JPH0518287B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an amplifier circuit, and more particularly to prevention of generation of a transient signal when power is turned on again after power is cut off in an amplifier circuit in which a buffer circuit is installed at the input section of a differential amplifier.

FIG. 1 shows this type of amplifier circuit. Each differential output generated by the Hall element 2 as a signal source is transmitted through a buffer circuit 4, a resistor 6, and a buffer circuit 8.
and the differential amplifier 1 via the capacitor 10 individually.
2 is entered. The buffer circuits 4 and 8 installed at the input section are installed so that the differential output generated by the Hall element 2 is not influenced by the differential amplifier 12 side. Further, the resistor 6 is an offset prevention resistor for equalizing input and output DC potentials of the differential amplifier 12, and the capacitor 10 is a DC blocking capacitor. That is, the Hall element 2 is
It is used to detect the rotation of a motor, etc., and its usage is often such that the detection output is amplified by the differential amplifier 12 and then input to a comparator or the like and converted into a pulse waveform. At this time, in order to determine the threshold level of the comparator, the output potential of the differential amplifier 12 needs to be fixed. Therefore, an offset prevention resistor 6 is used as a means to prevent unnecessary offset from occurring in the differential amplifier 12.

Furthermore, if the output potential of the buffer circuit 4 is _VB , this output potential _VB is set to the bases of the transistors 14 and 16 via the buffer circuit 4 and the resistor 6, and therefore,
It is equal to the base voltage of 6. Further, the resistance value of the resistor 6 is assumed to be R ₆ , and the resistance value of the feedback resistor 32 is assumed to be R ₃₂ . At this time, the current mirror circuit connected to the collector of each transistor 14, 16 allows
Since the collector currents of the transistors 14 and 16 are equal, the voltages between their bases and emitters are also approximately equal, both of which values are _VF , and the base currents _IB are also approximately equal for the same reason. Therefore, the voltage V ₀ at the output terminal 36 is V ₀ =V _B −I _B ×R ₆ −V _F +V _F +I _B ×R ₃₂ (1), and if R ₆ = R ₃₂ , then V ₀ = V _B (2), and the input and output DC potentials are made equal by the resistor 6.

The differential amplifier 12 includes transistors 14, 16,
18, 20, 22, 24, diode 26, 2
8. A constant current source 30 and a resistor 32 constitute a DC full feedback amplifier, and the feedback resistor 32 and the resistor 6 are set to have the same resistance value. That is, resistance 6
The resistance values of the feedback resistor 32 and the feedback resistor 32 are made equal to each other by:
This is to make Equation (3), which will be described later, hold true. A constant current source 3 is connected to the base of each transistor 20, 24.
0 and a diode 28 provide a constant DC bias, and its amplified output can be taken out from the output terminal 36.

This amplifier circuit also includes a power source 38 such as a battery.
is supplied via a power switch 40, and a capacitor 42 is installed between the power line and the reference potential point to suppress voltage fluctuations due to ripples and the like.

In such an amplifier circuit, the power switch 4
Even if 0 is opened and the power is cut off,
Since charge remains in the capacitor 42, when the terminal voltage of the capacitor 42 decreases due to its discharge, the transistors 14 and 16 become saturated by the transistors 18 and 22 and the diode 26, and each transistor 14 , ₁₆ increases, and the voltage drop given by the product of this voltage I _B and the resistance value R ₆ of the resistor 6 (=R ₆ · I _B ) increases, so the capacitor 10 is charged, That charge is maintained.

The reason why the transistors 14 and 16 become saturated is as follows. That is, if the power supply voltage is _Vcc and the base-emitter voltage of the transistor 22 is _VF , then
A voltage of V _cc −V _F is applied to the collector of the transistor 14 from the base of the transistor 22 .
Similarly, the collector of the transistor 16 is applied with a voltage of V _cc −V _F at the base of the transistor 18 or the cathode side of the diode 26 . on the other hand,
The base voltage V _B of the transistors 14 and 16, which is equal to the output potential of the buffer circuit 4, is 1/1 of the voltage V _cc .
Since the voltage value is set to 2, if the saturation voltage of the transistors 14 and 16 is Vsat, then V _B −V _F +Vsat=V _cc −V _F (3).

