JPH0766635A - Differential amplifier circuit - Google Patents

Abstract

PURPOSE:To protect a transistor for differential amplification and to improve the degree of freedom of inputted amplitude by providing a diode for current interruption on the emitter coupling part of a differential circuit and providing an extra constant current circuit. CONSTITUTION:Constant current I02 is supplied from each of constant current sources 7 and 9 and constant current I01 is supplied from a constant current source 8. The maximum collector current of transistors 1 and 2 at the operating point of a differential amplifier circuit becomes 1/2 I01+I02, and when inputted voltage Vin is large on a + side, each collector current of transistors 1 and 2 becomes I01+I02 and I02. At this time, reverse bias is applied to a diode 6 and a common connection point 2 is cut off from the transistor 2. When inputted Vin is large on a negative side, reverse bias is applied to a diode 5 and the point 2 is cut off from the transistor 1. Thus, the both transistors are always made an on state, the transistors are protected even if the amplitude of inputted voltage is large and the degree of freedom of the inputted amplitude can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a differential amplifier circuit, and more particularly to a differential amplifier circuit in which a differential transistor is protected even when the amplitude of an input voltage is large.

[0002]

2. Description of the Related Art A conventional differential amplifier circuit is shown in FIG. In the figure, reference numerals 1 and 2 denote transistors, which are emitter-coupled through resistors 3 and 4 to perform a differential operation. A constant current source 8 supplies a constant current to the transistors 1 and 2. Transistor 1
An input voltage (Vin) is applied to the base terminal of the other transistor, and the base terminal of the other transistor 2 is provided to the common potential point (ground potential point).

The operation of such a transistor will be described with reference to FIG. The horizontal axis represents the input voltage (Vin) and the vertical axis represents the collector current flowing through the transistors 1 and 2. The solid line shows the collector current of the transistor 1, and the broken line shows the operation of the collector current of the transistor 2. Due to the characteristics of the constant current source 8 and the transistors, the maximum collector current of the transistors 1 and 2 is I ₀ .

Therefore, when the collector current of the transistor 1 is I ₀ , that is, when the input voltage (Vin) has a large positive value, the collector current of the transistor 2 does not flow and the transistor 2 is turned off. As a result, a reverse bias voltage (NP
In the case of an N-transistor, the emitter voltage is higher than the base voltage), which may damage the transistor 2. The same applies when the input voltage (Vin) has a large negative value.

[0005]

Therefore, the conventional differential amplifier circuit has a problem that the amplitude value of the input voltage (Vin) is limited. An object of the present invention is to solve such a problem and to realize a differential amplifier circuit having a degree of freedom of amplitude of an input voltage (Vin).

[0006]

SUMMARY OF THE INVENTION According to the present invention, a pair of transistors, one base terminal of which is connected to a common potential point and the other base terminal of which an input is applied and whose collector serves as an output terminal, and a pair of transistors. A pair of current cut-off diodes, which are commonly connected to respective emitter terminals of the pair through resistors, and a first constant current source is connected to the common connection point, and a pair of diodes connected to the emitter terminals of the pair of transistors, respectively. The differential amplifier circuit includes the second and third constant current sources.

[0007]

By connecting a diode in series to the emitter coupling portion of the differential circuit and additionally providing a constant current source circuit, the transistors involved in the differential operation will not be turned off, and the differential amplifier circuit will not be turned off. The transistors for differential operation in are protected.

[0008]

1 is a block diagram of an embodiment of the present invention.
In the figure, the same parts as those in FIG. 4 are designated by the same reference numerals.
Reference numerals 5 and 6 are diodes, and reference numerals 7 and 9 are constant current sources. The constant current source 8 supplies the constant current I ₀₁ , and the constant current sources 7 and 9 supply the constant current I ₀₂ . The anode of the diode 5 is connected to the emitter of the transistor 1, and the anode of the diode 6 is connected to the emitter of the transistor 2. The cathodes of the diodes 5 and 16 are commonly connected via the resistors 3 and 4, and the common connection point is connected to the constant current source 8.

The operation in such a configuration will be described with reference to FIGS. 2 and 3. In FIG. 2, the horizontal axis represents the input voltage (Vin) and the vertical axis represents the collector current flowing through the transistors 1 and 2. The solid line shows the collector current of the transistor 1, and the broken line shows the operation of the collector current of the transistor 2. FIG. 3 shows the voltage generated at each point shown in FIG. 1 with the horizontal axis representing the input voltage (Vin).
Is a voltage generated at the emitter terminal of the transistor 1, is a voltage generated at the connection point of the resistors 3 and 4, and is a voltage generated at the emitter terminal of the transistor 2.

