_{1}+2R_{2})C} \\ &= \frac{1.44}{21000 \times 10^{-7}} \end{aligned} \]

which gives about 686 Hz. The factor of two on the second resistor is what accounts for it appearing in both the charge and the discharge interval, and omitting it is the usual error in this calculation.

Answer: B

Charging goes through both resistors while discharging goes through only the second, so the high time always exceeds the low time and the duty cycle cannot reach a half. The thresholds at one third and two thirds of the supply are symmetrical, and the output stage is not the issue. The standard fix is a diode across the second resistor so that charging bypasses the first, which makes the two paths equal - a good example of solving an asymmetry by giving each direction its own path.

Answer: C

A unijunction has a peak point beyond which its emitter current rises as its voltage falls, so once the timing capacitor reaches that point the device conducts heavily and discharges it. That negative resistance is what makes it a self-triggering switch rather than an amplifier. It has no useful current gain and provides no reference. A device whose characteristic has a negative-slope region can do things no ordinary three-terminal device can, and generating a sawtooth with two components is the clearest example.

Answer: D

The capacitor charges slowly through its resistor and discharges almost instantly through the device, giving a sawtooth, while the discharge current through the base resistor appears as a narrow pulse. That pulse is what made the circuit the standard trigger for thyristors. Neither a sine nor a symmetrical square wave arises, and a triangle would need equal charge and discharge paths. Reading a waveform off the charge and discharge paths is quicker than recalling it.

Answer: A

The two thresholds of a Schmitt trigger are exactly the upper and lower limits between which the capacitor charges and discharges, so the circuit needs nothing more than the trigger, a resistor and a capacitor. Its gain is not the point, the capacitor is essential, and amplitude follows the supply. A component whose defining property is a pair of thresholds is a natural fit for a circuit that needs exactly two, which is why the arrangement is so economical.

Answer: B

The period is a resistor times a capacitor, so the frequency inherits both components' tolerance and drift, and stabilities better than a per cent are hard to reach. The square output is often what is wanted, the component count is low, and these circuits work to megahertz. A frequency set by absolute component values can never compete with one set by a mechanical resonance, which is why every accurate clock in a system comes from a crystal and every cheap one from an RC.