Speed control apparatus for DC motor

Speed control apparatus for DC motor comprising a current mirror circuit including a first transistor 11 and second transistors 12-14 with their bases connected in common, the collectors of the second transistors 12-14 being in-common connected to one end of the DC motor 9 so as to control the motor current, the bases of the transistors being fed with such base current that saturates the transistors, thereby to raise their maximum controlling torque and starting torque of the DC motor.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates to an apparatus for speed control of a DC 
motor. 
More particularly, the present invention concerns a apparatus for speed 
control of a DC motor capable of operating the DC motor with a larger 
values of maximum controlling torque and starting torque than the prior 
arts. 
2. Background 
Conventional speed control apparatus for a small DC motor, so-called 
electronics governor apparatus, generally is designed to operate in a 
manner that a counter-electromotive force which is in proportion to the 
speed, of the motor is taken out across both ends of motor winding and is 
compared with a reference voltage thereby to produce a difference of the 
two voltage, then the DC motor is controlled by utilizing the difference 
of the two voltage. In such conventional apparatus, the problem is that 
the speed control is made by the difference voltage, which is between the 
reference voltage and the voltage obtained across both ends of the motor 
winding. In general, the latter voltage obtained across the motor winding 
is not exactly the counter electromotive force per se, but a voltage also 
including a voltage drop mainly due to ohmic resistance of the motor 
winding. Therefore, it has been general to construct the circuit to 
include a resistor bridge circuit in order to provide an accurate speed 
controlling by taking out and utilizing the pure counter-electromotive 
force across both ends of the motor winding. In the speed control 
apparatus including such resistor bridge circuit, it is an important 
problem that the resistance values of the resistors should be accurate as 
designed. Recently, many electronics circuit has been semiconductorized or 
formed on an integrated circuit, and such speed control apparatuses are 
also made in the form of semiconductor integrated circuit. However, since 
making of the accurate resistance values of the bridge resistors by 
semiconductor diffused regions is generally difficult, these bridge 
resistors are usually provided as an external resistors which is connected 
to the integrated circuit. Such use of external resistors with the 
integrated circuit inevitably requires a number of external connection 
wires from the integrated circuit. 
In order to remove the abovementioned inconvenience of a large number of 
wire connections, a conventional example of an integrated circuit type 
speed control apparatus shown in FIG. 1 has been already proposed and 
known. 
In the known speed control apparatus of FIG. 1, the parts encircled by the 
dotted lines are formed on a monolithic integrated circuit, wherein 1 
designates a controlling circuit, 2 a reference voltage generator, 3 a 
current limiting circuit, 4 a constant current circuit and 5 a current 
dviding circuit. A power source terminal 6, an output terminal 7 and an 
adjusting resistor terminal 8 for connecting an adjusting resisor 10 are 
provided as external connection terminals of the IC. Across the power 
source terminal 6 and the ground is connected a voltage source (not 
shown), across the output terminal 7 and the ground is connected a motor 
9, and across the adjusting resistor terminal 8 and the ground is 
connected the adjusting resistor 10, by changing the resistance of which a 
desired speed of the motor is set. 
In the abovementioned conventional speed control apparatus, the circuit 
operation is made in a manner that a reference voltage Vref which is 
proportional to the counter-electromotive force Ea of the DC motor is 
generated across the output terminal 7 and the adjusting resisjtor 
terminal 8, and a voltage drop in the adjusting resistor 10 is changed so 
as to be equal to the reference voltage Vref. For instance, when a voltage 
drop by an internal resistance of the DC motor 9 is changed by an external 
effect, the operation is made in a manner that by changing a current 
flowing through the adjusting resistor 10 by means of the current dividing 
circuit 5, the reference voltage Vref is hold to be equal to the 
counter-electromotive force Ea, so that the fluctuations of the revolving 
speed of the DC motor is suppressed. 
When speed control of a DC motor is made by the above-mentioned speed 
control apparatus, there is such problems that it is difficult to obtain a 
large starting torque, and therefore, it takes a considerable time until 
the revolution speed of the motor reaches a predetermined value, and the 
the maximum controlling torque is small. 
The present invention is made in view of the abovementioned problems of the 
conventional speed control appratus. 
SUMMARY OF THE INVENTION 
The present invention purports to provide an improved speed control 
apparatus for DC motor, the apparatus being capable of operating the DC 
motor with a larger values of maximum controlling torque and starting 
torque than the prior arts. 
