Patent GB-1489370-A (GB)
Title: Astable multivibrator circuits
IPC: H03K7/08
Filed: 1974-12-19
Abstract: 1489370 Astable variable mark-space multivibrator SONY CORP 19 Dec 1974 [24 Dec 1973] 55036/74 Headings H3T and H3P [Also in Division G3] In a cross-coupled astable variable pulse width but fixed frequencymultivibrator with two active devices, a control voltage source 12 is connected by means such as a resistor 11 to the control electrode of the second active device 2 and to a connection point between a capacitor 8 and unidirectionally conducting device 7 which are included in the coupling between the output electrode of the second device 2 and the control electrode of the first device 1. The active devices may be valves, field effect transistors, bipolar transistors and the like. As shown both devices are NPN transistors 1, 2, the control voltage source 12 being connected by two resistors 10, 11 respectively to the connection between a diode 7, forming the unidirectional conductive device, and to the base of the second transistor 2. The devices can have a common emitter resistance (29, Fig. 3, not shown), the control can be derived from a Zener diode (26) and the output taken via buffer 15 and a transformer 17. In use an increase in the magnitude of the control voltage changes the mark to space ratio of the generated waveform without changing the repetition rate of the waveform, the voltage at the collector of transistor 2 staying longer at a level approximately equal to Vcc as the voltage of 12 rises (Fig. 2G, not shown). The waveform at the other collector is in the other sense and does not have an abrupt rise to Vcc (Fig. 2F, not shown). With transistor 2 ON, transistor 1 OFF, the voltage VB1 at the base of 1 rises towards Vcc from a negative level, as capacitor 8 charges via Vcc, 9, 8, 7, 2 and ground. Simultaneously the voltage across capacitor 5, V5, discharges via Vcc, 3, 5, the emitter-base of 2 and ground and this reaches a negative voltage nearly (Fig. 2d, not shown) at the same time as V B1  reaches + V EB1  and then transistor 1 turns ON, and 2 turns OFF as its base falls to -Vccx. As 2 turns OFF, so its collector voltage Vc 2  rises to Vcc and reverse biases the diode 7. In the next period of time capacitor 8 discharges via the contol voltage source 12, resistor 10, capacitor 8 b-e of 1 and ground, causing the voltage V d1  at the anode of the diode 7 to rise, aiming at the voltage of the control source V 0 . Simultaneously capacitor 5 charges via the control voltage source 12, resistor 11 capacitor 5, c-e of 1 and earth and the voltage V 5  across the capacitor 5<SP>1</SP> rises towards V 0 . At some time t 2  (Fig. 2d, not shown) V 5  reaches V EB2  and 2 turns ON, at this time V 8  has reached a negative voltage -V 0X  (less than V 0 -V BE1 ) (Fig. 2c, not shown) and so V B1  falls from the V EB  to -V 0X  and 1 turns off. The cycle repeats. It is seen that the aiming potentials in one, half cycle are fixed at the supply voltage Vcc but in the other half they are variable, being the voltage of the control voltage source 12. If this voltage rises from V 0  to V 0 <SP>1</SP> the voltage across C 5  rises faster so that the time that O 1  is off is reduced, simultaneously C 8  discharges to a more negative voltage and since its subsequent charge is towards Vcc the time taken to reach V EB2  is increased. Suitable selection of values makes the sum of the two times a constant.