This equation (3) represents the collector potential when the transistor 14 is in a saturated state. That is, if the base potential of the transistor 14 is V _B and the base-emitter voltage is V _F , the emitter potential is:
When the transistor ₁₄ is in _a saturated state, the collector-emitter voltage V _CE is the saturation voltage V _sat of the transistor 14, so the collector potential of the transistor 14 is (V _B - V _F +
V _sat ), which becomes the left side of equation (3).

Further, equation (3) also represents the base potential of the transistor 22. That is, since the emitter potential is the power supply voltage _Vcc , the emitter-base voltage is _VF
Then, the base potential becomes (V _cc −V _F ),
This becomes the right-hand side of equation (3).

When these (V _B −V _F +V _sat ) and (V _cc −V _F ) are equal, the transistor 14 is saturated, and therefore, equation (3) holds true. If we rearrange this equation (3), we get V _cc = V _B + Vsat (4). Therefore, as is clear from equation (4),
When power supply voltage V _cc drops to a value of (V _B +Vsat), transistors 14 and 16 will enter a saturated state.

Furthermore, when the transistors 14 and 16 are saturated,
The reason why the base current increases is as follows. That is, with the emitter current I _E constant, transistor 1
When 4 and 16 are being driven, if the collector-emitter voltage V _CE is decreased, the current amplification factor β
decreases. When the transistor enters the saturation region, the current amplification factor β becomes extremely small, but since the emitter current is kept constant, the base current I _B
will increase.

To explain this phenomenon of increase in base current I _B with reference to the general static characteristic curve of transistors, the sixth
The figure shows a typical static characteristic curve of a transistor.
The vertical axis is the collector current Ic, and the horizontal axis is the collector-emitter voltage V _CE . In each characteristic curve, the base currents V _B = I _B1 , I _B2 , I _B3 , and I _B4 are each constant; The magnitude relationship is I _B1 < I _B2 < I _B3 < I _B4 .

Also, if we take the emitter current I _E on the vertical axis, I _E = I _c
+I _B , so if the base current I _B is added to each characteristic curve and a curve is drawn, the characteristic curve shown in FIG. 7 will be obtained.

Here, since the emitter current I _E of transistors 14 and 16 is a constant current source, in Fig. 7, the base-emitter voltage V _CE approaches 0 on a straight line with I _E = constant. The change in base current I _B during the process is as shown in Figure 8.
In other words, as the transistor approaches saturation, the base current I _B increases. This is the relationship between the saturation of the transistors 14 and 16 and the increase in the base current I _B.

A voltage equal to the output voltage of the Hall element 2 is applied to the base voltage of the transistor 14 by the buffer circuit 4 via the resistor 6.
In this case, the buffer circuits 4 and 8 are composed of transistors 3 and 3 having a common emitter, as shown in FIG.
A differential amplifier in which a constant current source 43 for supplying an operating current is connected to a differential pair consisting of transistors 9 and 41, and a current mirror circuit consisting of a diode 47 and a transistor 49 is connected to the collector side of transistors 39 and 41. At first, transistor 41
A full feedback amplifier is constructed by coupling the base and collector of the . The output of the Hall element 2 is applied to the input terminal 51, and the output taken out from the output terminal 53 is input to the differential amplifier 12.

Further, there is no AC feedback on the differential amplifier 12 side, and amplification is performed using the gain of the differential amplifier 12 at high frequencies. This operation will be explained with reference to the models shown in A and B of FIG. In A of FIG. 9, input terminals a and b are connected to the buffer output and therefore have very low impedance. Furthermore, when looking at this in terms of AC, the impedance of the capacitor on the input terminal b side is close to 0, so the AC equivalent circuit is represented by B in FIG. That is, a high-frequency AC signal whose impedance of the capacitor can be regarded as zero is amplified only by the so-called bare gain of the differential amplifier 12.

