1. Field of the Invention
The present invention relates to an oscillation circuit, and, in particular, to an oscillation circuit in which oscillation is performed as a result of two capacitors being alternately charged and discharged through transistors.
2. Description of the Related Art
FIG. 1 shows a circuit diagram of an example of an oscillation circuit in the related art.
The oscillation circuit 1 in the related art includes a power source 2 which supplies a driving voltage VD, a driving current generating circuit 3 which generates driving currents in accordance with the driving voltage VD supplied by the power source 2, capacitors C1, C2 which are charged by the driving currents generated by the driving current generating circuit 3, a discharging NPN transistor Q1 which discharges charge of the capacitor C1 in accordance with a charged voltage of the capacitor C2, a discharging NPN transistor Q2 which discharges charge of the capacitor C2 in accordance with a charged voltage of the capacitor C1, a diode component D1 which maintains the charged voltage of the capacitor C1 at a predetermined level and a diode component D2 which maintains the charged voltage of the capacitor C2 at a predetermined level. Thus, the oscillation circuit 1 forms a oscillation circuit using a multivibrator.
The driving current generating circuit 3 forms a current-mirror circuit, and includes PNP transistors Q3, Q4, Q5, Q6, Q7 and a constant-current source 4. The emitter of the transistor Q3 is connected to the positive voltage side of the power source 2, and the collector and the base of the transistor Q3 are connected with one another. The emitter of the transistor Q4 is connected to the positive voltage side of the power source 2, the base of the transistor Q4 is connected to the base and collector of the transistor Q3, and the collector of the transistor Q4 supplies a driving current. The emitter of the transistor Q5 is connected to the positive voltage side of the power source 2, the base of the transistor Q5 is connected to the base and collector of the transistor Q3, and the collector of the transistor Q5 supplies a driving current. The emitter of the transistor Q6 is connected to the positive voltage side of the power source 2, the base of the transistor Q6 is connected to the base and collector of the transistor Q3, and the collector of the transistor Q6 supplies a driving current. The emitter of the transistor Q7 is connected to the positive voltage side of the power source 2, the base of the transistor Q7 is connected to the base and collector of the transistor Q3, and the collector of the transistor Q7 supplies a driving current. The constant-current source 4 is connected to the collector and base of the transistor Q3, and draws predetermined currents from the collector and base of the transistor Q3, and the bases of the transistors Q4, Q5, Q6 and Q7.
The collector of the transistor Q4 of the driving current generating circuit 3 is connected to one end of the oscillation capacitor C1. The discharging transistor Q1 which discharges the capacitor C1 and the diode component D1 which maintains the voltage of the one end of the capacitor C1 at the predetermined level are connected between the connection point of the one end of the capacitor C1 and the collector of the transistor Q4, and the ground.
The collector of the discharging NPN transistor Q1 is connected to the connection point of the one end of the capacitor C1 and the collector of the transistor Q4, the emitter of the transistor Q1 is grounded, and the base of the transistor Q1 is connected to one end of the oscillation capacitor C2. Switching of the transistor Q1 is performed in accordance with the charged voltage of the capacitor C2 and thus the transistor Q1 controls charging and discharging of the capacitor C1. The diode component D1 is an NPN transistor Q8, and the base and the collector of the transistor Q8 are connected with one another, that is, diode component connection is performed. The base and collector of the transistor Q8 are connected to the connection point of the one end of the capacitor C1 and the collector of the transistor Q4, and the emitter of the transistor Q8 is grounded. The diode component D1 maintains the one end of the capacitor C1 at the forward voltage VF of the diode component D1 when the capacitor C1 is charged.
The collector of the transistor Q5 of the driving current generating circuit 3 is connected to the other end of the capacitor C1, and supplies a charging current to the other end of the capacitor C1.
The collector of the transistor Q6 of the driving current generating circuit 3 is connected to the one end of the capacitor C2, and supplies a charging current to the one end of the capacitor C2.
The collector of the transistor Q7 of the driving current generating circuit 3 is connected to the other end of the oscillation capacitor C2. The discharging transistor Q2 which discharges the capacitor C2 and the diode component D2 which maintains the voltage of the other end of the capacitor C2 at the predetermined level are connected between the connection point of the other end of the capacitor C2 and the collector of the transistor Q7, and the ground.
The collector of the discharging NPN transistor Q2 is connected to the connection point of the other end of the capacitor C2 and the collector of the transistor Q7, the emitter of the transistor Q2 is grounded, and the base of the transistor Q2 is connected to the other end of the capacitor C1. The diode component D2 is an NPN transistor Q9, and the base and the collector of the transistor Q9 are connected with one another, that is, diode component connection is performed. The base and collector of the transistor Q9 are connected to the connection point of the other end of the capacitor C2 and the collector of the transistor Q7, and the emitter of the transistor Q9 is grounded. The diode component D2 maintains the other end of the capacitor C2 at the forward voltage VF of the diode component D2 when the capacitor C2 is charged.
Operations of the oscillation circuit 1 will now be described. FIGS. 2A, 2B, 2C and 2D show operation waveforms of the above-described oscillation circuit 1. FIG. 2A shows the collector voltage of the discharging transistor Q1, FIG. 2B shows the collector voltage of the discharging transistor Q2, FIG. 2C shows the base voltage of the discharging transistor Q1, and FIG. 2D shows the base voltage of the discharging transistor Q2.
With reference to FIGS. 2A, 2B, 2C and 2D, at the time t1, the discharging transistor Q1 turns off and thereby the collector voltage of the transistor Q1 is changed to and maintained at a high level. Further, at the same time, the discharging transistor Q2 turns on and thereby the collector voltage of the transistor Q2 is changed to a low level. Thereby, as shown in FIG. 2C, the voltage of the one end of the oscillation capacitor C2 falls and charging is started.
At the time t2, the capacitor C2 has been charged via the one end of the capacitor C2 by the charging current supplied by the transistor Q6 of the driving current generating circuit 3. Then, when the voltage of the one end of the capacitor C2 becomes the turn-on voltage of the discharging transistor Q1, the transistor Q1 turns on and thereby the collector voltage of the transistor Q1 is changed to a low level as shown in FIG. 2A.
When the collector voltage of the discharging transistor Q1 is changed to the low level, the voltage of the other end of the oscillation capacitor C1 falls, and, as shown in FIG. 2D, the base voltage of the discharging transistor Q2 falls. As a result, the transistor Q2 turns off. The oscillation transistor C1 is charged since the base voltage of the transistor Q2 falls at the time t2. Then, when the voltage of the other end of the capacitor C1 becomes the forward voltage VF of the diode component D2 at the time t3, the transistor Q2 turns on. When the transistor Q2 turns on, as shown in FIG. 2B, the collector voltage of the transistor Q2 is changed to the low level. Thereby, as shown in FIG. 2C, the base voltage of the transistor Q1 falls, and the transistor Q1 turns off. Charging of the capacitor C2 is started again.
The operations between the times t1 and t2, and the operations between the times t2 and t3 are repeated. Thereby, the oscillation signals shown in FIGS. 2A and 2B are obtained from output terminals TOUT1 and TOUT2, respectively.
In the oscillation circuit 1 in the related art, as shown in FIGS. 2A and 2B, oscillation is performed as a result of switching of the discharging transistors Q1 and Q2 being performed alternately. When the transistors Q1 and Q2 turn on simultaneously due to noise or the like, the oscillation stops and it is necessary to disconnect the power source 2 to restart the oscillation.