JPH03218290A - Rotational speed control circuit for dc motor - Google Patents

Abstract

PURPOSE:To simplify the circuitry of an error amplifier by controlling the voltage between first and second terminals to a constant level at all times through stabilization of current flowing through a rotational speed setting resistor thereby performing constant current ratio control of first and second driving transistor groups. CONSTITUTION:One end of a DC motor M having the other end connected with an external power supply Vcc is connected to a first terminal T1, a shut resistor Rt having one end connected with the external power supply Vcc is connected to a second terminal T2, and the junction of two speed setting shut resistors Ra, Rb connected between the first and second terminals T1, T2 is connected to a third terminal T3 while furthermore the external power supply Vcc is connected to the input terminal VIN of the power supply. By such arrangement, the circuitry of an erroneous amplifier can be simplified without sacrifice of control characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION [Fields of Application in Industry] The present invention relates to improvements in the rotational speed control circuit of small DC motors used in tape recorders, radio-cassette players, and the like.

[Conventional technology]

In recent years, as tape recorders, video recorders, and the like have become smaller, there has been a demand for smaller, more stable rotational speed control circuits for DC motors.

In order to meet such demands, Japanese Patent Application Laid-Open No. 53142611
The publication discloses a three-terminal IC that constitutes a rotational speed control circuit for a DC motor.

Figure 2 shows this three-terminal IC.
OO has a grounding terminal pin ■, an output terminal pin ■, and a reference terminal pin ■. In the figure, 100a is an error amplifier with the reference voltage Eref connected to the inverting input terminal,
t is resistance, Rs is rotation speed adjustment resistance of DC motor M, Q
1 is a drive transistor, Q2 is a shunt transistor, and Ql and Q2 constitute a constant current ratio control circuit to which a current mirror circuit is applied.

By the way, the equivalent circuit of a DC motor is shown by a series connection of an internal resistance Ro and a back electromotive force Eo induced by the rotation of the DC motor M. When the number of turns of the wire rod is z1, the magnetic flux generated in the DC motor M is φ, the proportionality constant is K2, and the back electromotive force of the DC motor is EO, the following equation generally holds true.

Eo=N-K2・z・φ・・・・・・(1)
N=Eo/(K2・z・φ)・・・・・・(1)′ Also, if the armature current of DC motor M is Ia, the load torque of DC motor M is Td, and Kl is a proportional constant, then the load is Between torque Td and armature current Ia, Td=K1 ・Z−φ・■a ・・・・・・(2
)■a=Td/(K1・z・φ)・・・・・・(2)′
Furthermore, if the terminal voltage of the DC motor M is Vm, then the relationship between the back electromotive force Eo and the armature current Ia is Vm=E o +R
o ・I a − (3) E o =Vm−
R o ·I a −(3)' holds true.

Therefore, in such ICIOO, the ratio of the collector current ■1 of the drive transistor Q1 and the collector current ■2 of the shunt transistor Q2 is K=I 1/I 2
The area ratio of the drive transistor Q1 and the shunt transistor Q2 and the values of the resistors Rl and R2 are selected so as to satisfy (4). Error amplifier 100a
Since there is an imaginary short between the two input terminals B and C, the current Is flowing through the resistor Rs is I s = E ref /'R s
-...(5).

Therefore, if the voltage applied to the DC motor M is now Es, then from the above equations (4) and (5), Es=Eref+ (I
2+Is) ・Rt=Eref +(Rt/K) ・I
1+Eref ・(Rt/Rs) ・・−・−・(
f3).

Here, since the terminal voltage Vm of the DC motor M and the voltage Es applied to the DC motor M in the above equation (6) are equal, Eo=Vm
-Ro ・I a=Es-Ro ・I a-5- Therefore, the above formula (1)' N-Eo/(K2・z・φ) and (2
)' formula From engineering a=Td/(K1・z・φ), N=Eo/
(K2・z・φ) The relational expression K2・2・φ is derived.

Therefore, now in equation (7), R o − (R t /K) = 0 ・
- If Rt is set in advance so as to satisfy the equation (8), the second term becomes O, and the rotational speed N becomes independent of the load torque Td.

[Problem to be solved by the invention]

However, in such a three-terminal IC, the internal resistance RO of the DC motor M has a positive temperature characteristic, so even if Rt is set so that Equation 2 (8) holds true at a specific temperature, the temperature When , the above equation (8) no longer holds true.