The charging operation of the capacitor 10 is as follows. As shown in equation (4), when V _cc =V _B +Vsat, the transistors 14 and 16 become saturated, and their respective base currents I _B increase. Therefore, the base potential of the transistor 14 is (V _B −I _B
×R ₆ ). Since the differential amplifier 12 is provided with DC full feedback, the base potentials of the transistors 14 and 16 also become (V _B -I _B ×R ₆ ). The output potentials of the buffer circuits 4 and 8 directly output the output potential of the Hall element 2, and are approximately equal to _VB . In other words, a voltage of |(V _B −I _B ×R ₆ )−V _B |=I _B R ₆ ...(5) is generated across the capacitor 10, and the capacitor 10 is charged with this voltage. Become.

Therefore, when the transistors 14 and 15 are in a saturated state and the capacitor 10 is charged and its charge is maintained, when the power switch 40 is turned on again, the potential of the feedback system of the differential amplifier 12 is is lower, so capacitor 10
The charge becomes a discharge state, and this DC potential fluctuation occurs at the output terminal 36, which is generated as a transient signal and causes erroneous output.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an amplifier circuit that prevents erroneous charging of a DC cutoff capacitor and prevents the generation of a transient signal when the power is turned on again after being turned off.

That is, the amplifier circuit of the present invention includes a Hall element 2 that generates a Hall output according to a cross magnetic field, and is individually connected to each output terminal of this Hall element to receive the Hall output and perform impedance conversion, and first and second buffer circuits 4 and 8 that equalize and take out the potential in a DC manner, and first and second transistors 1 that have a common emitter.
A differential pair consisting of 4 and 16 transistors is installed, and the Hall output obtained through the first buffer circuit is connected to the base of the first transistor through the first resistor 6.
The Hall output obtained through the second buffer circuit is applied to the base of the second transistor of the differential pair through the capacitor 10, and the Hall output obtained through the second buffer circuit is applied to the base of the second transistor of the differential pair. A DC full feedback differential amplifier 12 in which the differential output obtained on the collector side of the transistor is fed back to the third transistor 22 and the second resistor 32.
, a constant current source 30 that generates a constant current; a diode 28 receives the constant current generated by this constant current source; and the constant current is applied to the emitter side of the first and second transistors of the differential amplifier. A current mirror circuit is used to supply an operating current to the differential amplifier through a fourth transistor 20 installed in the differential amplifier, and to supply an operating current to the third transistor through a fifth transistor 24. The charging voltage of this capacitor is connected to a voltage dividing circuit (resistors 48, 50).
a sixth transistor 47 applied to the base of the first and second transistors, the sixth transistor being conductive when the power supply voltage is at a value that does not saturate the first and second transistors; A reduced voltage detection circuit 44 detects a reduced power state of the power supply by turning off the sixth transistor when the value saturates the second transistor; and the reduced voltage detection circuit detects the reduced power state. At this time, a switch (4) cuts off the constant current to be supplied to the differential amplifier.
6 or 45).

Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings.

FIG. 2 shows an embodiment of the amplifier circuit of the present invention,
The same parts as those in the amplifier circuit of FIG. 1 are given the same reference numerals. In the figure, a Hall element 2 that generates a Hall output according to a cross magnetic field is installed as a signal source, and first and second buffer circuits are individually connected to each output terminal of the Hall element 2 and receive each Hall output. , impedance conversion is performed on the differential outputs of the Hall elements 2 which are DC-equalpotential, and the outputs are configured to be DC-equalpotential differential outputs.

The differential amplifier 12 also includes a current mirror circuit consisting of a transistor 18 and a diode 26 as an active load installed in a differential pair consisting of first and second transistors 14 and 16, and a current mirror circuit consisting of a transistor 18 and a diode 26 as an active load.
The output taken out from the collector of the buffer circuit 4 is fully fed back to the base side of the transistor 16 through the transistor 22 forming the third transistor and the second resistor 32, and the output of one buffer circuit 4 is fed back to the base side of the transistor 16 through the transistor 22 forming the third transistor and the second resistor 32. Since the output of the other buffer circuit 8 is directly connected to the base of the transistor 14 via the capacitor 10 and input to the base of the transistor 16, direct current feedback is achieved. It constitutes a type AC differential amplifier. In this differential amplifier 12, the resistance values of the resistor 6 and the resistor 32 are set to equal values.

