JPS6096009A - Emitter follower circuit - Google Patents

Abstract

PURPOSE:To increase each collector-emitter voltage of the 1st and the 2nd TR by connecting a voltage clamp element in parallel with the 1st TR and connecting a voltage drop element in series with the 2nd TR. CONSTITUTION:The voltage clamp element 21 and the voltage drop element 22 are added. When an input voltage Vin increased up to +0.5V, the collector-emitter voltage VCE1 of the 1st TR11 is decreased and the voltage VCR2 of the 2nd TR12 is increased and the possibility of destruction of a TR12 is caused. The voltage VCE2 is decreased by inserting the voltage drop element 22. When the voltage Vin is decreased to -7.0V, the voltage VCE2 of the TR12 is dropped, the voltage VCE1 of the TR11 is increased and the possibility of destruction of the TR11 is caused. Then the voltage clamp element 21 is inserted so as to prevent the potential at an intermediate connecting point P from being decreased from a prescribed level. The voltage to be clamped is set slightly higher than the collector-emitter dielectric strength BVCE1 of the TR11.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to emitter follower circuits, particularly constant current transistors. The present invention relates to an emitter follower circuit comprising:

Background of the Technology As is well known, emitter follower circuits have extremely high input impedance and are often used as a buffer for circuit coupling. For example, in semiconductor memory circuits, they are used in address input sections, data input sections, etc. Although the emitter follower circuit is basically composed of one stage of transistors, a constant current transistor is often used in series with the emitter follower circuit in order to stabilize the operation.

Prior Art and Problems FIG. 1 is a circuit diagram showing an example of a general follower circuit with a constant current transistor. In this figure, an emitter follower circuit 10 is made up of a series circuit of a first transistor 11 and a second transistor 12, receives an input voltage Vin at the base of the first transistor 11, and outputs an output voltage EEV from its emitter. Take out ut k. A second transistor 12, whose collector is connected to the emitter of the first transistor 11, receives a constant voltage at its base. Therefore, the constant current resistor 13 has (Va −VBK
A constant voltage J (VBIC is the base-emitter voltage of the second transistor 12, a constant value of about 0.7 V) is applied, and a constant current ■c flows. Thus, the second transistor 12 functions as a constant current transistor and contributes to stabilizing the operation of the emitter follower circuit 10.

Incidentally, in recent years, ICs such as semiconductor memory circuits are required to be more and more integrated and faster.

For this reason, the epitaxial layer for forming the base and emitter by diffusion, which is located on the buried N 2 layer forming the collector, tends to be thinner and thinner. However, on the other hand, if the layer thickness is made thinner, the breakdown voltage between the collector and emitter decreases, and in the worst case, the breakdown voltage of the first and second transistors is broken, a breakdown current flows, and the circuit function is lost. This is a problem, and it is thought that it will become more prominent in the future as epitaxial layers become thinner and smaller. Incidentally, as a factor for breaking the breakdown voltage, there can be mentioned a case where the input voltage Vin deviates from the range of the standard value.

OBJECTS OF THE INVENTION In view of the above-mentioned problems, the present invention proposes a follower circuit that can substantially increase the collector-emitter pressure of the first and second transistors. This is the purpose.

Structure of the Invention In order to achieve the above object, the present invention is characterized in that a voltage clamp element is connected in parallel with the first transistor, and a voltage drop element is connected in series with the second transistor. It is something.

Embodiment of the Invention FIG. 2 shows the basic configuration of an emitter follower circuit based on the present invention. FIG.

In this figure, voltage clamp element 21 and voltage drop element 2
2 is a newly added component, and the emitter follower circuit 20 is formed by including these components. These elements 2
Specific examples of Nos. 1 and 22 will be described later.

Regarding the input voltage VinIC in FIG. 2, its permissible level is usually determined separately into a rated value and a standard value. The rated value is a value that guarantees that the element will not be destroyed even if V in at a level within this range is applied.
In ECL IC, it is usually +0.5V to -7.0V. In addition, the standard value is a value that guarantees normal operation when operated at a level of ■50 within this range, and is usually -0.7.
V~-1.9v.

Therefore, the worst case is 100.5v~-7O■
must be designed to withstand

Conventional emitter follower circuits have a structure that can withstand the worst case. However, as described above, now that epitaxial layers tend to become thinner, there are cases where such a worst case can no longer be tolerated. If we consider the case where Vin rises to +〇, 5■, the potential at the intermediate connection point P in Fig. 2 also rises. Then, the collector-emitter voltage of the first transistor 11 becomes ■. Although IC1 decreases, the second transistor 12
collector-emitter voltage V. , 2 increases. This is because the emitter voltage (E) of the M2 transistor 12 is constant (VB - VBI). There is a possibility that the second transistor 12 may be destroyed. So, voltage drop element 22? ■ The voltage mentioned above that you are trying to increase by inserting it.

ln2'' to protect the second transistor 12.

