Oscillators and Triggers


Posted on Feb 6, 2014

Transistors are finding new and ever-increasing uses. Some of the circuits in which transistors are being applied are well-known through prior employment in conjunction with vacuum tubes. The applications shown in the chapters which follow illustrate some of the more unique ways in which to take advantage of the desirable properties of transistors


Oscillators and Triggers
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. The circuits that are described are practical, have been built, tested, and are now in use. THOUGH transistors generally demand their own circuitry, there are some good vacuum-tube circuits that function nicely when transistors are substituted. As the phase relations of a grounded-emitter transistor resemble those of a grounded-cathode vacuum tube, it is possible to use junction transistors for vacuum tubes in most oscillator circuits. The multivibrator and the two-terminal sine-wave generator perform well with junction transistors, with the transistorized two-terminal L-C having a great advantage over other transistor oscillators. As with vacuum tubes, the circuit oscillates on higher frequencies than single-transistor oscillators with the same components. While transistors must be selected for other oscillators, most transistors, even those not oscillating in single oscillators, work well on rf in this circuit. The multivibrator is in principle a two-stage R-C coupled amplifier (Fig. 501). The output voltage of the second tube is fed back to the grid of the first tube. Because of this feedback the circuit starts to oscillate at a frequency determined by the time constants of R and C. The circuit will also oscillate if you substitute an element with similar amplifying and phase-shift relations in place of the vacuum tubes. Before getting into further circuitry, let`s discuss the principle of analogy. Fig. 502 shows a...




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