The speed control apparatus for DC motor in accordance with the present 
invention comprises a current circuit indicating a first transistor 11 and 
second transistors 12-14 with their bases connected in common, the 
collectors of the second transistors 12-14 being connected in series to 
the DC motor 9 so as to control the motor current, the bases of the 
transistors being fed with such base current that saturates the 
transistors, thereby raising their maximum controlling torque and starting 
torque of the DC motor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of the present invention is now elucidated referring 
to FIG. 2, wherein a first transistor 11 and second transistors 12, 13, . 
. . 14 are of the same type (NPN type), have the same electric 
characteristics and connected in common at their bases. Emitters of these 
transistors 11, 12, 13, . . . 14 are connected through the emitter 
resistors of the same resistance, respectively, to the negative terminal 
30 of the power source, namely the ground terminal. The transistors 11 to 
14 of the common connected bases form a current mirror circuit, wherein 
the collector of the first transistor 11 is connected through a junction 
point 19 and a resistor 20 to a positive terminal 21 of the power source 
and the collectors of the second transistors 12 to 14 are connected in 
common to a junction point 22, between which and the positive terminal 21 
the DC motor 9 is connected. In the circuit, the total of the collector 
currents of the second transistors 12 to 14 flows through the DC motor, 
and accordingly, by adjusting the base currents of the second transistors 
12 to 14 the motor current Ia of the DC motor is adjusted and hence the 
revolution speed of the DC motor is controlled. The transistors 23 and 24 
are PNP transistors and form a diferential amplifier 3', wherein the base 
of the transistor 23 is connected to the junction point 22 and the base of 
the transistor 24 is connected to a negative terminal 251 of a reference 
voltage source 25. The emitters of the transistors 23 and 24 are connected 
in common and through a constant current circuit 101 to the junction point 
19. To the collectors of these transistors 23 and 24 are connected 
respective collectors of a pair of NPN transistors 26 and 27, which are 
connected so as to form a current mirror circuit 3", emitters of the 
transistors 26 and 27 being connected to the negative terminal 30 of the 
power source. A PNP transistor 28 is connected by its base to the negative 
terminal 251 of the reference voltage source 25, by its emitter to the 
junction point 22 and by its collector to the emitter of the first 
transistor 11. A junction point between the collectors of the transistors 
24 and the 27 is connected to the base of a PNP transistor 31, which is 
connected by its emitter through a constant current circuit 29 to the 
junction point 19 and by its collector to the power source negative 
terminal 30. A transistor 33 is connected by its base to the emitter of 
the transistor 31, by its collector to the junction point 19 and by its 
emitter to the bases of the first transistor 11 and the second transistors 
12-14, and also, through the resistor 32 to the negative terminal 30 of 
the power source. A variable resistor 34 as an adjusting resistor of the 
revolution speed is connected between the junction point 22 and a junction 
point 19, and a constant current circuit 35 is connected between the 
junction point 19 and negativae source terminal 30. The constant current 
circuit 35 is for suppressing change of current through the junction point 
19 at a change of collector current of the transistor 33. 
The automatic speed adjusting operation of the abovementioned FIG. 2 
example of speed control apparatus of the present invention at occurrence 
of revolution speed fluctuation is as follows: 
(1) Operation when motor speed becomes higher: When the motor speed 
fluctuates and becomes faster than a predetermined one, then the 
counter-electromotive force Ea of the DC motor 9 becomes higher than a 
predetermined one, and accordingly, the potential at the junction point 22 
is lowered below a predetermined one. By such lowering of the potential at 
the junction point 22, the base bias voltage of the transistor 23, which 
is a part of the differential amplifier 3', becomes increased, thereby 
making it larger than that of the other transistor 24, and thereby 
increasing collector current of the transistor 23 and hence increasing 
collector current of the transistor 26. Accordingly, the collector current 
of the transistor 27, which is a part of the current mirror circuit 3", 
also increases, thereby increasing base current of the PNP transistor 
31(,which base current flows from the base of the transistor 31 to the 
collector of the transistor 27), and hence increase emitter current of the 
transistor 31. Since the emitter current of the transistor 31 and the base 
current of the transistor 33 come from the common constant current circuit 
29, at the time of increase of the emitter current of the transistor 31 
the base current of the transistor 33 decreases, and thereby decreases 
base currents, hence collector currents, of the first transistor 11 and 
the second transistors 12 to 14. By such decrease of the collector 
currents of the second transistors 12 to 14, the motor current of the DC 
motor is decreased, and therefore, the revolution speed is lowered. Such 
control to lower the speed is made until the counter-electromotive force 
restores to the predetermined value, thereby making the base bias voltages 
of the transistors 23 and 24 of the differential amplifier 3' to be in an 
equibrium. 