A constant current source 30 is installed to supply an operating current to the differential amplifier 12, and the constant current generated by this constant current source 30 is passed through the diode 28, the fourth
transistor 20 and fifth transistor 24
The transistor 20 supplies a constant operating current to the differential pair of transistors 14 and 16, and the operating current is drawn into the transistor 24 from the collector side of the transistor 22 with a constant current. .

In this amplifier circuit, a voltage reduction detection circuit 4 detects a voltage reduction in the power supply voltage supplied from the power supply 38.
4 is installed between the terminals of the diode 28 of the current mirror circuit that flows the operating current to the differential amplifier 12, and forcibly amplifies the differential amplifier 12 in response to the output of the reduced voltage detection circuit 44. A switch 46 is installed as an operation stop circuit for stopping the operation.

The reduced voltage detection circuit 44 detects a reduced voltage state such as when the power supply voltage Vcc applied to the differential amplifier 12 is reduced by opening the power switch 40,
For example, it detects when the power supply voltage drops and the transistors 14 and 16 of the differential amplifier 12 begin to saturate, and generates a detection output. The switch 46 receives the detection output generated by this voltage reduction detection circuit 44.
It consists of a transistor or other electronic switch that is applied as a switching control output to close it.

With this configuration, based on the detection of the power supply voltage by the voltage reduction detection circuit 44, when the voltage decreases, the switch 46 is closed to cut off the supply of operating current to the transistors 14 and 16, and the differential amplifier 12 is operated. It is possible to forcibly control the system to a stopped state and prevent it from entering a saturated state as in the conventional case. As a result, it is possible to prevent erroneous charging of the capacitor 10 when the power is turned off, to prevent potential fluctuations caused by turning on the power switch 40 again, and to prevent the generation of transient signals. It is assumed that the switch 46 is opened when the differential amplifier 12 recovers full DC feedback capability.

FIG. 3 shows a specific example of the circuit configuration of the switch 40, and parts common to the amplifier circuit of FIG. 2 are given the same reference numerals. The reduced voltage detection circuit 44 has a sixth
It is composed of a transistor 47, a resistor 52 together with resistors 48 and 50 forming a voltage dividing circuit for the power supply voltage, and detects a reduced voltage state such as when the power switch 40 is opened. Further, the switch 46 is composed of a transistor 54 installed as a switching element,
A switching control input is applied to its base from the collector of the transistor 47 of the voltage reduction detection circuit 44.

According to such a configuration, when the power switch 40 is closed, the power supply voltage Vcc applied to the differential amplifier 12
When is a normal value, the transistor 47 of the voltage reduction detection circuit 44 becomes conductive because its base potential exceeds the threshold level.
Transistor 47 becomes non-conductive because its base potential decreases. For this reason, the supply voltage
When Vcc maintains a normal value, the differential amplifier 12 is in a steady state.

Further, when the power switch 40 is opened and the power supply voltage decreases due to the terminal voltage of the capacitor 42 and the transistors 14 and 16 become saturated, the transistor 47 of the voltage reduction detection circuit 44 detects the voltage reduction. The transistor 54 becomes non-conductive.
Set the circuit conditions under which the circuit becomes conductive.

In this way, the operation of the differential amplifier 12 can be controlled by detecting the voltage drop when the power is turned on/off and the voltage drop in the steady state. That is, when the power is cut off, the differential amplifier 12 is stopped when the transistors 14 and 16 tend to be saturated after the power is cut off due to the conduction state of the transistor 54. This prevents erroneous charging of the capacitor 10 due to the saturation of the transistors 14 and 16. This makes it possible to prevent the generation of transient signals due to potential fluctuations when the power is turned on again. Furthermore, the voltage reduction detection circuit 44 and the switch 46 can be formed together with the buffer circuits 4 and 8 and the differential amplifier 12 on a common substrate as a semiconductor integrated circuit.

