Third-order, high-pass filter used as the basis of a phase-shift oscillator. Here, three identical C-R high-pass filters are cascaded to make a third-order filter that is inserted between the output and input of the inverting (180° phase shift) amplifier the filter gives a total phase shift of 180° at a frequency, fo, of about 1/(14RC), so the complete circuit has a loop shift of 360° under this condition and oscillates at fo if the amplifier has enough gain (about x29) to compensate for the filter’s losses and, thus, give a mean loop gain fractionally greater than unity.įIGURE 2. The simplest C-R sine wave oscillator is the phase-shift type, which usually takes the basic form as shown in Figure 2. The frequency-selective feedback network usually consists of either a C-R or L-C or crystal filter practical oscillator circuits that use C-R frequency-selective filters usually generate output frequencies below 500 kHz ones that use L-C frequency-selective filters usually generate output frequencies above 500 kHz ones that use crystal filters generate ultra-precise signal frequencies. If the gain is below unity, the circuit will not oscillate, and if greater than unity, it will be over-driven and will generate distorted waveforms. The second requirement is that the amplifier’s gain must exactly counter the losses of the frequency-selective feedback network at the desired oscillation frequency, to give an overall system gain of unity, e.g., A1 x A2 = 1. Essential circuit and conditions needed for sine wave generation. Thus, if the amplifier generates 180° of phase shift between input and output, an additional 180° of phase shift must be introduced by the frequency-selective network.įIGURE 1. First, the output of its amplifier (A1) must be fed back to its input via a frequency-selective network (A2) in such a way that the sum of the amplifier and feedback network phase shifts equals zero degrees (or 360°) at the desired oscillation frequency, i.e., so that x° + y° = 0° (or 360°). To generate reasonably pure sine waves, an oscillator has to satisfy two basic design requirements, as shown in Figure 1.
#BIPOLAR SQUARE WAVE GENERATOR#
Next month’s edition of the series will deal with practical multivibrator types of bipolar waveform generator circuits. This month’s installment describes practical ways of using bipolars in the linear mode to make simple, but useful sine wave and white-noise generator circuits. Its RMS value is given in (11).The two most widely used types of transistor waveform generator circuits are the oscillator types that produce sine waves and use transistors as linear amplifying elements, and the multivibrator types that generate square or rectangular waveforms and use transistors as digital switching elements. Its RMS value can be calculated from equation (5), where D = 1/2. The square wave in Figure 3 is a pulse signal with 50% duty-cycle. Knowing the RMS value of a pulse waveform we can easily calculate the RMS value of a periodic square signal. The total RMS value of the bipolar pulse waveform is then calculated by applying the square root of the sum of squares of u11 RMS and u12 RMS.Īfter calculations, the RMS value of a bipolar pulse waveform isĪs you can see, the bipolar pulse RMS value does not depend on its duty-cycle, and it is equal with its amplitude. In a similar way, we can calculate the RMS value of u12(t): The RMS value of u11(t) is identical with the one shown in equation (3). Where with u11(t) and u12(t) I noted the two sections of the waveform in Figure 2. To calculate its RMS value, let’s split the signal in two: from 0 to t1 and from t1 to T as in (6). In this case we should expect that the negative section of the signal to also contribute to the energy delivered to the load.