(2) Operation when motor speed becomes lower: When the motor speed 
fluctuates and becomes slower than the predetermined one, then the 
counter-electromotive force Ea of the DC motor 9 becomes lower than 
predetermined, and accordingly, the potential at the junction point 22 is 
raised than predetermined. By such rising of potential at the junction 
point 22, the base bias voltage of the transistor 23, which is a part of 
the differential amplifier 3', becomes decreased, thereby making it 
smaller than that of the other transistor 24, and thereby decreasing its 
collector current and hence decreases collector current of the transistor 
26. Accordingly, the collector current of the transistor 27, which is a 
part of the current mirror circuit 3", decreases also, thereby decreasing 
base current of the PNP transistor 31, and hence decreases emitter current 
of the transistor 31. By means of the decrease of the emitter current of 
the transistor 31, the base current of the transistor 33 increases, and 
thereby increases base currents, hence collector currents of the first 
transistor 11 and the second transistors 12 to 14. By such increase of the 
collector currents of the second transistors 12 to 14, the motor current 
is increased and the revolution speed is raised. Such control to 
accelerate the speed is made until the counter-electromotive force 
restores to the predetermined value, thereby making the base bias voltages 
of the transistors 23 and 24 of the differential amplifier 3' to be in 
equibrium. 
(3) Reason of large maximum controlling torque: 
The feature of the apparatus of the present invention is that the maximum 
controlling torque of the DC motor controlled by the apparatus of the 
present invention is very large. The maximum controlling torque is the 
maximum value of the torque which retains revolution at a rating speed. 
The case when the maximum controlling torque becomes maximum is the case 
when the potential at the junction point 22 is minimum. Therefore, when 
the second transistors 12 to 14 are saturated, then the maximum 
controlling torque becomes large. 
In the abovementioned apparatus of FIG. 2, to the junction point 19 is 
connected the collector of the transistor 33, which is for feeding through 
its emitter the base currents of the transistors 11 to 14. Furthermore, 
since the reference voltage generator 25 of the voltage Vref is connected 
across the junction point 19 and the base of the tranistor 28, emitter of 
which is connected to the junction point 22, the potential at the junction 
point 19 becomes higher than that of the junction point 22, by the 
reference voltage Vref (usually Vref is about 1.2 V). Therefore, the 
collector potential of the transistor 33 is higher than that of the second 
transistors 12 to 14. Accordingly the transistor 33 can feed such enough 
currents to the bases of the second transistors 12 to 14 that can saturate 
the latter. Therefore, by such saturation of the transistors 12 to 14, a 
large motor current is fed as follows: 
The maximum controlling torque .PHI.max for the abovementioned enough base 
current for saturation is given by the following formula (1): 
##EQU1## 
wherein Kt . . . torque constant of the DC motor 9, 
Vcc . . . source voltage impressed across the terminals 21 and 30, 
Eao . . . counter-electromotive force at the rating revolution speed, 
Vce . . . collector-emitter saturation voltage of the transistors 12 to 14, 
Ra . . . internal resistance of the DC motor, and 
Reo . . . equivalent resistance of the resistors 16 to 18 (that is, one 
n-th fraction of resistance of each of the resistors 16 to 18 for number n 
of the resistors 16 to 18). 
For an example that, in the speed control apparatus of FIG. 2, 
Kt=96 gcm/A, 
Vcc=4.5 V, 
Vce=0.5 V, 
Eao=2.45 V, 
Ra=6.3.OMEGA., and 
Reo=1.OMEGA., 
the maximum controlling torque .PHI.max is computed as follows: 
##EQU2## 
This value of maximum controlling torque for such DC motor is as about 
twice large as that obtained by using the conventional speed controlling 
apparatus. 
