JPS60160707A - Push-pull type output circuit - Google Patents

Abstract

PURPOSE:To obtain a push-pull type output circuit providing a high impedance to its output by switching a voltage at an input terminal and an internal constant current source. CONSTITUTION:In forming a high output impedance circuit, an input 12 is grounded so as to zero a current value of the constant current source 7. Since a charge/discharge current of a capacitor flows to a capacitor 4, resistors 6, 14 and an output terminal 11 in this order in this case, a base voltage of a transistor (TR) 8 is divided by the resistors 6, 14 and when the divided voltage is set to a value being <= twice the diode voltage, no base current to a TR9 through the TR8 is produced. Then the output current is only said charge/discharge current and the output impedance is kept high. On the other hand, in using the titled circuit as a conventional output circuit, the potential difference of the resistor 14, that is, a potential difference between the output terminal 11 and a base terminal of the TR8 is small and the resistance value is comparatively high, then the current is small and the circuit acts like the conventional output circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a transistor output circuit applicable to, for example, an audio output circuit of audio equipment.

Conventional configuration and its problems FIG. 1 shows a push-pull type output circuit that is often used in audio amplifiers and the like.

The collector of an NPN transistor 1 whose base terminal is the input terminal 12 is connected to a power supply terminal 13, and the base of an NPN transistor 3 and a resistor 2 whose other end is grounded are connected to its emitter. A constant current source 3 connected to the power supply terminal, a diode 5, a transistor 8, and a resistor 6 are connected to the collector of NPN) transistor 3 as shown in the figure, and the base of NPN) transistor 1 and the collector of NPN) transistor 3 are connected to each other. A phase compensation capacitor 4 is connected between them. resistance 6
is usually around 60Ω. The collector of the output NPN) transistor 9 is connected to the power supply terminal, and its emitter is connected to the emitter of the PNP) transistor 10 whose collector is grounded. The bases of NPN) transistor 9 and PNP) transistor 1o are respectively connected to the anode of diode 5,
and NPN) connected to the emitter of transistor 8. 11
is the output terminal.

Recently, in radio cassette recorders, VTRs, and the like, there has been an increasing demand for amplifiers that can switch the output impedance between high impedance and low impedance for the purpose of increasing functionality and reducing external components. Even in the push-pull output type output circuit shown in FIG. 1, the output impedance of the output terminal 11 can be increased to a certain degree by grounding the input terminal 12 and simultaneously reducing the current value of the constant current source 7 to zero. . However, in the circuit shown in FIG. 1, the phase compensation capacitor 4 is connected to the base of the PNP output transistor 10, and when the amplitude of the signal applied from the outside to the output terminal 11 is larger than a certain level, the charging/discharging current of the capacitor increases. Since this becomes the base current of the output transistor, distortion or clipping occurs in the waveform of the output terminal 11.

That is, when the potential of the output terminal becomes high, the capacitor is charged along the path from the emitter of the transistor 10 to the base of the transistor 1o to the capacitor, while when the potential of the output terminal becomes low, the path from the capacitor to the resistor 6 to the transistor 8 is charged.
base → collector of transistor 8 → transistor 9
The capacitor is discharged along the path from the base of the transistor 9 to the emitter of the transistor 9. For this reason, the conventional push-pull type output circuit is inappropriate as an output circuit of an amplifier capable of providing a high impedance output.

OBJECTS OF THE INVENTION An object of the present invention is to provide a push-pull type output circuit that can make the output high impedance.

Structure of the invention: a signal input section consisting of a first emitter-follower-coupled transistor; a series body of a current source, a forward diode, and a first resistor; an anode point, a cathode point of the diode, and the other end point of the first resistor. an excitation circuit section consisting of a third transistor having a load section consisting of a second transistor whose collector, base and emitter are respectively connected to the excitation circuit section; and an output consisting of a push-pull coupled pair driven by the signal of the excitation circuit section. a capacitor between the base of the first transistor and the collector of the third transistor; and a capacitor between the base of the first transistor and the collector of the third transistor.
This is a push-pull type output circuit in which a second resistor is inserted between the base of the transistor and the output terminal of the output transistor, and this allows the voltage at the input terminal and the internal constant current source to be adjusted appropriately. By switching to this, a high output impedance circuit can be realized.

DESCRIPTION OF EMBODIMENTS FIG. 2 shows an output circuit of the present invention, in which circuit elements 1 to 10 correspond to 1 to 1o in FIG. corresponds to 12.11.13 in FIG. In this circuit, a resistor 14 is newly connected between the output terminal 11 and the base of the transistor 80. The resistor 14 is approximately 10Ω.

The circuit operation of the present invention will be explained with reference to FIG.

When it is desired to make the output impedance of this circuit high, the input 12 is grounded, and at the same time, the current value of the constant current source 7 is made zero. In this case, in the conventional example, since the charging/discharging current of the phase compensation capacitor becomes the base current of the output transistor 9, a current that is hFE times the charging/discharging current flows to the output terminal, but in the present invention, the charging/discharging current of the capacitor but,
The voltage applied to the base of the NPN transistor 8 is divided by the resistor 6 and the resistor 14, and this voltage is 2VD.
(VD: diode voltage) or lower, no base current of the PNP output transistor 9 is generated through the transistor 8b. Therefore, the current flowing to the output terminal is only the charging/discharging current of the capacitor, and the output impedance can be kept high.

On the other hand, when this circuit is used as a normal push-pull type output circuit, the current flowing through the newly provided resistor 14 becomes a problem. The difference is small, and
Since the resistor 14 is relatively high, on the order of 10Ω, the current flowing through the resistor 14 is small, and as a result, the present circuit exhibits circuit operation similar to a conventional push-pull type output circuit.

Effects of the Invention As is clear from the above explanation, the push-pull type output circuit of the present invention can operate as a normal push-pull type output circuit and as a high output impedance circuit depending on the voltage at the input terminal. This can be achieved by appropriately switching the internal constant current source.

[Brief explanation of drawings]

FIG. 1 is a conventional output circuit diagram, and FIG. 2 is an output circuit diagram of an embodiment of the present invention. 1.3, 8.9...NPN) transistor, 2゜6.14... Resistor, 4... Phase compensation capacitor, 5... Diode ,7...
・Constant current source, 1o...PNP transistor. - Name of agent: Patent attorney Toshio Nakao and one other person Figure 1 Figure 2

Claims (1)

[Claims] a signal input section including a first emitter-follower-coupled transistor; a series body of a current source, a forward diode, and a first resistor; a collector connected to an anode point, a cathode point of the diode, and the other end point of the first resistor; an excitation circuit section consisting of a third transistor having a load section consisting of a second transistor whose base and emitter are respectively connected; and an output transistor consisting of a push-pull combination driven by a signal from the excitation circuit section. and a capacitor between the base of the first transistor and the collector of the third transistor, and a capacitor between the base of the first transistor and the collector of the third transistor.
A push-pull type output circuit in which a second resistor is inserted between the base of the transistor and the output terminal of the output transistor.