JPS6364407A - Emitsuta follower circuit - Google Patents

Abstract

PURPOSE:To keep high input resistance and protect a constant current source transistor by providing a clamping circuit that restrict collector potential of the constant current source transistor to the output terminal of an emitter follower circuit. CONSTITUTION:A transistor 6 in which the base is connected to the constant voltage point 23 of a constant voltage circuit 25, the collector is connected to a DC power source 30 and the emitter is connected to an output terminal 50 is provided. When signals to an input terminal 40 are lower than the constant voltage point 23, an element 6 becomes conductive, and potential of the output terminal 50 is clamped to potential lower than the constant voltage point 23 by VBE of the element 6. Accordingly, when voltage drop of a constant voltage element 7 is set to VBE of the element 6, potential of the terminal 50 does not become lower than a constant voltage point 21, and operation of a constant current source transistor 2 becomes stable. When the element 6 is in the state of conduction, an emitter follower element 1 is in the state of cut off, and input resistance of a terminal 40 becomes very large value, and becomes far larger than input resistance at the time of conduction of the element 1.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an emitter follower circuit, and particularly to an emitter follower circuit suitable for use on an integrated circuit.

[Conventional technology]

FIG. 2 shows a well-known emitter follower circuit.

In FIG. 2, the collector of the transistor 1 is connected to a DC power supply 30, the base to an input terminal 40, and the emitter to an output terminal 50. Also, transistor 2
The collector is the output terminal, the base is the connection point (constant potential point) 21 between the resistor 13 and the diode 3, and the emitter is the resistor]1
are connected to one end of each, and the other end of the resistor 11 is connected to L(!
It is connected to l potential. Furthermore, one end of the resistor 1:3 is connected to the DC power supply; 30, the other end of the resistor 13 is connected to the anode of the diode: 3, and the cathode of the diode 3 is connected to the resistor 1.
2 and the other end of the resistor 12 is connected to ground potential. Here, a constant current circuit 1 is made up of a transistor 2 and a resistor 11.
A constant voltage circuit 20 is configured by a resistor 13, a diode 33, and a resistor 1-2.

The emitter follower circuit shown in FIG. 2 transmits the voltage signal applied to the input terminal 40 to the output terminal 50 as a voltage signal lowered by the pace emitter voltage of transistor 1, but the input resistance is high and the output It is characterized by low resistance. In particular, as for the input resistance, an extremely high value (approximately several hundred ohms to several Mohms) is expected since it is a constant current load.

By the way, in a bipolar integrated circuit that uses a general PN junction isolation method, the collector potential of the NPN transistor is lower than the base potential, and the difference is about 0.5.
~0.7v, integrated circuit board (P-type semiconductor)
It is known that a parasitic PNP transistor consisting of the base and collector of the NPN transistor is activated and generates a substrate current, sometimes inducing phenomena that lead to latch-up and IC destruction. Therefore, when employing the emitter follower circuit shown in FIG. 2 on an integrated circuit, it is necessary to take measures to prevent the collector potential of the constant current source transistor 2 from falling below the base potential.

Conventionally, there is a structure shown in FIG. 3 as an emitter follower circuit provided with the above pairing. What are the characteristics of the emitter follower circuit shown in Figure 3?

A clamp circuit consisting of a transistor 4 and a diode 5 whose base potential is the constant voltage point 22 of the constant voltage circuit 24 is added to the input terminal 40.

According to this configuration, the input voltage signal can be limited so that the collector potential of the transistor 2 does not fall below the base potential. In addition, the operation of this is based on the Special Publication Publication No. 60-4073.
It is detailed in Publication No. 0.

[Problem that the invention seeks to solve]

In the configuration shown in Figure 3 above, when a low-potential input signal is applied, transistor 4 and diode 5 become conductive, resulting in a decrease in input resistance (approximately several Ω to several tens of Ω), which adversely affects the preceding stage circuit. There is a problem.

An object of the present invention is to provide an emitter follower circuit that maintains a high input resistance and can protect the operation of a constant current source transistor. This is achieved by attaching a different number of clamp circuits that limit the collector potential of the constant current source transistor to the output terminal of the follower circuit.

[For production]

The clamp circuit uses the base potential of the constant current source transistor as a reference and operates so that the collector potential of the transistor does not fall below a certain constant potential.

Therefore, the parasitic PNP transistor of the transistor is not activated and stable operation can be ensured. In addition, only the base of the emitter follower transistor is connected to the input terminal, so high input resistance can be expected.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a configuration diagram of an embodiment of an emitter follower circuit according to the present invention, and the same parts as in FIGS. 2 and 3 are designated by the same reference numerals and the explanation thereof will be omitted. The features of FIG. 1 include a constant voltage circuit 25 composed of a diode 3, a resistor 12, 16, and a constant voltage element 7; a base is connected to the constant voltage point 23 of the constant voltage circuit 25; The transistor 6 is provided with a collector connected to the terminal 50 and an emitter connected to the output terminal 50.

In FIG. 1, when the input voltage signal applied to the input terminal 40 drops below the constant voltage point 23, the transistor 6 becomes conductive, and the potential at the output terminal 50 decreases from the constant voltage point 23 by the base-emitter voltage of the transistor 6. Clamped to a reduced potential.

Therefore, for example, if the voltage drop of the constant voltage element 7 is set equal to 1 to the pace emitter voltage of the transistor, the potential of the output terminal 50 will not drop below the constant voltage point 21, and the constant current source transistor 2 will stabilize. Operation is guaranteed.

Furthermore, when the input voltage signal decreases and the transistor 6 is conductive, the emitter follower transistor 1 is in a cutoff state, and the input resistance of the input terminal 40 becomes an extremely large value (several MΩ or more), so that the transistor 1 is in a conductive state. It increases from the input resistance value when it is at .

〔Effect of the invention〕

As described above, according to the emitter follower circuit of the present invention, it is possible to protect the operation of the constant current source transistor while maintaining a high input resistance.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the emitter follower circuit of the present invention, FIG. 2 is a block diagram of a well-known emitter follower circuit,
FIG. 3 is a configuration diagram of a conventional emitter follower circuit to which a constant current source transistor operation protection circuit is added. 1... Emitter follower transistor. 2... Constant current source transistor, 3... Diode, 6... Clamping transistor, 7... Constant voltage element, 11, 12.16... Resistor,
", 0...constant current circuit, 25...constant voltage circuit, 2
1.23... Constant voltage point, 30... DC power supply, 40
...Input terminal, 50...Output terminal. Figure 1 Figure 2 Figure 3 ')2

Claims (1)

[Claims] (1) A first transistor whose collector is connected to a DC power supply, a pace connected to an input/output terminal, and an emitter connected to an output terminal, and a second transistor whose collectors are connected to a current output terminal, and the first transistor and the output terminal In the emitter follower circuit, the base is connected to a constant potential point with respect to the base potential of the second transistor, the collector is connected to the DC power supply, and the emitter is connected to the output terminal. An emitter follower circuit comprising a third transistor connected to the emitter follower circuit.