For this reason, it is possible to make the resistance Rt have a positive temperature characteristic so that the equation (8) is always satisfied even when the value of RO fluctuates, but if this is done, the above (7) Since the first term of the equation includes Rt, the rotation speed N will fluctuate due to temperature changes. Further, since the magnetic flux φ decreases as the temperature rises, the first term of the above equation (7) changes both the resistance Rt and the magnetic flux φ, and the rotation speed N changes depending on the temperature.

As a result, a three-terminal IC as shown in Figure 2 was created.
In this case, the rotation speed N is not compensated for temperature changes, and as can be seen from the first term of equation (7) above, in this circuit, the rotation speed N is compensated for changes in the rotation speed adjustment resistance Rs. Since N does not change linearly but shows a hyperbolic change,
It has problems such as difficulty in setting the rotation speed and difficulty in using it.

Furthermore, unless the circuit configuration of the error amplifier 100a is complicated, the control characteristics of the motor will deteriorate.

That is, when the rotational speed N of the DC motor M is set low, the load torque Td becomes lighter and the voltage drop across the resistor Rt becomes smaller, but the input voltage to the error amplifier 100a is approximately equal to the reference voltage Eref than the power supply voltage. It's getting lower. On the other hand, when the power supply voltage drops or when the load torque Td becomes heavy, the drive transistor Q1 is sufficiently driven to maintain the rotational speed N of the DC motor M, and the collector voltage drops to approximately the ground potential.

In this way, the input voltage of the error amplifier 100a changes over a wide range, and in particular drops to approximately the GND potential. However, in reality, in the practical circuit of the error amplifier 100a, there is a restriction on the lower limit of the input voltage, so there is a problem that the control characteristics deteriorate when the power supply voltage is low or when the load torque Td is heavy.

Therefore, in order to solve the above-mentioned disadvantages of the 3-terminal IC,
For example, in Japanese Utility Model Application Publication No. 5 6-3 8 5 9 9,
A speed control IC having two terminals has been proposed.

FIG. 3 is a circuit diagram of a device using this five-terminal IC.

This IC 200 includes a reference terminal (2), a detection terminal (3) for generating a voltage between the terminals (2) and (2) equal to the back electromotive force Eo of the DC motor, an output terminal (2), and a ground terminal (3).

Incidentally, since the power supply terminal is not shown in this figure, by providing the power supply terminal as ■, it becomes a five-terminal type IC.

To explain the structure of the internal circuit, between the reference terminal ■ and the ground terminal ■, there is a collector of the shunt transistor QO.
The emitters are connected in the forward direction via a resistor Ro, and between the output terminal ■ and the ground terminal ■, there are a plurality of transistors Q1 to
The collector and emitter of Qn are connected to resistors R1 to R in the forward direction, respectively.
connected via n. In addition, the bases of these transistors Qo and Q1 to Qn are all connected to the output terminal of the error amplifier 200a, and a reference voltage having no temperature characteristics is connected between the inverting input terminal of the error amplifier 200a and the reference terminal. E ref is provided. Further, the non-inverting input terminal is connected to the reference terminal ■.

In such an IC 200, a DC motor M is connected between a power supply terminal ■ and an output terminal ■, a shunt resistor Rt is connected between a power supply Vcc and a reference terminal ■, and a shunt resistor Rt is connected between a power supply terminal ■ and an output terminal ■. A series circuit of speed setting voltage dividing resistors Ra and Rb is connected between the terminals (2), and the divided voltage at the connection point is input to the detection terminal (2).

In the DC motor speed control circuit configured as above,
By combining the shunt transistor Qo and the transistors Q1 to Qn, a constant current ratio control circuit 200b applying a current mirror circuit is configured, and the transistor Qo
When K is a (constant) ratio of the collector current IB' of the transistors Q1 to Qn and the sum IA' of the collector currents of the transistors Q1 to Qn, IA' = K・IB' (9
) The resistors RO and R1 to Rn are set to 101. Furthermore, if the current flowing through the resistors Ra and Rb through the resistor Rt is Is, then since the input terminal of the error amplifier 200a is in an imaginary short state, Is = Eref/Ra . . . -
・(to) is established, and a current that is the sum of It and Is flows through the resistor Rt, so the power supply Vcc and the error amplifier 20
The voltage VAB between the inverting input terminals of 0a is VAB=Eref
+(Is+IB)·Rt −···−αD.

