JPS60153207A - Differential amplifier circuit - Google Patents

Abstract

PURPOSE:To remove the generation of an unnecessary output at the time of no input by supplying idling current to the signal input side of a differential amplifier by a constant current circuit to which operating current common to the differential amplifier is applied. CONSTITUTION:Since current corresponding to a half of current flowing into a Tr8 of the differential amplifier 2 is made to flow into Trs 4, 6, the value of current applied from a Tr56 of the constant current circuit 48 to the Tr6 is set up to 1/2hFE under the consideration of current amplification factor hFE, so that the emitter area of the Tr54 is set up to twice the Tr56. Consequently, constant idling current 1/2hFE is applied to the base of the Tr6 independently of the existence of the signal input and the base bias of the Tr6 is increased, so that no output is generated until a signal is applied to an input terminal 26.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a differential amplification circuit, and particularly to an improvement in differential amplification suitable for a measurement circuit such as a level meter circuit.

FIG. 1 shows a differential amplifier circuit used in a level meter circuit. That is, the differential amplifier 2 is connected to the transistor 4
．． 6.8.10, consists of a diode 12 and a resistor 14.16, the operating current is set by the transistor 8, and the base of the transistor 4 has a resistor 18 and a diode 20.
．． A constant bias is applied at 22 and a constant voltage is applied to terminal 24. Further, an input terminal 26 is formed at the base of the transistor 6 via a resistor 16, and an input signal is applied thereto. In this differential amplifier 2, the transistor 10 and the diode 12 constitute a current inversion circuit, and the differential output is taken out from the collector of the transistor 6 and applied to the output circuit 28.

The output circuit 28 includes transistors 30, 32, 34, 36
．． 38 and a resistor 40. A driving power source of voltage Vcc is connected between the reference potential point terminal 42 and the power source terminal 44, and the output is taken out from an output terminal 46 formed at the collector of the transistor 36.

In such a differential amplifier circuit, even when there is no input due to a mismatch in the characteristics of the transistors 4 and 6 constituting the differential amplifier 2, a slight current flows, and this flows into the output circuit 2.
Occurs from 8.

For example, when applying an AC signal shown in FIG. 2A to the input terminal 26 and taking out a half-wave rectified output from the output terminal 46, a slight current flows through the resistor 40 in a section that should normally have no output, and as shown in FIG. 2B. A voltage drop VL given by the product occurs as shown in FIG.

Such erroneous output occurs in the case of level meter circuits, etc.
This causes a slight fluctuation in the meter and lighting of the display element, which causes a decrease in measurement accuracy.

An object of the present invention is to provide a differential amplifier circuit that suppresses unnecessary output.

This invention installs a constant current circuit that generates a current output when given an operating current common to that of a differential amplifier, and uses the output current of this constant current circuit to flow an idling current to the signal input side of the differential amplifier. , which prevents unnecessary output from occurring.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 3 shows an embodiment of the differential amplifier circuit of the present invention.
The same parts as those in the circuit shown in the figure are given the same reference numerals.

This differential amplifier circuit is provided with a constant current circuit 48 that supplies an idling current to the input side of the differential amplifier 2. This constant current circuit 48 is supplied with an operating current common to that of the differential amplifier 2, and applies a predetermined constant current to the base of the transistor 6. That is, this constant current circuit 48 includes transistors 50.
52, 54, 56 and a resistor 58, and a bias current is applied to the base of the transistor 52 and the base of the transistor 8 in common. Transistor 50 is connected in series with transistor 52, and transistor 50 is connected in series with transistor 52.
4.56 constitutes a current inversion circuit, which inverts the base current of the transistor 50 and applies it to the base of the transistor 6. - The operation will be explained based on the above configuration. Assuming that the current flowing through the transistor 8 is ■, a current of I/2 flows through the transistor 4.6, so the value of the idling current applied from the transistor 56 to the transistor 6 is 1/2, taking into account the current amplification factor hFE. It is set to 2hFE. To set this current, the emitter area of transistor 54 is twice that of transistor 56.
Set to double.

In this way, the base of the transistor 6 has a constant idling current r/2hp regardless of the presence or absence of a signal input.
E is applied, and the base bias of transistor 6 becomes deep, so that no output is produced unless a signal is applied to input terminal 26. As a result, the level meter circuit does not require zero point adjustment, and a highly accurate measurement output can be obtained.

Furthermore, when such a differential amplifier circuit is constructed using a semiconductor integrated circuit, it is possible to improve the consistency of the constituent transistors, so even if the current amplification factor hFE changes relatively due to mass production, the same disadvantages in terms of circuit operation can be avoided. It does not cause

As described above, according to the present invention, unnecessary output generation during unmanned operation can be effectively eliminated.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a conventional differential amplifier circuit, FIG. 2 is an explanatory diagram showing its operating waveforms, and FIG. 3 is a circuit diagram showing an embodiment of the differential amplifier circuit of the present invention. 2... Differential amplifier, 48... Constant current circuit.

Claims (1)

[Claims] (11) A constant current circuit that is supplied with an operating current common to the differential amplifier and generates a current output is provided, and an idling current is caused to flow through the signal input side of the differential amplifier by this constant current circuit. A differential amplifier circuit characterized by: (2) The idling current is set to a value obtained by dividing 1/2 of the operating current flowing through the differential amplifier by the current amplification factor of the transistors forming the differential amplifier. A differential amplifier circuit according to claim 1, characterized in that: