TRANSISTOR AMPLIFIER CIRCUIT

Abstract: A transistor amplifier circuit provided with an oscillation preventing countermeasure. When a transistor (TR₁) having a relatively high input capacitance is driven by another driving transistor (TR₂), an oscillation preventing diode (D₁ and/or 50) is connected to the connecting wire of both the transistors (TR₁, TR₂). Current communication necessary between the capacitance between the collector and the base of the transistor (TR₁) and the inductance due to the length of the wire can be prevented by the rectifying action of the oscillation preventing diode (D₁, 50), thereby preventing oscillation of the transistor amplifier circuit.

Description: SPECIFICATION TRANSISTOR AMPLIFIER CIRCUIT TECHNICAL FIELD The present invention relates to a transistor amplifier circuit which has adopted oscillation preventive measures. TECHNICAL BACKGROUD A transistor amplifier circuit designed for driving a transistor of relatively large input capacitance by another driving transistor may sometimes oscillate if the wire length between the both transistors is large. For example, in a power amplifier circuit for driving a servomotor, a Darlington transistor TR1 constituted by a Darlington connection of an output stage transistor Q1 and a driving stage transistor Q2 is driven by another driving transistor TR2 so as to obtain a high DC amplification factor, for instance, as shown in Fig. 1. In this case, the Darlington transistor TR1 is usually packaged on a radiator because it calls for cooling, whereas the driving transistor TR2 is packaged on a printed circuit board on the side of control equipment. Consequently, the wire length between the both transistors becomes large and, in some cases, resonance occurs between the inductance owing to this wire length and the collector-base capacitance of the Darlington transistor TR1, resulting in such oscillation as mentioned above. In Fig. 1, reference character PS indicates a driving power wource, R a resistor, D a diode, C1, B1 and E1 the collector, base and emitter of the Darlington transistor TR1, and C2, B2 and E2 the collector, base and emitter of the driving transistor TR2. The aforementioned oscillation could be prevented by the use of such a Darlington transistor which is constituted by a Darlington connection of three transistors on one chip as shown in an electric circuit diagram of Fig. 2, for instrance; however, such a Darlington transiistor is not manufactured for the reason that the switching time would be long. Heretofore, the oscillation has been prevented by connecting a capacitor CP of a suitable capacitance between the base and emitter of the Darlington transistor TR1 as shown in an electric circuit diagram of Fig. 3 in which the Darlington transistor in Fig. 1 is indicated by a black box, or by connecting a resistor RG between the base B1 of the Darlington transistor TR1 and the emitter E2 of the driving transistor TR2 as shown in Fig. 4; however, the former method has the defect that a transistor of large current capacity is required as the driving transistor TR2 because a current flows through a capacitor C, and the latter method has the defect that a forward voltage drop of the Darlington transistor TR1 becomes large, resulting in increased loss. DISCLOSURE OF THE INVENTION The present invention is intended to overcome such defects of the prior art, and has for its object to prevent the oscillation with a simple arrangement without causing any trouble. The invention will hereinafter be described in detail in connection with its embodiment. According to the present invention, in a transistor amplifier circuit of such an arrangement that a transistor of relatively large input capacitance is driven by another driving transistor, the both transistors are interconnected via an oscillation preventive diode and, by the rectifying operation of the oscillation preventive diode, transmission and reception of a current necessary for oscillation is inhibited, ensuring to prevent oscillation. The current capacity value of the driving transistor need not be increased and loss does not increase, either. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an electric circuit diagram showing a servomotor driving power amplifier circuit; Fig. 2 is an electric circuit diagram of a Darlington transistor employing three transistors; Figs. 3 and 4 are electric circuit diagrams of conventional transistor amplifier circuits; and Fig. 5 is an electric circuit diagram illustrating the principal part of a circuit embodying the present invention. PREFERRED EMBODIMENTS OF THE INVENTION Fig. 5 is an electric circuit diagram illustrating the principal part of an embodiment of the present invetion. The parts corresponding to those in Fig. 1 are identified by the same reference characters. Reference character D1 indicates a diode for preventing oscillation, PCB a printed circuit board, and RD a radiator. According to this embodiment, as shown in Fig. 5, in a power amplifier circuit in which the Darlington transistor TR1 is packaged on the radiator RD for cooling it and the driving transistor TR2 therefor is packaged on the printed circuit board PCB on the side of the control equipment, the oscillation preventive diode D1 of a polarity shown in connected between the collector C1 of the Darlington transistor TR1 and the collector C2 of the driving transistor TR2. With such an arrangement, in a closed loop CrDrC 2 E2)Bl+Cl, current flows only in the forward direction of the oscillation preventive diode D1 and, consequently, transmission and reception of current necessary for oscillation cannot be effected between the collector-base capacitance of the Darlington transistor TR1 and the inductance based on the wire length, thus ensuring to prevent oscillation. As will be appreciated from the principle of the present invention, it is sufficient if the oscillation preventive diode D1 is connected at least either between the collectors of the Darlington transistor TR1 and the driving transistor TR2 or between the base B1 of the Darlington transistor TR2 and the emitter E2 of the driving transistor TR2 as indicated by the broken line 50 in Fig. 5, and its packaging is carried out on the printed circuit board PCB in practice. However, sometimes a circuit for increasing the switching speed of the Darlington transistor TR1 is connected between the base B1 of the Darlington transistor TR1 and the emitter E2 of the driving transistor TR2 and, in such a case, it may be difficult or improper to connect the oscillation preventive diode D1 at the position 50; in this case, the oscillation preventive diode D1 is connected between the collectors of the Darlington transistor TR1 and the driving transistor TR2. While in the foregoing embodiment the present invention has been described as being applied to a transistor amplifier circuit arranged to drive a Darlington transistor by another transistor, the invention is not limited specifically thereto but may also be applied to a transistor amplifier circuit which drives a transistor of relatively large input capacitance by another transistor and is liable to oscillate due to large wire length between the both transistors, and the same effect as mentioned previously can be obtained. As has been described in the foregoing, according to the present invention, in a transistor amplifier circuit arranged so that a transistor of relatively large input capacitance is driven by another driving transistor, an oscillation preventive diode is inserted in the line interconnecting the both transistors; therefore, the present invention has the advantage that oscillation can be'prevented with an extremely simple arrangement, without the necessity of increasing the current capacity value of the driving transistor as in the case of the prior art circuit shown in Fig. 3 and without incurring an increase in loss as in the conventional circuit depicted in Fig. 4. Accordingly, the present invention, when applied to a servomotor driving power amplifier or the like, enhances its reliability, and hence is of great utility.

Claims: CLAIM 1. A transistor amplifier circuit in which a transistor of relatively large input capacitance is driven by another driving transistor, characterized in that the both transistors are interconnected via an oscillation preventive diode. 2. A transistor amplifier circuit according to claim 1, characterized in that the transistor of relatively large input capacitance is a Darlington transistor and packaged on a radiator for cooling, and the driving transistor is packaged on a printed circuit board. 3. A transistor amplifier circuit according to claim 2, characterized in that the oscillation preventive diode is connected between the collector of the Darlington transistor and the collector of the driving transistor in a forward direction with respect to a drive current.

Inventors: KOBARI, KATSUO, KOBAYASHI, TOSHIO, SAKAMOTO, KEIJI
Applicants: FANUC LTD
Application year: 1981
Main classification: H03F1
IPC codes: H03F1