Further, the voltage VAC between the power supply voltage Vcc and the non-inverting input terminal of the error amplifier 200a is applied to the armature current Ia of the DC motor M.
However, since the current Is is flowing through the resistor Rb, the formula Is = E. V from ref/R a
AC=Eo+Ro ・I a-Rb ・I sEo1R
o ・I a−Rb ・ (Eref/R a) −・・Q21 holds true, but in steady state control, the above VAB and VAC are equal due to the imaginary short, so θD, θ Ku equation and 00) equation I 6 =Eref/R a and (
9) From the formula IA' = K・IB', therefore, 0■
Equation and (1)' Equation N=Eo/(K2・z・φ) (2)' Equation a
=Td/(K1・z・φ) and twist hold true.

Here, in the above 0◇' formula, Ra
By making the value sufficiently large, the second term becomes sufficiently small and can be ignored, and as a result, the above equation (14) becomes as follows.

Therefore, the resistance Rt is adjusted according to the temperature characteristics of the internal resistance RO of the DC motor M so that Ro - (Rt/K)=O is satisfied.
By selecting the resistor material of , and making the temperature characteristic of the resistor Ra in the first term of equation 041 larger than the temperature characteristic of the resistor Rb to cancel out the temperature characteristic of the magnetic flux φ, the first term also has a temperature characteristic. Therefore, the rotation number N becomes stable without being affected by the load torque Td and having no temperature characteristics.

In this case, the temperature coefficients of the resistors 'Rt, Ra, and Rb can be adjusted using metal film resistors if a resistor with a small temperature coefficient is required, or carbon film resistors if a resistor with a larger temperature coefficient is required. This can be achieved by appropriately combining resistors, linear resistors, etc.

However, even in the five-terminal speed control IC as described above, there remains a drawback that the circuit configuration of the error amplifier 200a must be complicated.

[Means to solve the problem]

In the present invention, in the configuration shown in FIG. 3, the collector and base are commonly connected to the first terminal side, and each emitter is connected to the ground terminal via a resistor.
A first drive transistor group Qll to Qln is provided, which is a combination of a plurality of transistors with uniform characteristics, and the first
The common collector of the drive transistor group Qll to Qln is connected to the first terminal, and the second resistor is connected to the non-inverting input terminal of the error amplifier through the intermediary 141-. , whose collector is connected to the second terminal,
Its base is the output terminal of the error amplifier, and
At least one or more second drive transistors Q21 connected to a common base of the group of drive transistors and whose emitters are connected to a ground terminal via a resistor are provided,
On the third terminal side, connect the collector to the power input terminal,
The base of a second transistor whose emitter is connected to the inverting input terminal of the error amplifier is connected, and further,
” The emitter of a first current mirror circuit configured by combining two transistors with the same characteristics is connected to the power supply input terminal mentioned above, and one of the input terminals is connected to the above-mentioned first current mirror circuit.
, the other output terminal is connected to the other end of a second resistor connected to the non-inverting input terminal side of the error amplifier, and the base of the first transistor is connected to a reference voltage circuit. is connected to one input terminal of a second current mirror circuit configured by combining two 1 transistors with the same characteristics through a third resistor, and the other input terminal is connected to the above-mentioned It is characterized in that it is connected to the emitter of the second transistor connected to the inverting input terminal of the error amplifier, and its two emitters are commonly connected to the ground terminal.

[Effect]

In the DC motor rotation speed control circuit of the present invention, a voltage 2 equivalent to the voltage v1 generated across the first resistor provided on the emitter side of the first transistor is applied to both ends of the second resistor using the first current mirror circuit. , while causing a current equal to the collector current of the first transistor to flow through the second transistor using a second current mirror circuit.
The temperature characteristics of the base-emitter voltages of the first transistor and the second transistor are made equal, and the voltage across the second resistor and the base-emitter voltage of the second transistor are added together through an error amplifier. By this,
The voltage between the first terminal T1 and the third terminal T3 is stabilized as a reference voltage having no temperature characteristics.

