Because the circuit in Fig. The circuit can be enhanced to oscillate at frequencies from less than Hz to UHF Ultra High Frequency frequencies with a laminated iron-core transformer. The same circuit will oscillate satisfactorily in the UHF regions with an air-core transformer. Classic LC Oscillators: Figure 6 illustrates the Hartley Oscillator , which is a variation of the tuned-collector oscillator that was shown in Fig.
This oscillator is recognizable by the tapped coil in its tuned resonant circuit. Oscillation of the Hartley oscillator circuit depends on phase-splitting autotransformer action of the tapped coil in the tuned resonant circuit. The positive power supply is connected to the tap to obtain the necessary autotransformer action. The signal from the top of the coil is coupled to the base input of Q1 through isolating capacitor C2.
The oscillator will oscillate at a center frequency determined by its LC product. The Colpitts Oscillator shown in Fig.
It is identified by the voltage divider in its tuned resonant circuit. With the component values shown, this Colpitts circuit will oscillate at about 37KHz. Capacitor C1 is in parallel with the output capacitance of Q1, and C2 is in parallel with the input capacitance of Q1.
This shift can be minimized for high frequency stability by selecting values of C1 and C2 that are relative to the internal capacitances of Q1. The Clapp Oscillator , a modification of the Colpitts oscillator, shown in Fig. This is achieved by adding capacitor C1 in series with the coil in the tuned resonant tank circuit. It is selected to have a value that is small with respect to C2 and C3. As a result of the presence of this capacitor, the resonant frequency of the tank and oscillator will be determined primarily by the values of L1 and C1.
With the component values shown, the Clapp oscillator oscillates at about 80KHz. Figure 9 shows the classic Reinartz Oscillator. In this circuit, tuning coil L1 in the collector circuit and the tuning coil L2 in the emitter circuit are inductively coupled to tuning coil L3 in the resonant tank circuit. Positive feedback is obtained by coupling the collector and emitter signals of the transistor through windings L1 and L2, and inductively coupling both of these coils to L3.
This Reinartz oscillator oscillates at a frequency determined by L3 and trimmer capacitor C2. With the values and turns ratios given in Fig. Modulation: The LC oscillator circuits shown in Figs. Figure 10 is the schematic for a beat-frequency oscillator BFO. It is based on the tuned-collector circuit of Fig. The value of emitter-decoupling capacitor C1 was selected to present a low impedance to the KHz carrier signal, while also presenting a high impedance to the low-frequency modulation signal.
Figure 11 shows how the BFO circuit in Fig. Tuning is adjusted by trimmer potentiometer R5. Silicon diode D1 functions as an inexpensive varactor diode. Astable Oscillators: Conventional oscillator circuits produce sinewaves, but repetitive square waves are important in electronics. One way to generate them is with the astable multivibrator circuit shown in Fig. This multivibrator is a self-oscillating regenerative switch whose on and off periods are controlled by the time constants obtained as the products of R2 and C2, and R3 and C1.
The waveforms taken at the collector and base of transistors Q1 and Q2 are shown in Fig. The frequency of the astable multivibrator in Fig. The frequency can be varied with dual-gang variable resistors placed in series with 10K limiting resistors in place of R2 and R3.
The operating frequency can, if required, be synchronized to that of a higher-frequency signal by coupling part of the external signal into the timing networks of the astable circuit. Outputs can be taken from either collector of the circuit, and the two outputs are in opposite in phase. The upper voltage limit is set by inherent transistor behavior: as the transistors change state at the end of each half-cycle, the base-emitter junction of one transistor is reverse biased by a voltage that is about equal to the supply voltage.
Consequently, if the supply voltage exceeds the reverse base-emitter breakdown voltage of the transistor, circuit timing will be affected. This characteristic can be overcome with the circuitry modifications shown in Fig.
A silicon diode is connected in series with the base input terminal of each transistor to raise the effective base-emitter reverse breakdown voltage of each transistor to a value greater than that of the diode. This variation can be further reduced to 0.
Multivibrator Variations: The basic astable multivibrator shown in Fig. Some modifications are shown in Figs. A shortcoming of the multivibrator shown in Fig. The lower the values of timing resistors R2 and R3 with respect to collector load resistors R1 and R4, the more pronounced will be this waveform rounding. This is an astable multivibrator, as it continually switches itself on and off. A bistable multivibrator remains either on or off after you switch it. While some oscillators and multivibrators have a fixed frequency, sometimes having a general-purpose signal generator with a frequency you can tune for different applications is handy.
Variable-frequency multivibrators are easy to design; a variable resistor in series with the capacitor slows down its charge rate by turning the resistor knob. A variable-frequency oscillator is harder to design.
Its resonance circuits are finely adjusted, so changing the frequency can compromise the resonance if not done carefully. Some oscillators incorporate a quartz crystal in the resonant circuit. The crystal has a resonant frequency of its own, and its natural stability allows for great precision.
Radios, computers, clocks and watches use quartz-crystal stabilized oscillators as precise frequency sources. Multivibrators generally do not use quartz crystals. An oscillator produces a sine wave as its primary output. Quartz crystal is sometimes featured within the resonant circuit of an oscillator. This design is often used for watches, radios and computers as a method for obtaining precise frequency. Multivibrators generally do not incorporate quartz crystal into their circuit.
Multivibrators and oscillators also have a difference in how the switch works for producing an electronic signal. Both produce the electronic signal using this method. For an oscillator, the switch turns on when there is a low current in the circuit, which sends a charge to the capacitor. The switch turns off again when the current level is back to normal.
0コメント