JPH037167B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a ground separation circuit between circuits, and more particularly to a ground separation circuit in a circuit system in which each circuit is individually grounded.

For example, in car audio equipment, a signal source such as a cassette deck or tuner and an amplifier for amplifying the signal output are often installed separately to form a so-called component car stereo system. Therefore, if there is impedance in the ground system that connects these components, various noise currents such as alternator noise, ignition noise, and horn noise generated from the car will flow there, and this noise will be transferred to the amplifier input along with the signal. The components are applied in a superimposed manner, causing noise.

In order to prevent the generation of noise due to the noise current generated in such a ground path, there is a method of separating the ground between each component using a line transformer or photo coupler, but line transformers have problems in terms of weight and cost. However, the photo coupler has the disadvantage that it picks up noise by itself and outputs it. Therefore, a method of removing noise using a ground separation circuit as shown in FIG. 1 has been adopted.

In other words, signal source circuit 1 such as tuner or deck
A differential amplifier 3 is provided between the amplifier 2 and the output terminal of the signal source circuit 1, and the output terminal of the signal source circuit 1 is connected to the positive phase input of the differential amplifier 3, and the ground terminal of the signal source circuit 1 is connected to the The differential amplifier 3 is connected to the phase inputs, and the output end and common end (ground end) of the differential amplifier 3 are connected to the input end and the ground end of the amplifier 2, respectively. In addition, 4 is a speaker, +B
indicates karbattuteri.

By doing this, if a noise voltage e _N is applied between the ground terminal of the amplifier 2 on the output side and the ground terminal of the signal source 1 on the input side, the positive-sequence input voltage of the differential amplifier 3 will be e _S +e _N (e _S is the signal voltage) and the negative phase input voltage is e _N , so the gain of the differential amplifier 3 is A
Then, the differential amplifier output e ₀ , that is, the input of the output amplifier 2 becomes e ₀ =A(e _S +e _N −e _N )=Aeg (1). Therefore, the noise voltage existing between the ground is removed.

However, the CMRR in differential amplifier 3
(common-mode component rejection ratio) is limited to about 50 dB to 60 dB, so the noise component e _N applied in the same phase to the differential input
It is impossible to completely eliminate this, and the number of circuit elements increases, resulting in high costs.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a ground separation circuit which isolates the ground between each component in order to remove the noise component generated between the ground terminals of each component with an extremely simple configuration.

The earth separation circuit of the present invention is intended for use as an earth separation circuit in a circuit system in which a signal source circuit and a signal processing circuit that performs processing such as amplification on the output of this signal source are individually earthed. A first transistor to which an input is applied to the base, a second transistor whose base input is the output of this first transistor and whose conductivity type is opposite to that of the first transistor, and a constant ratio between these first and second transistors. and a current supply means for supplying a current and outputting another current having a fixed ratio relationship with these currents, and a grounding point for the current flowing through the first and second transistors and a grounding point for the other current. This is due to the separation of the

The present invention will be explained below using the drawings.

FIG. 2 is a circuit diagram of an embodiment of the present invention.
Parts equivalent to those in the figures are designated by the same reference numerals. A ground separation circuit 5 according to the present invention is provided between a signal source circuit 1 such as a tuner or deck and an amplifier 2. This circuit 5 includes a PNP transistor Q ₁ whose base receives the signal from the signal source 1, an NPN transistor Q ₂ whose base input is the emitter output of this transistor Q ₁ , and furthermore, both transistors Q ₁ and Q _2. and a current mirror circuit that supplies a constant ratio of current to the current. The current mirror circuit is a PNP transistor with diode connection configuration.
Q ₄ , a PNP transistor Q ₃ having a common base with the base of this transistor Q ₄ , and resistors R ₂ , R ₃
, the current at a constant ratio (including 1) to the collector current of transistor Q ₂ is
It is now being supplied to _Q1 . The collector of transistor Q ₁ is directly connected to the ground side of signal source 1, and the emitter of transistor Q ₂ is connected to resistor R _1.
It is connected to the ground side of the signal source 1 via.