Further, FIG. 4 shows another embodiment of the amplifier circuit of the present invention, in which parts common to the amplifier circuit of FIG. 2 are given the same reference numerals. In the amplifier circuit of this embodiment, a switch 45 is installed between the common emitter of transistors 14 and 16 and the collector of transistor 20 to cut off the operating current flowing through transistor 20, and the operation of switch 46 shown in FIG. On the contrary, the opening and closing are made in response to the detection output of the voltage reduction detection circuit 44. Even with this configuration, when the power is cut off, the differential amplifier 12
It is possible to stop the operation of the transistors 14 and 16, thereby preventing erroneous charging of the capacitor 10 due to saturation of the transistors 14 and 16.

In each of the embodiments, a Hall element is used as a signal source, and the amplifier circuit is configured to amplify the differential output generated by the Hall element. A similar effect can be expected when applied to the case of amplifying the dynamic output.

As explained above, according to the present invention, when the power is turned off, the operation of the differential amplifier is stopped and the erroneous charging of the DC cutoff capacitor is prevented, so that fluctuations in the DC potential are prevented when the power is turned on again. It is possible to reliably prevent the generation of transient signals when the power is turned on again after being turned off.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional amplifier circuit, FIG. 2 is a circuit diagram showing an embodiment of the amplifier circuit of the present invention, FIG. 3 is a circuit diagram showing a specific example of the circuit configuration, and FIG. FIG. 5 is a circuit diagram showing a specific example of the configuration of the buffer circuit in the amplifier circuit shown in FIGS. 1 to 4, and FIG. 6 is a circuit diagram showing another embodiment of the amplifier circuit of the present invention. Static characteristics (I _C −
Figure 7 is a diagram showing the static characteristics (I _E - V _CE ) curve of the transistor, Figure 8 is a diagram showing the base current characteristic ₍ I _B - V _CE ) of the transistor,
FIG. 9 is a circuit diagram showing an equivalent circuit of a differential amplifier. 2... Hall element, 4... First buffer circuit, 6... First resistor, 8... Second buffer circuit, 10... Capacitor, 12... Differential amplifier,
14...first transistor, 16...second transistor, 20...fourth transistor, 22
... third transistor, 24 ... fifth transistor, 28 ... diode, 30 ... constant current source, 32 ... second resistor, 42 ... capacitor,
44...Low voltage detection circuit, 45...Switch, 4
6...Switch, 47...Sixth transistor.

Claims (1)

[Scope of Claims] 1. A Hall element that generates a Hall output according to a crossed magnetic field; and a Hall element that is individually connected to each output terminal of the Hall element to receive the Hall output and convert the impedance, and converts the Hall output into a DC converter. A differential pair consisting of first and second buffer circuits that equalize and take out potentials, and first and second transistors having a common emitter, is installed, and the first buffer circuit is connected to the base of the first transistor. The Hall output obtained through the circuit is applied to the base via a first resistor, and the Hall output obtained through the second buffer circuit is applied to the base of the second transistor via a capacitor. a differential amplifier in which the differential output obtained on the collector side of the first transistor is fully fed back through a third transistor and a second resistor; a constant current source that generates a constant current; The constant current generated by the source is received by a diode, and the constant current operates the differential amplifier through a fourth transistor installed on the emitter side of the first and second transistors of the differential amplifier. A current mirror circuit that allows a current to flow and an operating current to flow through a fifth transistor to the third transistor; a capacitor that is charged by the supply of power; and a charging voltage of this capacitor that is applied to the base through a voltage divider circuit. the sixth transistor is turned on when the power supply voltage is at a value that does not saturate the first and second transistors;
and a reduced voltage detection circuit that detects a reduced voltage state of the power supply by cutting off the sixth transistor when the value saturates the second transistor; An amplifier circuit comprising: a switch that cuts off the operating current to be supplied to the differential amplifier when detected.