(4) Reason of large starting torque: 
The other feature of the apparatus of the present invention is that the 
starting torquae of the DC motor controlled by the apparatus of the 
present invention is very large. The reason why such large starting torque 
is obtainable is elucidated as follows: 
In the abovementioned example of FIG. 2, when the DC motor is locked, the 
counter-electromotive force of the DC motor becomes zero, and therefore 
the potential at the junction point 22 is considerably raised. As a result 
of the rising of the potential, the base bias of the transistor 23 is 
decreased, and therefore its collector current decreases, and hence the 
collector current of the transistor 26 decreases, and accordingly, the 
collector current of the transistor 27 decreases. And hence the base 
current of the transistor 31 is decreased. Since the total of the currents 
of the emitter of the transistor 31 and the base of the transistor 33 is 
controlled constant by the constant current circuit 29, the base current 
of the transistor 33 is increased by the decrease of the emitter current 
of the transistor 31. Therefore, the base currents of the first transistor 
11 and the second transistors 12 to 14 increase, and resultantly lowers 
the potential at the junction point 22. Accordingly, a large motor current 
flows through the DC motor 9. 
Provided that the ratio of the total value Ict of the collector currents of 
the second transistors 12 to 14 and collector current Ic11 of the first 
transistor 11 is K, namely K=(Ict/Ic11), then a current of about one K-th 
fraction of the motor current flows through the junction point 19 (and to 
the collector of the transistor 11) (Here, it is provided that the total 
of currents through the constant current circuits 29 and 101, the current 
through the collector of the transistor 33 and the current through the 
reference voltage generator 25 is negligidly small in comparison with the 
collector current of the first transistor 11.). Accordingly, by selecting 
the resistance value of the resistor 20 to be equal to K.times.Ra (Ra is 
the resistance of the DC motor), the potentials of the junction points 19 
and 22 can be made equal, since, by so selecting, the voltage drops across 
the resistor 20 and across the DC motor 9 becomes equal with each other. 
In such case, the base potentials of the transistors 23 and 24 are not 
equal with each other. Namely, the potential of the base of the transistor 
24 becomes lower than that of the transistor 23 by the value of the 
reference voltage Vref generated by the reference voltage generator 25. By 
such lowering of the base potential of the transistor 24, the base 
potential of the transistor 28 is also lowered, and resultantly increases 
base bias voltage of the transistor 28, hence increasing its collector 
current. Since the collector current of the transistor 28 flows into the 
emitter resistor 15 of the first transistor 11, the emitter potential of 
the first transistor 11 is raised. Accordingly, the condition for the 
current mirror circuit, which have been formed by the transistors 11 to 14 
having the same emitter potential, is broken. Thus, the collector current 
of the first transistor 11 becomes smaller than each one of the second 
transistors 12 to 14. By such decrease of the collector current of the 
transistor 11, the voltage drop across the resistor 20 is lowered than 
that across the DC motor 9. This raises the potential at the junction 
point 19 to be higher than that of the junction point 22. According to the 
rising of the potential at the junction point 19, the collector voltage of 
the transistor 33 is increased, and therefore, the collector current of 
the transistor 33 is greatly increased. As a result of the increase of the 
collector current of the transistor 33, it becomes possible to feed such 
an enough base currents to the bases of the transistors 12 to 14 as to 
make the transistors 12 to 14 saturated. Accordingly, the collector 
currents of the second transistors 12 to 14, namely the motor current of 
the DC motor 9 greatly increases. 
The starting torque .PHI.s of the DC motor 9 is given by the following 
formula (3): 
##EQU3## 
Provided the circuit constants are identical to those given in computing in 
the formula (2), the starting torque .PHI.s for such condition is: 
##EQU4## 
This figure is as about twice large as the conventional starting torque. 
FIG. 3 shows changes of torque responding with revolution speed of DC 
motors. Curve A shows a change of the motor speed controlled by the speed 
control apparatus in accordance with the prior art, and Curve B shows a 
change of the motor speed controlled by the speed control apparatus in 
accordance with the present invention. 
Abscissa indicates torque in gcm and ordinate indicates number of 
revolution (speed) in revolution per minute. As seen in FIG. 3, according 
to the present invention, the maximum controlling torque, over the value 
of which the number of revolution drops out from the rating number of 
revolution, is as almost twice high as that of the conventional apparatus. 
Moreover, the starting torque, namely the torque at the rising up of the 
curve, is also as almost twice high as that of the conventional one. 
