JPH0220906A - Emitter follower circuit - Google Patents

Abstract

PURPOSE:To equalize base-emitter voltages of input and output side transistors and to reduce a distortion between input and output voltages by supplying a current having the same magnitude as that of a current flowing to the input side transistor, to the output side transistor by a current mirror circuit. CONSTITUTION:An input voltage, an output voltage, a base-emitter voltage of an input side transistor 3, a base-emitter voltage of an output side transistor 4, a current flowing to the input side transistor 3, and a current flowing to the output side transistor 4 are denoted as VIN, VOUT, VBEIN, VBEOUT, IIN and IOUT, respectively. In this case, when an input side resistance 5 is denoted as RIN, IIN=(VIN-VBEIN)/RIN is derived, and depends on the input side resistance 5. When a current amplification factor hFE is the infinite, since each base-emitter voltage of the input side and the output side transistors 3, 4 in a current mirror circuit is equal, IOUT becomes equal to IIN. Accordingly, VBEIN and VBEOUT also become equal, and VOUT becomes equal to VIN. In such a way, a distortion between input and output voltages can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to emitter follower circuits in the field of semiconductor circuit technology.

BACKGROUND OF THE INVENTION Conventional emitter follower circuits have a configuration in which one NPN type transistor and one PNP type transistor are connected to two stages, one on the input side and one on the output side, in order to equalize the input and output DC voltage levels. The configuration and operation of a conventional emitter follower circuit will be described below.

FIG. 2 is a circuit diagram showing a conventional emitter follower circuit. In Fig. 2A and B, 17 and 25 are input terminals;
18.26 is the output terminal, 19 and 27 are the input side transistors, 20.28 is the output side transistor, 21.29 is the input side resistance, 22.30 is the output side resistance, 23 is the current flowing through the input side transistor, 24 is the This is the current flowing through the output transistor. In addition, the circuit configurations shown in Figure 2A and Figure 2B are based on the NPN type and PNP type transistors having opposite polarities, and the operating principles are basically the same, so the circuit configurations shown in Figure 2A are as follows. I will explain about this.

In the emitter follower circuit configured as described above, the input voltage is VIN - the output voltage is T, the base-emitter voltage of the input transistor is vBEINI, the base-emitter voltage of the output transistor is vBEOUT
Then, vOUT becomes as follows.

When vOUT=vIN−vBEIN+vBEOUT,
vBEIN'! : vBEOUT 7'', hoho, etc. Therefore, vOUT is approximately equal to IN, and the output of the emitter follower circuit is obtained.

However, it is assumed here that the characteristics of the input and output transistors are the same.

Problems to be Solved by the Invention However, in the above configuration, the current 23 flowing through the input side transistor depends on the input side resistor 21, and the current 24 flowing through the output side transistor depends on the output side resistor 22, so that the current 23. 24 are not necessarily equal in size. Therefore, when vBEIN and vBEOUT are different, there is a problem that distortion occurs between the output voltage voUT and the input voltage vIN.

In view of this, an object of the present invention is to provide an emitter follower circuit with low distortion between input and output.

Means for Solving the Problems In order to achieve the above object, the present invention is characterized in that it includes a current mirror circuit that makes the amount of current flowing through the output side transistor equal to the amount of current flowing through the input side transistor.

According to the above-described structure, the present invention supplies a current having the same magnitude as the current flowing through the input transistor to the output transistor using a current mirror circuit, thereby reducing the voltage between the base and emitter of the input and output transistor. be equal,
Distortion between input and output voltages can be reduced.

Embodiment FIGS. 1A and 1B are circuit diagrams showing an emitter follower circuit in an embodiment of the present invention. Figure 1A.

In B, 1 and 1o are input terminals, 2 and 11 are output terminals, 3 and 12 are input side transistors, 4 and 13 are output side transistors, 5 and 14 are input side resistors, and 6.16 is the input of the current mirror circuit. 7.16 is the output side transistor of the current mirror circuit, 8 is the current flowing through the input side transistor, and 9 is the current flowing through the output side transistor.

Also, the circuit configurations in Figures 1A and 1B are of NPN type and PN type.
Since this is a P-type transistor with the polarity reversed, and the principle of operation is basically the same, FIG. 1A will be explained below.

In the emitter follower circuit configured as above, the input voltage is vIN, the output voltage is voOUT, the base-emitter voltage of the input transistor is vBEIN, the base-emitter voltage of the output transistor is VBEOUT,
Assume that the current flowing through the input side transistor is EN, and the current flowing through the output side transistor is 10UT. Further, it is assumed that the current amplification factor hFE of each transistor is infinite, that the characteristics of the transistors on the input side and the output side are the same, and that the characteristics of the transistors on the input side and the output side in the current mirror circuit are the same.

At this time, if the input resistance is RIN, then IN: (vIN-VBEIN)/RIN, which depends on the input resistance. If the current amplification factor hFE is infinite, the voltage between the pace emitters of the input side and output side transistors in the current mirror circuit is equal, so l0UT
becomes equal to IIN. Therefore vBEIN and vBEOU
T also becomes equal, and vOUT becomes equal to vIN. As described above, according to this embodiment, the distortion between the input and output voltages can be reduced by providing the current mirror circuit.

As described in detail, according to the present invention, an emitter follower circuit with low distortion between input and output can be realized, and its practical effects are significant.

[Brief explanation of the drawing]

1A and 1B are circuit diagrams showing an emitter follower circuit according to an embodiment of the present invention, and FIGS. 2A and 2B are circuit diagrams showing a conventional emitter follower circuit. 1.1o...Input terminal, 2,11...
Output terminal, 3.12...Input side transistor,
4.13...Output side transistor, 5,14...
...Input side resistance, 6.16...Input side transistor of current mirror circuit, 7.16...
・Output side transistor of current mirror circuit.

Claims (1)

[Claims] One NPN type and one PNP type transistor on the input side,
An emitter follower circuit configured by connecting two stages on the output side, the emitter follower circuit comprising a current mirror circuit that makes the amount of current flowing through the output side transistor equal to the amount of current flowing through the input side transistor. .