Digital oscillators, timers and dividers
Figure 14.1(a) shows an astable constructed from bipolar junction transistors (BJTs). The circuit is clearly seen to be two RC coupled inverting ampliﬁ er stages, with the output of the second stage fed back to the input of the ﬁ rst. Figure 14.1(b) shows the more usual way of drawing the astable with cross-coupling. The frequency of oscillation depends on the values of the time constants, formed here by the capacitors and resistors marked C1, C2, R1 and R3. The output of this circuit is approximately a square wave, and by using more advanced circuitry, the shape of the wave can be almost a perfect square shape, with very low rise and fall times.