That is, by stabilizing the current flowing through the rotational speed setting resistors Ra and Rb, the voltage between the first terminal T1 and the second terminal T2 is always controlled to be constant, and the voltage between the first drive transistor group and the second terminal T2 is always controlled to be constant. Since the drive transistor No. 2 is controlled by a constant current ratio, the voltage across the shunt resistor and the DC motor M
By setting the shunt resistance value so that the voltage drop is equal with the internal resistance of the first terminal T1 and the second terminal T2
The voltage between the terminals is always equal to the back electromotive force of the motor, and by stabilizing this voltage between the terminals, the back electromotive force of the motor, in other words, the rotational speed of the motor is stabilized.

1. Therefore, in this circuit, the rotational speed of the DC motor depends only on the rotational speed setting resistor, which is an external resistor, and the magnetic flux of the DC motor, so by adjusting the temperature coefficients of these rotational speed setting resistors, they can be adjusted to each other. Fluctuations due to the temperature characteristics of the rotor can be canceled out, thereby providing a stable rotational speed with temperature compensation.

〔Example〕

17 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing the configuration of the present invention.

A is a speed control IC of the present invention, and a power input terminal VI
N, ground terminal VG, first, second, and third terminals Tl,
It has T2 and T3. The first terminal T1 is connected to the other end of the DC motor M whose one end is connected to the external power supply Vcc, the second terminal is connected to the shunt resistor Rt whose one end is connected to the external power supply Vcc, and the third The terminal T3 has the first,
The connection point between the two speed setting voltage dividing resistors Ra and Rb connected between the second terminals Tl and T2 is connected, and the external power supply Vcc is connected to the power input terminal VIN.

The inside of the IC circuit includes the following circuit.

That is, a constant current circuit C is connected to the power input terminal VIN side.
r and a reference voltage circuit 1 whose one end is connected to the ground terminal are connected in series, the base of the first transistor Q1 is connected to the output terminal of the reference voltage circuit 1, and the emitter of the first transistor Q1 is further connected to the output terminal of the reference voltage circuit 1. is connected to the ground terminal VG via the first resistor R1.

In addition, the above 1. A first drive transistor group 31, which is a combination of a plurality of transistors Qll to Qln with the same characteristics, is provided on the terminal Tl side of the transistor, and the collector and base of each transistor are connected in common, and the emitter of each transistor is connected to a resistor Rll. ~Rln to ground terminal V
Connected to G. This first drive transistor Qll
The base of ~Qln is connected to the error amplifier 2 output terminal in the same manner as the second drive transistor Q21 described later.

The second drive transistor Q21 uses a transistor having the same characteristics as the first drive transistors Qll to Qln, and its collector is connected to the second terminal T2, and its emitter is connected to the ground terminal V via a resistor R21.
Connected to G.

In this embodiment, the first transistors Qll to Qln
The constant current ratio control circuit 3 is configured by combining the second transistor Q21 and the second transistor Q21.

On the other hand, 2 is an error amplifier that controls the constant current ratio control circuit 3, and its non-inverting input terminal is connected to the second terminal T1.
The inverting input terminal is connected to the second transistor Q whose base is connected to the third terminal T3.
2 (the collector of this transistor Q2 is connected to the power supply input terminal VIN described above) and the collector of a transistor Q5 constituting a second current mirror circuit 42 to be described later. Each of the first and second current mirror circuits 41 and 42 is a known transistor in which the bases of two transistors Q3 and Q4, Q5 and Q6, which have the same characteristics, are connected in common, and the bases and collectors are connected. As shown in the figure, the first current mirror circuit 41 connects the emitters of two transistors Q3 and Q4 to the power input terminal VIN, and
The collector of one transistor Q3 is connected to one end of the second resistor R2, which is connected to the non-inverting input terminal of the error amplifier 2 described above.

On the other hand, the second current mirror circuit 42 connects the collector of one transistor Q6 to the base of the first transistor Q1 via a third resistor R3, and the reference voltage output Eref is marked and the other The collector of the transistor Q5 is connected to the inverting input terminal of the error amplifier 20 to which the emitter of the second transistor Q2 is connected, and the emitters of the two transistors Q5 and Q6 are connected to the ground terminal VG.
is connected to.

Note that in the circuit of the embodiment, the first and second transistors Q
l, Q2 and first and second current mirror circuits 41.4
All the characteristics of transistors Q3 to Q6 that constitute 2 are
The values of the resistors R1 to R3 are all set to be the same.

