JPH0432812Y2 - - Google Patents

Description

[Detailed Description of the Invention] Technical Field The present invention relates to an earth isolation amplifier that relays signals between devices installed at separate locations and having different grounding systems.

BACKGROUND ART Signal relay between audio signal processing devices, such as an in-vehicle audio device, where one is installed in a dash panel and the other is installed in a trunk room or the like away from the dash panel, is performed while suppressing the mixing of ground noise into the signal. For this reason, an earth isolation amplifier as shown in FIG. 9 is used.

In FIG. 9, a current limiting resistor R ₃ is connected to one end A of the resistor voltage divider circuit composed of resistors R ₁ and R ₂ .
A voltage Vcc is applied from a bias power supply Vcc through the resistor voltage divider circuit, and the other end of the resistor voltage divider circuit is connected to a ground point B that is common to the ground system of an input signal supply source (not shown). Further, a bypass capacitor _C1 is connected in parallel to the resistor voltage divider circuit, so that the AC potentials at points A and B are made equal. The divided voltage of the resistor voltage divider circuit is applied to the base of the PNP transistor _Q1 . This base is supplied with the input signal via a coupling capacitor _C2 . The emitter of the transistor _Q1 is connected to the emitter resistor _R4 , and the other end C of the resistor _R4 is connected to the connection point A between the resistors _R1 and _R3 .
The collector of the transistor _Q1 is connected to one end of a coupling capacitor _C3 and a collector resistor _R5 ,
The other end of the capacitor C ₃ is connected to the center conductor 2 a of the signal transmission line 2 . In this way, the isolation amplifier 1 is constructed. Collector resistance R ₅
The other end is connected to the shield conductor 2b of the signal transmission line 2. The other end 3a of the center conductor 2a is connected to an input end of an output circuit (not shown), and the other end 3b of the shield conductor 2b is connected to a ground point D of the output circuit. Here, inter-ground noise caused by the potential difference between the ground point D and the ground point B is indicated as a noise source h.

Next, the operation of this circuit will be explained. First, the isolation amplifier 1
operates as a common emitter amplifier circuit using a well-known PNP type transistor, and the potential of the input signal is
When the value of the emitter resistance R ₄ is R ₄ , the emitter current ie becomes ie≈hii/R ₄ . Also the collector resistance
If the value of _R5 is set to _R5 and the output side circuit 3 is made to have a high input impedance, the emitter current ie will almost flow through the collector resistor _R5 , so the voltage Hc applied to the collector resistor _R5 will be Hc≒ It becomes ie・R ₅ . Therefore, the gain of such an amplifier circuit is expressed as R ₅ /R ₄ .
In addition, since the transistor _Q1 acts equivalently as an AC current source and its impedance is very large, the noise current in from the noise source hin hardly flows to the collector resistor _R5 , and the noise current in from the noise source hin flows to the output signal from the noise source hin. Noise contamination can be ignored. Therefore,
This circuit suppresses noise from entering the signal due to differences in grounding systems (hereinafter referred to as interference) (hereinafter referred to as noise suppression effect) and prevents interference in signal relays between installations located far apart from each other. It is possible to do so.

However, as the frequency of the noise hin increases, the impedance between the base and collector decreases as the frequency increases due to the base-collector capacitance of the transistor _Q1 , and the noise suppression effect is impaired and the noise is passed through the resistors _R2 and _R5 . As a result, a noise loop current in is generated. In addition, if the output admittance of the transistor is not small enough, the noise suppression effect will not be sufficient and a noise loop current will be generated from a relatively low frequency, and ground noise will enter the output signal and cause interference. S/N of output signal
There is a problem where the value decreases.

Summary of the invention Therefore, an object of the invention is to provide an earth isolation amplifier that can exhibit a noise suppressing effect over a wide band.

In order to achieve the above object, the earth isolation amplifier of the present invention includes at least a two-stage amplifier circuit using an emitter-grounded transistor amplifier circuit, the front-stage amplifier circuit is connected to the first grounding system, and the rear-stage amplifier circuit is connected to the second grounding system. a transistor multi-stage amplifier circuit connected to the circuit and whose collector output of the front-stage amplifier circuit serves as a base input of the rear-stage amplifier circuit, and an AC voltage divider circuit connected between the first and second ground systems,
The divided voltage output is supplied to the collector circuit of the front-stage amplifier circuit and the emitter circuit of the rear-stage amplifier circuit.

Embodiment Hereinafter, an embodiment of the present invention will be described with reference to FIG. In FIG. 1, an isolation amplifier 1 includes a grounded emitter transistor amplifier circuit 10 as a front stage amplifier circuit, a grounded emitter transistor amplifier circuit 20 as a rear stage amplifier circuit, and a resistive voltage divider circuit 30. The amplifier circuit 10 includes a base bias circuit consisting of resistors R ₁₁ and R ₁₂ to which a bias voltage Vcc is applied to one end A via a resistor R ₃ and the other end connected to a ground point B, and this base bias circuit. An NPN transistor Q ₁₀ to which a predetermined voltage is applied to the base, a resistor R ₁₃ connected between the emitter of this transistor Q ₁₀ and the ground point B, and one end connected to the transistor Q 10
It is formed by a resistor _R14 connected to the collector of _Q10 . The input signal is supplied to the base of transistor _Q10 via a coupling capacitor _C10 . A resistor R ₃₁ is connected between the other end C of the resistor R ₁₄ and one end A of the base bias circuit, and the one end A is AC grounded to the ground point B by a bypass capacitor C ₁₁ .

The collector output of transistor _Q10 is amplifier circuit 2
0 to the base of a PNP transistor 20. The emitter of the transistor 20 is connected to the other end C of the resistor _R14 via a resistor _R21 , and the collector is connected to the ground point D via a resistor _R22 . The voltage applied across the resistor R ₂₂ is supplied as an output signal to the output terminal 3a via the coupling capacitor C ₂₀ .

A resistor R ₃₂ is connected between the other end C of the resistor R ₁₄ and the ground point D via a bypass capacitor C ₂₁ (DC blocking capacitor). Resistors R ₃₁ and R ₃₂ form a resistive voltage divider circuit 30 . In addition, the resistor voltage divider circuit 30
forms an AC voltage divider circuit with capacitors C ₁₁ and C ₂₁ . The values of these resistors R ₃₁ and R ₃₂ are the same as those of resistors R ₁₄ and
It is set to a value sufficiently smaller than the value of _R21 .
Capacitor _C11 and
A noise voltage hin is applied through _C21 , and the divided noise component is applied to the collector circuit _R14 of the transistor _Q10 and the emitter circuit _R21 of the transistor _Q20 .

Next, the operation of the circuit will be explained. Amplifier circuit 1
Since 0 and 20 are emitter grounded amplifier circuits, the values of resistors R ₁₃ , R ₁₄ , R ₂₁ and R ₂₂ are respectively
Assuming R ₁₃ , R ₁₄ , R ₂₁ and R ₂₂ , the input signal is multiplied by R ₁₄ /R ₁₃ by the amplifier circuit 10, further multiplied by R ₂₁ /R ₂₂ by the amplifier circuit 20, and the output signal is sent to both ends of the resistor R ₂₂ . It is derived as By the way, the transistors Q ₁₀ and Q ₂₀ have a collector-base capacitance Cob and an output admittance hoe, and cannot serve as perfect current sources, and noise between the ground and the ground intrudes into the signal system. A resistive voltage divider circuit 30 is provided to suppress this. The noise suppression effect of the present invention will be explained below with reference to FIG.

The circuit shown in FIG. 2 is an AC equivalent circuit of the circuit shown in FIG. n and the noise voltage (1-t) applied between the connection points C and D.
Here, t is the partial pressure ratio, R ₁₇ / (R ₁₇ + R ₁₈ )
It is expressed as Resistor R ₃₀ represents a composite resistance in which resistors R ₁₇ and R ₁₈ are connected in parallel. Front stage amplifier circuit 1
0 has the same configuration as a conventional isolation amplifier, and its transfer function for the noise component is expressed in the form A+jωB. Here, A and B are real numbers, ω is the frequency of noise n, and j is an imaginary unit. Therefore, the noise voltage applied to the resistor _R14 due to the noise voltage thin becomes thin(A+jωB). Further, the transfer function for the noise component of the rear-stage amplifier circuit 20 is expressed as C+jωD. Here C and D are real numbers. Therefore, the noise voltage (1
The noise voltage applied to the resistor R ₂₂ by -t)hin becomes (1-t)hn(C+jωD). In addition,
The above A, B, C and D are 0 or a value larger than 0. Furthermore, the rear stage amplifier circuit 20 includes a resistor
The noise voltage applied across _R14 is amplified and output across resistor _R22 . Assuming that the amplification factor of the second-stage amplifier circuit 20 is 1 and applying the superposition principle in consideration of the output phase, the noise output voltage H o applied across the resistor R ₁₆ is H o = (1 - t) hin(C+jωD)−thin(A+
jωB) (dB). If the transfer function indicating the relationship between the noise voltage h and the output voltage h is expressed on a complex plane as h, it will look like the vector diagram in FIG. 3. In the figure, h ₁ =A+jωB, h ₂ =C+
jωD, (1-t)<1. Here, the transfer function h
=(1-t)(C+jωD)-t(A+jωB), if the value of t is set to t=D/(D+B), then ho=(B・C−D・A)/(D+B) and becomes an imaginary number The parts cancel each other out, frequency dependence is eliminated, and the value of the real part also decreases. Therefore, noise suppression is achieved. On the other hand, the possible value |h| of the transfer function h with t as a variable is |h|≦|h ₁ | or |h|≦
|h ₂ | Therefore, at least the noise suppression effect of the isolation amplifier of the present invention is such that even if the setting of the voltage division ratio t is not appropriate, one of the amplifier circuits constituting the two-stage amplifier circuit has a worse noise suppression effect. It is preferable in terms of circuit reliability and is not inferior to the latter. Furthermore, by adjusting the ratio between the real part and the imaginary part of the transfer function h so that A/B=C/D, it is possible to make the value of the transfer function h almost 0, and the noise voltage h component is removed.

When the amplification degree of the second-stage amplifier circuit 20 is α, the above transfer function h(α) is h(α)=(1-t)(Cα+jωDα)-αt(A+
jωB). However, Cα+jωDα is a transfer function for the noise component of the rear stage amplifier circuit 20. Even in such a case, there is a t whose imaginary component is 0. Therefore, by appropriately setting the pressure ratio of the voltage dividing circuit 30, it is possible to suppress a decrease in the noise suppression effect in the high frequency range.

Figure 10 shows the frequency characteristic A of the conventional isolation amplifier and the frequency characteristic B of the isolation amplifier according to the present invention, where the vertical axis shows the frequency characteristic when 0 [dBV] is applied as the noise voltage. The horizontal axis represents the frequency [KHz] of the noise voltage hin, which represents the leakage to the output terminal. As is clear from this figure, the intrusion of ground noise into the output end is significantly suppressed over a wide range of radio frequencies, and the noise suppression effect is particularly improved in the high frequency range.

Other embodiments of the invention are shown in FIGS. 4 to 8. In the circuit shown in each figure, the parts corresponding to the circuit shown in FIG. 1 are denoted by the same reference numerals, and the explanation of these parts will be omitted.

In the circuit of Figure 4, the transistor _Q10
PNP type transistor, NPN in transistor Q ₂₀
It uses a type transistor. Resistor R ₄₀ is a decoupled spring resistor that isolates the ground system.

In the circuit of FIG. 5, PNP type transistors are used for transistors _Q10 and _Q20 . Capacitor C ₂₂ supplies the divided voltage output (AC component) of the voltage dividing circuit (resistors R ₃₁ and R ₃₂ ) to the emitter circuit of transistor Q ₂₀ .

In the circuit of FIG. 6, FETs are used as transistors _Q10 and _Q20 . Resistor R ₄₁ is the gate bias resistance of transistor Q ₂₀ . Capacitor _C12 is a coupling capacitor.

The circuit of FIG. 7 is an example in which the positions of the resistor R ₃₁ of the voltage dividing circuit and the bypass capacitor C ₁₁ of the voltage dividing circuit are changed in the circuit of FIG. 1, and both are equivalent as an AC circuit.

The circuit in Figure 8 is a transistor of the circuit in Figure 1.
This is an example in which Q ₁₀ is a FET and the resistor R ₃₁ and capacitor C ₁₁ are connected with their positions reversed.

The AC equivalent circuit of each circuit is equivalent to the AC equivalent circuit of the circuit shown in FIG. 1, and the same noise suppressing effect can be obtained.

Note that the resistive voltage divider circuit 30 can be configured to be a variable resistor or include a variable resistor, and in such a case, it is possible to adjust differences in characteristics of each circuit due to variations in the values of circuit elements. Further, the AC voltage divider circuit that divides the voltage of the AC component is not limited to a resistive voltage divider circuit, but may be configured by an impedance circuit.

Effects of the invention As explained above, the earth isolation of the invention uses a grounded emitter multistage amplifier circuit in which the first stage is connected to the first grounding system and the subsequent stage is connected to the second grounding system, and the ground-to-ground noise voltage. The divided voltage output of this voltage dividing circuit is supplied at an appropriate value to the collector circuit of the front-stage amplifier circuit and the emitter circuit of the rear-stage amplifier circuit, so that the combined transmission of both circuits to the ground-to-ground noise is achieved. Since the configuration is such that the frequency dependence and gain of the function are reduced, it is preferable that ground noise components are suppressed from being mixed into the output signal from low to high audio frequencies.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG.
The figure is a circuit diagram showing an AC equivalent circuit of the circuit shown in Fig. 1, Fig. 3 is a vector diagram for explaining the operation of the circuit shown in Fig. 2, and Figs. 4 to 8 9 is a circuit diagram showing another embodiment of the present invention, FIG. 9 is a circuit diagram showing an example of a conventional circuit, and FIG. 10 is a characteristic curve diagram showing frequency characteristics of the conventional circuit and the circuit of the present invention. 10... front stage amplifier circuit, 20... rear stage amplifier circuit, 30... voltage dividing circuit.

Claims (1)

[Scope of utility model registration request] a first transistor amplifier circuit with an emitter-grounded configuration having a first grounding system; and a second transistor amplifier circuit with an emitter-grounded configuration that uses the collector output of the first transistor amplifier circuit as a base input and has a second grounding system. The isolation amplifier includes an AC voltage divider circuit that is connected between the first grounding system and the second grounding system and outputs only an AC component, and the divided voltage output of the AC voltage divider circuit is connected to the AC voltage divider circuit. 1st
An earth isolation amplifier characterized in that the earth isolation amplifier is supplied to a collector circuit of a transistor amplifier circuit and an emitter circuit of the second transistor amplifier circuit.