FIG. 4 shows a modified example. In the circuit of FIG. 4, the circuit 
construction is more simple than in FIG. 2, but the operation is 
substantially the same with the example of FIG. 2. Principal difference is 
that a PNP transistor 36 is used instead of the NPN transistor 33 combined 
with the PNP transistor 31 and the constant current circuit 29. Also the 
transistor 28 of FIG. 2 is omitted. The bases of the transistors 23 and 24 
in the differential amplifier circuit 3' are connected to the negativae 
terminal 251 of the reference voltage generator 25 and to the junction 
point 22, respectively. The transistor 36 is connected by the base to the 
collectors of the transistors 24 and 27 and by the emitter to the junction 
point 22. 
The automatic speed adjusting operation of the abovementioned FIG. 4 
example of speed control apparatus of the present invention at occurrence 
of revolution speed fluctuation is as follows: 
(1') Operation when motor speed becomes higher: When the motor speed 
fluctuates and becomes faster than a predetermened one, then the 
counter-electromotive force becomes higher than a specified one, and 
accordingly, the potential at the junction point 22 is lowered below a 
specified one. By such lowering of the potential at the junction point 22, 
the base bias voltage of the transistor 24 of the differential amplifier 
3' becomes increased, thereby making the base bias voltage larger than 
that of the other transistor 23. Therefore, the collector current of the 
transistor 24 increases and hence the collector current of the transistor 
23 decreases, thereby decreasing the collector currents of the transistors 
26 and 27. The decrease of the collector current of the transistor 27 
makes the base current of the transistor 36 decrease, thereby decreasing 
its collector current and hence decreasing the base currents of the first 
transistor 11 and the second transistors 12 to 14. By such decrease of the 
base currents, the collector currents of the second transistors 12 to 14 
decreases, and therefore the motor speed is lowered. Such control to lower 
the speed is made until the counterelectromotive force restores to the 
predetermined value, thereby making the base bias voltages of the 
transistors 23 and 24 of the differential amplifier 3' to be in an 
equibrium. 
(2') Operation when motor speed becomes lower: 
When the motor speed fluctuates and becomes slower than a predetermined 
one, then the counter-electromotive force becomes lower than a specified 
one, and accordingly, the potential at the junction point 22 is raised 
over a specified one. By such raising of the potential at the junction 
point 22, the base bias voltage of the transistor 24 of the differential 
amplifier 3' becomes decreased, thereby making the base bias voltage 
smaller than that of the other transistor 23. Therefore, the collector 
current of the transistor 24 decreases and hence the collector current of 
the transistor 23 increases, thereby increasing the collector currents of 
the transistor 26 and 27. The increase of the collector current of the 
transistor 27 makes the base current of the transistor 36 increase, 
thereby increasing its collector current and hence increasing the base 
currents of the first transistor 11 and the second transistors 12 to 14. 
By such increase of the base currents, the collector currents of the 
second transistors 12 to 14 increases, and therefore the motor speed is 
accelerated. 
(3) Reason of large starting torque: 
In the example of FIG. 4, when the DC motor is locked, the 
counter-electromotive force of the DC motor becomes zero, and therefore 
the potential at the junction point 22 is considerably raised. As a result 
of the raising of the potential, the base bias of the transistor 24 is 
decreased, and therefore its collector current decreases. Accordingly, the 
collector current of the transistor 23 increases, and hence, the collector 
currents of the transistors 26 and 27 are increased. Therefore, the base 
current of the transistor 36, which flows into the collector of the 
transistor 27, increases. The current Io of the constant current circuit 
38 is so adjusted that the current makes the transistor 36 saturated. When 
the base currents of the first transistor 11 and the second transistors 12 
to 14 increase as a result of the saturation of the transistor 36, the 
collector currents of the transistors 11 to 14 also increase. Accordingly, 
the potential at the junction point 22 is lowered, thereby feeding the DC 
motor 9 with a large current. 
Provided that the ratio of the total value Ict of the collector currents of 
the second transistors 12 to 14 and collector current Ic11 of the first 
transistor 11 is K, namely K=(Ict/Ic11), then a current of about one K-th 
fraction of the motor current flows through the junction point 19 (and to 
the collector of the transistor 11) (Here, it is provided that the total 
of the current through the constant current circuit 38 and the current 
through the reference voltage generator 25 is negligibly small in 
comparison with the collector current of the first transistor 11.). 
