JPH06101651B2 - Isolation amplifier circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an isolation amplifier circuit, and more particularly to an audio amplifier device which is installed at positions distant from each other and which is coupled to each other with different grounding systems. 2. In regard to the above, it relates to improvement of an isolation amplifier circuit suitable for repeating and amplifying an audio signal.

[Conventional technology]

The isolation amplifier circuit is used for the purpose of interposing between a pair of circuits or devices to be relayed while amplifying a signal and electrically insulating the two. That is, for example, in an audio system for an automobile, the constituent devices of the audio system may be installed at positions apart from each other (for example, a dashboard and a trunk room) due to restrictions on installation space and the like. In such a case, it is necessary to ground each device in order to amplify and transmit the signal between the two devices that are spaced apart from each other. Regarding the grounding, generally, so-called body grounding is performed by connecting to the body near the installation location of the device. However, the car itself can be regarded as a kind of noise source. This is because noise is generated from the alternator, ignition, etc. because the engine is driven. Such noise has different potentials at each place,
The transmission signal is mixed from the body ground that connects the devices that are arranged apart from each other, and the S / N ratio is deteriorated. Therefore, an isolation amplifier circuit is interposed in the signal transmission paths of both devices to electrically insulate the signal to prevent noise from entering.

FIG. 5 shows an example of a conventional isolation amplifier circuit 50. As shown in FIG. 5, the audio signal sent from the first signal processing circuit (not shown) is input to the input terminal IN, and the PNP transistor Q is input via the coupling capacitor C _1.
Supplied on base of ₁ . The emitter of the transistor Q ₁ is connected to the DC power supply V _CC via the emitter resistor R _E. Between the DC power supply V _CC and the first ground system G ₁ is connected resistors R _1, R ₂ connected in series, the connection point of the resistors R _1, R ₂ is connected to the base of the transistor Q _1. The collector of the transistor Q ₁ is connected to the output terminal OUT and is also connected to the second ground system G ₂ via the collector resistance R _C.
Here, it is assumed that the first grounding system G ₁ and the second grounding system G ₂ are present at positions distant from each other, such as the above-described automobile dashboard and trunk room.

With respect to the first ground system G ₁ of the isolation amplifier circuit 50 having such a configuration, the vehicle noise causes the entire signal voltage to be oscillated by an inductive effect, so-called common mode noise V _{N, which} is the first ground system G ₁ and the second ground system. May be applied across system G ₂ .

Next, the operation of the isolation amplifier circuit 50 will be described.
First, when common mode noise is not applied, the isolation amplifier circuit 50 is known to the input signal.
It operates as a grounded-emitter amplifier circuit using a PNP transistor. That is, assuming that the input signal voltage value is v _i and the resistance value of the emitter resistance R _E is R _E , the emitter current i _e is i _e = v _i
Expressed as / R _E. Also, change the resistance value of the collector resistance R _C to R _C
Assuming that the second signal processing circuit (not shown) connected to the output terminal OUT has a high input impedance, the emitter current i _e almost flows through the collector resistance R _C, and therefore is applied to the collector resistance R _C. The applied voltage v _c is v _c = i _e × R _C
Is expressed as Therefore, the gain A of the isolation amplifier circuit 50 is A = v _c / v _i = R _C / R _E , and the isolation amplifier circuit 50 operates as an amplifier circuit.

FIG. 6 shows an AC equivalent circuit of the isolation amplifier circuit 50 including the common mode noise V _N when common mode noise is applied.

Here, C _ob1 is the output capacitance existing between the base and collector, r ₀₁ is the reciprocal of the input open output admittance h _oe when the emitter is grounded in the case of h parameter display, and functions as the output impedance. GT ₁ and GT ₂
Are terminals connected to the first ground system G ₁ and the second ground system G ₂ , respectively. If the AC equivalent circuit shown in FIG. 6 is further simplified, the equivalent circuit shown in FIG. 7 is obtained. Where R _T
Is the total resistance value of the input side load (including the first signal processing circuit) viewed from the base of the transistor Q ₁ , and
Since the resistance r ₀₁ > the emitter resistance R _E , the emitter resistance R _E is ignored.

In FIG. 7, ignoring the existence of the processing capacitance C _{ob1 and letting} the voltage value of the common mode noise V _{N be} V _N , the noise
The noise output value caused by V _N is expressed by the following equation.

Noise output value = R _C · V _N / (r ₀₁ + R _C ) = R _C · V _N / [(1 / h
_oe ) + R _C ] ... As is apparent from this equation, the noise output value is smaller than the voltage value V _N of the common mode noise V _N. Therefore, the mixing of common mode noise V _{N into} the audio signal is
_If the value of C _ob1 is small, it is suppressed to some extent.

[Problems to be Solved by the Invention]

However, in reality, when the output capacitance C _ob1 is a size that cannot be ignored, the output capacitance C _ob1 and the output resistance r ₀₁
Since they are connected in parallel with each other, the total output impedance decreases, the noise output value increases accordingly, and the isolation effect decreases. Especially the transistor Q ₁
When the amplification degree is large (gain A), the total output capacitance becomes C _ob1 × (1 + A) due to the Miller effect, and the isolation effect further decreases. Therefore, suppression of common mode noise becomes insufficient, and noise sound may be mixed into the output audio signal.

Therefore, it is an object of the present invention to provide an isolation amplifier circuit that prevents the isolation effect from being lowered due to the output capacitance between the base and collector of a transistor.

[Means for Solving the Problems]

In order to solve the above-mentioned problems, the present invention provides that a base (B) which is an input terminal is connected to a first ground system (G1) side and a collector (C) which is an output terminal is the first ground system (G1). In a base-grounded isolation amplifier circuit having a transistor (Q1) connected to a second grounding system (G2) side electrically separated from the second grounding system via a collector resistance (R _c ), Between the collector (C) and collector resistance (R _c ) of the transistor (Q1), a transistor (Q ₂ ) whose base (B) is grounded to the first ground system (G1) in an alternating manner is inserted. , The output terminal is derived from the collector of the transistor (Q ₂ ).

[Action]

With this configuration, since the input portion (between the emitter and the base) of the base-grounded transistor amplification circuit which is the load of the grounded-emitter transistor amplification circuit is a low impedance, the output capacitance of the grounded-emitter transistor is The Miller effect due to is small, and the output capacitance (between collector and base) of the base-grounded transistor amplifier is essentially small because the base is grounded. Therefore, even if it is used as an amplifier circuit, it is hardly affected by the Miller effect.

When common mode noise is applied between the first ground system and the second ground system, the first ground system → base / collector of grounded-emitter transistor → emitter / collector of grounded-base transistor → collector resistance → second A noise current tries to flow through the path of the ground system. In this case, since the output resistance of the grounded-base transistor amplification circuit is connected in series to the output resistance of the grounded-emitter transistor amplification circuit, the total output resistance becomes extremely large.
The ratio of collector resistance and total output resistance becomes extremely small. Therefore, since the noise current flowing through the collector resistance is reduced, the common mode noise is suppressed from being mixed into the output signal.

〔Example〕

 Next, an embodiment of the present invention will be described with reference to the drawings.

First Embodiment FIG. 1 shows the isolation amplifier circuit 10 of the first embodiment of the present invention, FIG. 2 shows its AC equivalent circuit, and FIG. 3 shows a further simplified equivalent circuit. In addition, the same reference numerals are given to the portions described in FIGS. 5 to 7, and the description thereof is incorporated.

1 is different from FIG. 5 in that a base-grounded transistor amplifier circuit 1 is added.

As shown in FIG. 1, the grounded-base transistor amplifier circuit 1 is connected as the collector load of the transistor Q ₁ forming the grounded-emitter transistor amplifier circuit. That is, the collector of the transistor Q ₁ is the emitter of the PNP-type transistor Q ₂ to which constitutes the common base transistor amplifier circuit 1 is connected with the collector of the transistor Q ₂ is connected to the output terminal OUT, and the collector resistor R _C It is connected to the second grounding system G ₂ via. The base of the transistor Q ₂ is connected to the positive electrode of the battery E as a bias power source,
The load of the battery E is connected to the first ground system G ₁ . Here, as is well known, the grounded base transistor amplifier circuit is
High output impedance and low input impedance.

Next, the operation of the isolation amplifier circuit 10 will be described with reference to FIGS. 2 and 3.

Load of transistor Q ₁ (ie grounded base transistor
Since the input impedance of Q ₂ ) is low, the output capacitance C _ob1
The capacitance value of is reduced, and the Miller effect due to this output capacitance C _ob1 is reduced. Further, the output capacitance C _ob2 of the transistor Q ₂ shown in FIG. ₂ is originally extremely small because it is configured by grounding the base. Therefore, even if the isolation amplifier circuit 10 is used as an amplifier circuit, two types of output capacitance C _{ob1
And the} effect of the mirror effect based on C _ob2 is small.

Further, the output resistance r ₀₂ in the base ground is expressed by using the h parameter in the grounded emitter as follows.

r ₀₂ = (1 + h _fe ) / h _oe where h _fe is the output short-circuit current amplification factor when the emitter is grounded.

From the simplified equivalent circuit diagram shown in FIG. 3, the noise output value due to the common mode noise V _N is expressed by the following equation.

Noise output value = R _C · V _n / (r ₀₁ + r ₀₂ + R _C ) = R _C · V _n / {[(1+
h _fe ) / h _oe ] + R _C } ... In the formula, r ₀₂ > r _01, so r ₀₁ is ignored.

Comparing the denominator [(1 / h _oe ) + R _C ] of the above formula and the denominator {[(1 + h _fe ) / h _oe ] + R _C } of the formula, the denominator of the formula is larger. Therefore, the noise output value of the equation is smaller than the noise output value of the equation. Therefore,
Based on the operation of the isolation amplifier circuit 10, the mixing of the common mode noise V _{N into} the audio signal is suppressed.

Further, when the common mode noise is not applied between the first grounding system and the second grounding system, the current boosting rate α of the grounded base is approximately 1, so that the original amplification effect of the audio signal is little affected.

Second Embodiment Next, a second embodiment is shown in FIG. The present embodiment is a circuit diagram in conformity with the actual embodiment, and the difference from the first embodiment is that the power supply of the grounded base transistor Q ₂ is performed via the dividing resistors R ₃ and R ₄ . The same parts as those in FIG. 1 or 5 are designated by the same reference numerals, and the detailed description thereof is cited.

As shown in FIG. 4, a DC power supply V _CC is connected to the base of the grounded base transistor Q ₂ of the isolation amplifier circuit 20.
Are supplied by being divided by resistors R ₃ and R ₄ . Between the DC power supply V _CC and the first ground system G ₁ is a capacitor C ₂
Are connected. A capacitor C ₃ connected in parallel with the dividing resistor R ₄ is for AC bypass for operating the transistor Q ₂ with the base grounded. The transistor Q ₃ is a grounded-emitter first-stage amplifier circuit.

With this configuration, it can be applied to, for example, an audio system for automobiles.

〔The invention's effect〕

As described above, according to the present invention, the grounded base transistor amplifier circuit is added to the collector load of the grounded emitter transistor amplifier circuit to reduce the Miller effect due to the output capacitance regardless of the amplification degree.
Since the value of the output impedance becomes large, the applied noise is reduced at the time of output. Therefore, the isolation amplifier effect can be improved.

[Brief description of drawings]

1 is an electric circuit diagram of the first embodiment of the present invention, FIG. 2 is an AC equivalent circuit diagram thereof, FIG. 3 is an equivalent circuit diagram obtained by further simplifying FIG. 2, and FIG. 2 is an electric circuit diagram of the second embodiment, FIG. 5 is an electric circuit diagram showing a conventional example, FIG. 6 is an AC equivalent circuit diagram thereof, and FIG. 7 is an equivalent circuit diagram obtained by further simplifying FIG. 1 ... Base-grounded transistor amplifier circuit 10, 20 ... Isolation amplifier circuit Q ₁ ... PNP transistor for grounded emitter Q ₂ ... PNP transistor for grounded V _N ...... Common mode noise C _ob1 ...... Output capacitance of transistor Q ₁ C _ob2 …… Output capacitance of transistor Q ₂ G ₁ …… First ground system G ₂ …… Second ground system

Claims (1)

[Claims] 1. A base constituting a circuit input terminal of a transistor constituting a grounded-emitter transistor amplifier circuit is based on a first ground system, and a collector side of the transistor is an isolator based on a second ground system via a collector resistor. In the ratio amplifier circuit, a transistor (Q2) whose base (B) is AC-grounded to the first grounding system (G1) between the collector (C) and collector resistance (R _c ) of the transistor (Q1). Is inserted, and the output end is led out from the collector of the transistor (Q2).