The previous current mirror circuit consists of transistor _Q4
PNP with base and common connected base
has a transistor Q ₅ and its emitter resistor R ₄ ,
It outputs a current at a constant ratio (including 1) to the current of transistor _Q4 . This current output is connected to flow into the ground side of the amplifier 2 via resistors R ₅ and R ₆ . Furthermore, resistance
R ₆ is the impedance of the signal ground and can be ignored.

Assuming that the signal output voltage of the signal source 1 is e _S and the noise voltage e _N superimposed on the ground line, the base voltage V _B of the transistor Q ₁ becomes V _B = e _S + e _N (2). The emitter voltage V _E of the transistor Q ₂ is V _E = V _B + V _BE1 − V _BE2 = e _S + e _N + V _BE1 − V _BE2 (3). Here, V _BE1 and V _BE2 are transistors Q ₁ ,
Let it be the voltage between each base and emitter of Q ₂ .

Here, V _BE of a transistor is generally expressed as V _BE = _klo (Ic/Is+1) (4). k is a constant, Ic is collector current, Is
is the base-emitter reverse saturation current. It can be considered that Ic/Is≫1, and if transistors with equal Is are used as transistors Q ₁ and Q ₂ , then V _BE1 − V _BE2 = _klo (Ic ₁ / Ic ₂ ) …(5) becomes. Ic ₁ and Ic ₂ are respective collector currents of both transistors Q ₁ and Q ₂ . Since Ic ₁ /Ic ₂ is now constant due to the current mirror circuit, equation (5) is expressed by a constant value α. Furthermore, Ic ₁ / Ic ₂
If is 1, α=0. Therefore, equation (3) becomes V _E = e _S + e _N + α (6).

Here, Ic ₂ is calculated as Ic ₂ = (V _E −e _N ) / R ₁ = (e _S + e _N + α−e _N ) / R ₁ = (e _S + α) / R ₁ …(7) It will be done. Also, due to the current mirror effect of transistors Q ₄ and Q ₅ , Ic ₂ and transistor
Since the output Ic ₃ of Q ₅ is a constant ratio, this circuit 5
The output voltage e ₀ of is expressed as e ₀ = (R ₅ + R ₆ ) Ic ₃ = (R ₅ + R ₆ )·βIc ₃ (8). β is a constant value. Substituting equation (7) into equation (8), e ₀ = R ₅ + R ₆ /R ₁ ·β (e _S + α) (9), and no noise will occur in the output.
In addition, as a special case, when α=0 and β=1, e ₀ =R ₅ +R ₆ /R ₁ ·e _S (10).

As described above, according to the present invention, noise caused by ground impedance between input and output can be effectively removed with an extremely simple configuration, and therefore the present invention is particularly suitable for use in in-vehicle audio equipment, but is not limited thereto. There isn't. Furthermore, since the V _BE component of the bipolar transistor constituting the earth separation circuit does not appear in the output, there is an advantage that no signal distortion occurs due to the addition of the earth separation circuit.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional earth separation circuit, Figure 2
The figure is a circuit diagram of an embodiment of the present invention. Explanation of symbols of main parts, 1...Signal source circuit, 2
...Amplifier, 5...Earth separation circuit, Q ₁ ...First transistor, Q ₂ ...Second transistor, Q ₃
~Q ₅ ...Current mirror transistor.

Claims (1)

[Claims] 1. A ground separation circuit in a circuit system in which a signal source circuit and a signal processing circuit that processes the output of the signal source are individually grounded, the first transistor having an input applied to its base; The output of the first transistor is used as a base input, and a second transistor of a conductivity type opposite to that of the first transistor is supplied with a constant ratio of current to the first and second transistors, and a constant ratio relationship with these currents is provided. and a current supply means for outputting a different current, the earthing separation being characterized in that a grounding point for the current flowing through the first and second transistors and a grounding point for the another current are separated. circuit. 2. The earth separation circuit according to claim 1, wherein the current supply means is a power supply mirror circuit.