Accordingly, by selecting the resistance value of the resistor 20 to be 
equal to K.times.Ra (Ra is the resistance of the DC motor), the potentials 
of the junction points 19 and 22 can be made equal, since, by so 
selecting, the voltage drops across the resistor and across the DC motor 9 
becomes equal with each other. In such case, the base potentials of the 
transistors 23 and 24 are not equal with each other. Namely, the potential 
of the base of the transistor 23 becomes lower than that of the transistor 
24 by the value of the reference voltage Vref generated by the reference 
voltage generator 25. By such lowering of the base potential of the 
transistor 23, the differential amplifier 3' operated to make and hold the 
transistor 23 ON and the transistor 24 OFF. 
Provided that, in the case of locking the motor, 
VL . . . voltage across the junction point 22 and the terminal 30, 
Vcc . . . voltage impressed across the power source terminals 21 and 30, 
Reo . . . equivalent resistance of the resistors 16 to 18 (that is, one 
n-th fraction of resistance of each of the resistors 16 to 14 for number n 
of the resistors 16 to 18), 
Ra . . . internal resistance of the DC motor, 
Vben . . . base-emitter voltage of the second transistors 12 to 14 when all 
of them are made ON, 
Vces36 . . . collector-emitter voltage at the saturated state of the 
transistor 36, 
then, the voltage VL is given by the following formula: 
##EQU5## 
By arranging the formula (5) with respect to VL, the following equation 
holds: 
##EQU6## 
By another arranging, the voltage (across the junction point 22 and the 
terminal 30) is given as follows: 
##EQU7## 
For an example that, in the speed control apparatus of FIG. 4, wherein 
Ra=6.3.OMEGA., 
Reo=1.0.OMEGA., 
Vben=0.8 V, 
Vces36=0.1 V, 
Vcc=4.5 V, 
the voltage VL across the points 22 and 30 is 
##EQU8## 
The lock current IL at the time of the locking is 
##EQU9## 
By substituting the figures of Vcc=4.5 V, VL=1.39 V and Ra=6.3.OMEGA. in 
the formula (9) the lock current of such a large current as IL=0.494 A is 
obtainable. 
Starting torque .PHI.s of the DC motor is given as follows by multiplying 
the torque constant Kt=96 gcm/A and the lock current IL: 
EQU .PHI.s=Kt.multidot.IL=96.times.IL (10) 
By substituting the figures of IL=0.494 A in the formula (10), the starting 
torque is given as: 
EQU .PHI.s=96.times.0.494=47.4 gcm. 
This starting torque of 47.4 gcm is much grater than average value of about 
20 gcm obtained by using conventional apparatus. 
When embodying the circuit of FIG. 4 in a semiconductor intergrated 
circuit, the PNP transistor 36 must be made in a known vertical type 
transistor, namely in a manner that collector, base and emitter are 
arranged in a vertical order with respect to the principal face of the 
semiconductor substrate, in order to give a satisfactory saturation of the 
second transistors 12 to 14, hence enough lock current to the DC motor. 
The reason why the vertical arrangement is necessary for such purpose is 
as follows: In order to give a large motor current, the base currents of 
the secondary transistors 12 to 14 becomes large, and therefore the 
collector current of the transistor 36 must be large. For one example, 
provided that the h.sub.FE s of the second transistors 12 to 14 are 50 or 
higher, for the total motor current at the locked state of 0.494 A, the 
total of the base current of the transistors 12 to 14, given by 
IL/h.sub.FE, must be about 9.9 mA. In order to allow such large base 
currents to the transistors 12 to 14, the collector or emitter current of 
the transistor 36 must be 9.9/h.sub.FE36 mA, where h.sub.FE36 is the 
current amplification factor of the transistor 36. If a known laternal 
type PNP transistor, which laternal type transistor is known as generally 
to have very low current amplification factor, is used for the transistor 
36, a feeding of a large base current to the transistor 36 from the 
differential amplifier circuit is necessary, and this is a difficult 
problem. Instead, if the vertical type PNP transistor, which is known as 
generally to have sufficiently high current amplification factor, the 
value of the base current of the transistor 36 can be considerably small. 
By employing the speed control apparatus of the present invention, for a DC 
motor of a record player, an advantage of very quick rise up of the 
revolution speed, since maximum controlling torque is very high with this 
apparatus. 
Also, by employing the apparatus for a tape recorder, an advantage that 
even a considerable increase of the load by a large friction of the tape 
would not lock the motor, since the starting torque is considerably high 
with this apparatus.