Conversely, if we consider the case where the voltage Vin drops to 17OV, the potential at the intermediate connection point P in FIG. 2 also drops. Then, the voltage of the second transistor 12 is -. Although o2 decreases (the voltage VzFi is constant), the voltage of the first transistor 11 is -. 81 increases. here the first
There is a possibility that the transistor 11 may be destroyed. Therefore, a voltage clamp element 21 is inserted to prevent the potential at the intermediate connection point P from dropping below a certain level. At this time, the voltage to be clamped is set slightly higher than the collector-emitter breakdown voltage Bvo]e1 of the first transistor 11.

In this way, the voltage clamping element 21 is able to control the voltage when Vin falls deeply to -ZO■ (the difference from the lower limit of the standard value is about 5.1
(7,0-1,9)V), and its role is more important than that of the voltage drop element 22. This is because when Via rises to +〇, 5■ with 22 voltage drop elements (the difference from the upper limit of the standard value is about 1
．． 2 (0,5+0.7) V) to protect the second transistor 12.

That is, the voltage drop element 22 is 1. While it is sufficient to perform a voltage drop function of less than several 7, the tJaF clamp element 21
5. Level differences below the equation 7 must be clamped.

3A, 3B and 30 respectively show the first. FIGS. 4A, 4B and 40 are diagrams showing second and third specific examples, respectively. FIGS. It is a figure which shows the 2nd and 3rd specific example. In the first specific example 21-1 of the voltage clamp element 21, a third transistor 31 is used, and a predetermined clamp voltage VOL is applied to the base of the third transistor 31.
(Focusing on the lower limit of the standard value of Vin -1.9V, for example, VoL knee 2.1V) is given. Second specific example 21
-2 is one in which a plurality of diodes 32 are connected in series. The third specific example 21-3 combines a level shift using a diode 33 and a voltage drop using a resistor 34. A first specific example 22-1 of the voltage drop element 22 is a diode 41 that performs level shifting, and a second specific example 22-1 is a diode 41 that performs level shifting.
Using the voltage drop caused by the -2tfi resistor 42, the third specific example 22-3 consists of a diode 43 and a resistor 44, which are a combination of the above-mentioned 22-1 and 22-2fe.

DETAILED DESCRIPTION OF THE INVENTION As described in detail, according to the present invention, it is possible to realize an emitter follower circuit which improves the apparent withstand voltage between the collector and emitter and does not cause any problem in thinning the epitaxial layer.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an example of an emitter follower circuit with a general constant current transistor, FIG. 2 is a circuit diagram showing the basic configuration of an emitter follower circuit based on the present invention, and FIG.
, 3B and 3C respectively show the first . The diagrams 4A, 4B and 4C show the second and third specific examples, respectively. It is a figure which shows the 2nd and 3rd specific example. 11...First transistor, 12...
Second transistor, 13...constant current resistor, 2
0...Emitter funnel lower circuit, 21...
・Voltage clamp element % 22... Voltage drop element, 31... Third transistor, 32, 33, 4
1.43...Diode% 34,42.44
······resistance. Patent Applicant Fujitsu Limited Patent Attorney Akira Aoki Patent Attorney Kazuyuki Nishidate (1) Yukio Patent Attorney Akiyuki Yamaguchi Figure 1 Figure 2 Figure 3B Figure 30 Figure 111 1] -4) 1 □-" Z Figure 4B " 21 ■ j 2 Figure 4C: 2 □-1" Yoge) 10→Voul −-1-Vouj

Claims (1)

[Claims] 1. A first transistor that receives input voltage as a base, and a second transistor connected in series with the first transistor and provided with a constant current resistor for forming a constant current; An emitter follower circuit configured to take out an output voltage from an intermediate connection point between the first and second transistors, comprising: a voltage clamp element connected in parallel to the first transistor; and a voltage drop element connected in series to the second transistor. Furthermore, it features an emitter follower circuit. 2. The emitter follower circuit according to claim 1, wherein the voltage distribution clamping element comprises a third transistor receiving a predetermined constant voltage at its base. 3. The emitter follower circuit according to claim 1, wherein the voltage clamp element comprises a plurality of series-connected diodes. 4. The emitter follower circuit according to claim 1, wherein the voltage clamp element comprises a diode and a resistor connected in series. 5. The emitter follower circuit according to claim 1, wherein the voltage drop element comprises a diode. 6. The emitter follower circuit according to claim 1, wherein the voltage drop element comprises a resistor. 7. The emitter follower circuit according to claim 1, wherein the distribution voltage drop element comprises a diode and a resistor connected in series.