Next, it will be explained that this circuit has compensated temperature characteristics.

The reference voltage output E ref from the reference voltage circuit 1 having no temperature characteristics is applied to the base of the first transistor Q1, and is connected between the collector and emitter of the transistor Q1 through the toe 21 of the first current mirror circuit 41 and the transistor Q4. A current of 1 flows, producing a voltage of v1 across the resistor R1. Also, transistor Q
3 and Q4 constitute the first current mirror circuit 41, so the collector current 2 flowing to the second resistor R2 through the transistor Q3 is equal to 1, so that V
The voltage of 2 is V2=I2・R2=I1・'R 1 =V
1...00, the voltage v1 across the resistor R1 is equivalent to the voltage ■2 across the resistor R2, and this voltage ■2 is applied to the non-inverting input terminal of the error amplifier 10a. become.

On the other hand, the reference voltage Eref is applied to the second current mirror circuit 4 formed by the transistors Q5 and Q6 through the resistor R3.
2 to the collector and base of transistor Q6. Since the third resistor R3 and the resistors Rl and R2 are all equal, the current 3 flowing through the resistor R3 is I 3 = (Eref-VBEQ6) /R 3
Also, the current flow 1 flowing through the first resistor R1 is I
1=(Eref-VBEQI)/Rl...-a
? ) -221, and furthermore, since the current factor 3 and the collector current factor 4 of the transistor Q5 are equal, in the end, I 1 = I 2 = I 3 = I 4 .
...08) holds true.

Further, the collector of the second transistor Q2 is connected to the input power supply terminal VIN, the base is connected to the third terminal T3, and the emitter is connected to the collector of the transistor Q5 that constitutes the second current mirror circuit and the inverting input terminal of the error amplifier 10a. Therefore, when the rotational speed N of the DC motor M increases and the back electromotive force Eo increases, the potential of the third terminal T3 decreases, but the potential across the resistor R2 is always equivalent to ■1, so there is no error. The potential of the non-inverting input terminal of the amplifier 10a decreases and the first
The currents of the transistors Qll to Qln of the drive transistor group are suppressed, and the rotational speed N is maintained at a constant value.

Here, the resistors R1 to R3 have a positive temperature characteristic of about 3000 PPM because diffused resistors are used in a normal IC, and furthermore, the base-emitter voltage VB of the transistors Q1 to Q6 is
Since E has negative temperature characteristics, the voltage across resistor R1 is
1 has positive temperature characteristics. Therefore, the voltage v2 across the resistor R2 also exhibits similar temperature characteristics. In this case, the resistance Rl,
R2 and R3 have the same resistance value, are formed on the same pellet, and have the same temperature characteristics, so if the resistance value changes due to temperature characteristics, the current changes to compensate for the change in resistance. Therefore, since the voltage v2 across resistor R2 and VBEQI of transistor Q1 have negative temperature characteristics, they have positive temperature characteristics.

On the other hand, the collector current (2) of the transistor Q2 is equal to the collector current (1) of the transistor Q1 since all of the factors 1 to (4) are equal according to formula 00, and due to the temperature characteristics of VBEQ2, the first terminal T1 and the third terminal The voltage between V13
is V 1 3 = VBEQ2 + V 2 = VBEQ1 + V
1 = Eref (I9), and a stable voltage without temperature characteristics is obtained.

Furthermore, in this circuit, the first drive transistors Qll~
Qln and the second drive transistor Q21 are combined to constitute a constant current non-control circuit 3 applying a current mirror circuit, and now the first drive transistors Qll to Ql
Let the collector currents flowing through n and the second drive transistor Q21, respectively, be IA and IB, and their ratio is K (constant).
If we put, IA=K・IB ・・・・・・(
2+), the values of the resistor R21 and Rll to Rln are selected so that

On the other hand, in the steady state control state, the input terminal of the error amplifier 2 is in an imaginary short state, so the voltage V13 between the first terminal T1 and the third terminal T3 becomes V 1 3 =VBEQ2+V 2 . , Eref=VBEQ1+V1-12]), so,
If the current flowing through the resistors Ra and Rb is Is, then 25 Is = Eref/Rb...Ci! Z, and if the voltage between the power supply voltage Vcc and the inverting input terminal of the error amplifier 2 is VAB, then the above equation and the above C
From the formula, VAB=It-Rt+Is-Ra+VBEQ2(
IB+Is) ・Rt+Is −Ra+VBEQ2・
...(2) holds true.

