JPH0334682B2 - - Google Patents

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 the power supply terminal 13, and the base of the NPN transistor 3 and a resistor 2 whose other end is grounded are connected to the emitter. A constant current source 7 connected to the power supply terminal, a diode 5, a transistor 8, and a resistor 6 are connected to the collector of the NPN transistor 3 as shown in the figure, and between the base of the NPN transistor 1 and the collector of the NPN transistor 3, ,
Connect the phase compensation capacitor 4. The resistor 6 is usually about 50 kΩ. Connect the collector of the output NPN transistor 9 to the power supply terminal, and connect its emitter to the
Connect to the emitter of the collector-grounded PNP transistor 10. The bases of NPN transistor 9 and PNP transistor 10 are connected to the anode of diode 5 and the emitter of NPN transistor 8, respectively. 11 is an output terminal.

Recently, radio cassette recorders, VTRs, etc. are being made with higher functionality and fewer external parts.
There is an increasing demand for amplifiers whose output impedance can be switched between high impedance and low impedance. Also in the push-pull output type output circuit of 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.
If the amplitude of the signal externally applied to the output terminal 11 is greater than a certain level, the charging/discharging current of the capacitor becomes the base current of the output transistor, causing distortion or clipping in the waveform of the output terminal 11.
In other words, 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 10 to the capacitor.
On the other hand, when the potential of the output terminal becomes low, the capacitor is discharged along the path of capacitor→resistor 6→base of transistor 8→collector of transistor 8→base of transistor 9→emitter of 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 including 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 having a collector, base and emitter connected to each other; and an output transistor consisting of a push-pull coupled pair driven by a signal from the excitation circuit section. and,
and a capacitor between the base of the first transistor and the collector of the third transistor, and a second capacitor between the base of the second transistor and the output terminal of the output transistor.
This is a push-pull type output circuit in which two resistors are inserted respectively, and by appropriately switching the voltage at the input terminal and the internal constant current source, 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 10 in FIG. correspond to 12, 11, and 13 in FIG. In this circuit, a new resistor 14 is added to the output terminal 11.
and the base of transistor 8. The resistor 14 is approximately 10kΩ.

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, the charging/discharging current of the phase compensation capacitor becomes the base current of the output transistor 9, so 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 flows in the path of capacitor 4 → resistor 6 → resistor 14 → output terminal 11, so that a voltage sufficient to make each of the transistors conductive is not generated between the base and emitter of output transistor 9 or 10. 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, but there is a voltage difference between both ends of the resistor 14, that is, the output terminal 11 and the base terminal of the transistor 8 Since the resistor 14 has a relatively high resistance of about 10 kΩ, the current flowing through the resistor 14 is small, and as a result, this 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 input terminal voltage and internal By appropriately switching the constant current source of
realizable.

[Brief explanation of the drawing]

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, 10...
...PNP transistor.

Claims (1)

[Claims] 1 A first NPN transistor whose base is connected to an input terminal, whose collector is connected to a power supply terminal, and whose emitter is grounded through a first resistor; whose emitter is grounded and whose base is connected to the emitter of the first NPN transistor. a first PNP transistor whose collector is grounded, whose emitter is connected to the output terminal, and whose base is connected to the collector of the second NPN transistor;
a third NPN transistor whose emitter is connected to the collector of the second NPN transistor; and a second NPN transistor whose one end is connected to the emitter of the third NPN transistor and whose other end is connected to the base of the third NPN transistor. a resistor, a diode whose cathode is connected to the base of the third NPN transistor and whose anode is connected to the collector of the third NPN transistor, whose collector is connected to the power supply terminal, whose emitter is connected to the output terminal, and whose base is connected to the base of the third NPN transistor. a fourth NPN transistor connected to the collector of the third NPN transistor; a current source having one end connected to the power supply terminal and the other end connected to the collector of the third NPN transistor; and one end connected to the input terminal. and the other end is connected to the collector of the second NPN transistor, one end is connected to the base of the third NPN transistor, and the other end is connected to the output terminal, at high output impedance. A push-pull type output circuit characterized by being provided with a resistor for charging and discharging a capacitor.