First, FIG. 2 will be described. The maximum collector current of the transistor 1 and the transistor 2 at the operating point (input voltage (Vin) = 0V) of the differential amplifier circuit shown in FIG. ) × I ₀₁ + I ₀₂ . When the input voltage (Vin) has a large value on the + side,
The collector current of the transistor 1 is (I ₀₁ + I ₀₂ ).
At this time, the collector current of the transistor 2 becomes (I ₀₂ ). At this time, the diode 6 is reverse biased,
Since the diode 6 is cut off, the transistor 2
This is because the current appearing at the emitter terminal of is only that supplied from the constant current source 9.

Referring to FIG. In the figure, (a)
If the input voltage (Vin) is large negative value,
The section (1) shows the range of the input voltage (Vin) in which the differential operation is performed in the conventional example, and the section (c) shows the case where the input voltage (Vin) is large to a positive value. At this time, the voltage appearing at the emitter terminal of the transistor 1 shown by is always supplied with the current I ₀₂ by the constant current source 7, so that the input voltage (V
in) from the base-emitter voltage (V
_BE ).

Further, the voltage appearing at the emitter terminal of the transistor 2 indicated by is a value obtained by subtracting the base-emitter voltage (V _BE ) from the ground voltage because the current I ₀₂ is supplied by the constant current source 9. Appears.

The operation of the voltage appearing at the connection point of the resistors 3 and 4 shown in FIG. Input voltage (V
In the section (a) where in) is large as a negative value, reverse bias is applied to both ends of the diode 5 as understood from the fact that the value of the point is large as a negative value.

Therefore, since the diode 5 disconnects the point portion from the transistor 1,
The voltage generated at the point is based on the base-emitter voltage (V _BE ) of the transistor 2 and further the voltage drop (V _D ) due to the diode 6 and the voltage drop due to the current flowing through the resistor 4. . Therefore, [-V
_BE −V _D −I ₀₁ × R] will appear.

In the section (c) where the input voltage (Vin) is large in the positive value, the reverse bias is applied across the diode 6 as can be understood from the fact that the value of the point is large in the positive value. Takes.

For this reason, since the diode 6 disconnects the point portion from the transistor 2,
The voltage generated at the point is the base of transistor 1
It becomes a value based on the voltage drop (V _D ) due to the diode 6 and the voltage drop due to the current flowing through the resistor 4 from the voltage between the emitters (V _BE ). Therefore, [V
_{in-V BE -V D -I 01} × R] that, the input voltage (Vin)
A value obtained by subtracting the voltage drop of the resistor 4 and the diode 6 from the above appears.

The input voltage (Vin) in the range (b) in which the differential operation is performed need not be particularly described. If the characteristics of the resistance 3 and the resistance 4, the diode 5 and the diode 6 are the same, it changes following the input voltage (Vin) with a gradient of half of the voltage value indicated by the point.

That is, the point is [V] = [Vin-V
When represented by _BE], point [V] = [(1/2) × Vin-V BE -V D - (1/2) becomes × I ₀₁ × R] ... and represented it.

In this case, since the differential operation is performed, both the transistors 1 and 2 are both in the ON state and there is no fear of destruction.

In this way, as can be understood from FIG. 2, both transistors 1 and 2 are always in the ON state, so that there is no fear of destruction even when an input voltage (Vin) having a large amplitude is input.

[0021]

As described above in detail, according to the present invention, even when the amplitude of the input voltage (Vin) is large, the transistor that performs the operation for the differential amplification is protected, so that the amplitude of the input is free. The degree improves. Due to its structure, a transistor having good high-frequency characteristics has a very low withstand voltage when a reverse bias is applied to the base-emitter voltage.
Therefore, the diode is reverse-biased, the voltage is cut off from the input, and the current is directly supplied to turn on the transistor, so that the differential amplifier circuit is protected very effectively.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of the operation of the present invention.

FIG. 3 is an explanatory diagram of the operation of the present invention.

FIG. 4 is a configuration diagram of a conventional example of the present invention.

FIG. 5 is an explanatory diagram of an operation of a conventional example.

[Explanation of symbols]

 1, 2 Transistors 3, 4 Resistances 5, 6 Diodes 7, 8, 9 Constant current source

Claims (1)

[Claims] 1. A pair of transistors each having one base terminal connected to a common potential point and an input applied to the other base terminal and having a collector as an output terminal, and resistors connected to the emitter terminals of the pair of transistors respectively. And a common constant connection point and a first constant current source connected to the common connection point, and second and third constant current sources respectively connected to the emitter terminals of the pair of transistors. And a differential amplifier circuit.