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Basic Characteristics and OperationAuthor: Leonard Krugman One of the most inviting applications of the negativeresistance oscillator is as a relaxation type, particularly since its power requirements are low. Transistor relaxation oscillators have almost limitless use where a complex waveform, pulse generation, triggered output or frequency division is required. Like the equivalent vacuumtube types, the periodic operation of the transistor relaxation oscillator usually depends on a RC or RL combination for the storage and release of signal energy. For this reason, they are characterized by abrupt changes from one operating point to another. This makes relaxation oscillators particularly useful for generating sawtooth waveforms. Fig. 617. (A) Bask emittercontrolled relaxation oscillator with (B) idealized characteristic, and (C) waveforms. Figure 617 represents the basic emittercontrolled relaxation oscillator and its idealized currentvoltage characteristic. The location of the frequencydetermining network in the emitter circuit provides the largest measure of control. This basic type, therefore, is the most useful. The fundamental operation is involved, but not difficult to understand. For simplicity, assume the operation starts at point A (Figure 617B). At this point the transistor is cut off, since the emitter is biased in the reverse direction (E_{A}). Because of this reverse bias, the input circuit offers a high resistance path. The charge on capacitor C_{E} (equals E_{A}) has to leak off through R_{E}, and the rate of discharge is determined by the time constant R_{E}C_{E}.
When the current reaches point C, operation passes from the saturation region to the negativeresistance region. Instability in this area causes the current to drop instantaneously to its value at point B. Because of this rapid drop, the condenser voltage does not change. The operating point returns to point A, and the condenser discharge action starts the cycle again. Note that there are two time constants during a complete cycle. The first one T_{1} = R_{E}C_{E} controls the discharge rate of the condenser when operation moves from point A to point D. The second time constant controls the charging rate when operation moves from point D to point A. The sawtooth voltage generated by this circuit is illustrated in Fig. 617 (C). The frequency of operation is approximately . The frequency of the current wave is the same, but the waveform approximates a pulse, since the current only flows during the period when the condenser is charging (T_{2}). This simple oscillator, then, is useful as a voltage sawtooth or a current pulse generator. The following problem will be used as a numerical example of basic relaxation oscillator design. Assume that a sawtooth voltage wave is required for use in a sweep circuit, and that the following characteristics are specified; frequency is 5 kc; the charging rate interval T_{2} is limited to 10% of the total cycle; R_{B} is 2,000 ohms, required for sustained oscillation; E_{c} is fixed at 12 volts. The numerical values of the major operating points shown on Fig. 617 (B) are: for point A, I_{E} = 0.1 ma, E_{E} = 10 volts; for point B, I_{E} = 0.01 ma, E_{E} = 2 volts; for point C, I_{E} = 3 ma, E_{E} = 10 volts; and for point D, I_{E} = 5 ma, E_{E} = 2 volts. From the preceding analysis, Thus at point D, = 2,400 ohms The overall time constant T_{o} = T_{1} + T_{2} = = 200 μ seconds, T_{2} = 10% (T_{o}) = .10 (200) = 20 μ seconds, and T_{1} = T_{o}  T_{2} = 200  20 = 180 μ seconds. Since , Since T_{a} = R_{E}C_{E},


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