Furthermore, if the voltage between the power supply voltage Vcc and the non-inverting input terminal of the error amplifier 2 is VAC, then VAC=E o +R o-I a-V 2 -
(2@ holds true.

By the way, on the circuit, since the above VAB and VAC are equal, the above C■, QΦ formula and the above 07) formula V 2 3 = VBEQ2 + V 2 =
From VBEQ1+V IEref and I1=Ia+Is, 26 Ro-1 a+V2 Therefore, (251 formula and (1)' formula N=Eo/(K2・z・φ) and (2)' formula ■a=Td/(K1・z・φ) The twist holds true.

However, in the above (20), by setting Rb sufficiently larger than Rt, the second term becomes sufficiently small and can be ignored, so the rotation speed N of the motor M is, however, R t / from the above equation (8). K = Ro...
......(8), so even if the temperature changes (27
In order to maintain the second term in Equation 1 at 0, the resistance material of the resistance Rt is selected according to the temperature characteristics of the internal resistance of the DC motor, and the temperature characteristics of the resistance Ra in the first term of Equation (to) is changed to R
By making it larger than the temperature characteristic of b and canceling it out with the temperature characteristic of magnetic flux φ, the first term also no longer has a temperature characteristic,
The rotational speed N is not affected by the load torque Td} and is stable without temperature characteristics.

[Effect of idea]

According to the circuit of the present invention having such a configuration, the reference voltage circuit 1
are the first and second drive transistors Qll to Qln, Q2
A power input terminal VI that is not affected by the control current flowing through
Since the configuration is connected to the constant current circuit Or, the current flowing through the constant current circuit Or can be set to a value with more margin than in the case of the conventional circuit. In addition, in the present invention, the upper limit of the input voltage to the error amplifier 2 is set to VBEQ of the transistor Q2 rather than the power supply voltage.
2, and since only the voltage drop at the resistor R2 remains at the lower limit, there is an advantage that the control characteristics do not deteriorate even if the configuration of the error amplifier 2 is simplified.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is an example of a conventional 5-terminal IC circuit for speed control of a DC motor, and Fig. 3 is a circuit diagram showing an embodiment of the present invention.
The figure shows an example of a three-terminal IC circuit for controlling the speed of a DC motor. A... 5-terminal IC circuit of the present invention, 1...
・Reference voltage circuit, 2...Error amplifier, 3...
...Constant current ratio control circuit 29, 41...First current mirror circuit, 4
2...Second current mirror circuit, Ql...
...First transistor, Q2...Second transistor, Rl...First resistor, R2...
...Second resistance, M...DC motor, Rt.
・・・・・・Shunt resistor, Ra, Rb・・・・Voltage dividing resistor for speed setting, VIN・・・・Power input terminal, VG・
...Ground terminal, TI, T2, T3...
-First, second, and third terminals.

Claims (1)

[Claims] A power supply terminal, a reference terminal, a first terminal to which the DC motor is connected at one end to the external power supply, a second terminal to which the torque control resistor is connected, and a series connection between the first and second terminals. a third terminal to which the connection point of the two connected speed setting voltage dividing resistors is connected, and a series circuit of a current source circuit and a reference voltage circuit connected between the power supply terminal side and the reference terminal; , a first transistor having a base connected to the output terminal of the reference voltage circuit and having an emitter resistance between it and the reference terminal; and a collector-emitter current path connected between the first terminal and the reference terminal. a second transistor having a base connected to the output of the error amplifier; a first resistor connected between the first terminal and the non-inverting input terminal of the error amplifier; a third transistor 2 having a collector-emitter current path connected between the terminal and the reference terminal and having a base connected to the output of the error amplifier; a base connected to the third terminal and the output of the error amplifier; a second transistor having an emitter connected to an inverting input terminal; a current mirror circuit having an input terminal connected to the collector of the first transistor and an output terminal connected to the non-inverting input terminal of the error amplifier; a fourth transistor having an end connected to the base of the first transistor via a second resistor and an output connected to the inverting input of the error amplifier.