Constant Voltage Circuit

This invention relates to a constant voltage circuit for minimizing the voltage variation of a power supply due to a temperature variaiton.

An exposure meter using a light receiving element such as CdS etc. finds a wide application in a camera and a mercury cell is often used as a power supply for the exposure meter. The mercury cell can advantageously supply a stable voltage of about 1.3 volts, but it is liable to cause air pollution. For this reason it is desirable to use the other small type of cell. For example, a manganese cell can be used for this object. Since, however, the output voltage of the manganese cell is greatly varied, the manganese cell is proved unsatisfactory as a power supply for an exposure meter requiring a high precision.

It is accordingly the object of this invention to provide a constant voltage circuit capable of supplying a low stable output voltage.

According to one embodiment of this invention there is provided a constant voltage circuit comprising first and second circuit terminals; a first transistor of one conductivity type having a collector coupled to said first circuit terminal; a second transistor of said one conductivity type having a collector and base respectively connected to the emitter and collectors of said first transistor and an emitter connected to said second circuit terminals; a third transistor of said one conductivity type having a base and emitter respectively coupled to the base and emitter of said first transistor and a collector coupled to said first circuit terminal; a fourth transistor of said one conductivity type having a base coupled to the collector of said third transistor and a collector and emitter respectively coupled to said first and second circuit terminals; and bias means for applying a base voltage to the bases of said first and third transistors.

This invention will be further described by way of example by referring to the accompanying drawings in which:

FIG. 1 shows a constant voltage circuit according to one embodiment of this invention;

FIG. 2 shows an improved constant voltage circuit for providing a more stable output voltage with respect to a temperature variation;

FIG. 3 shows a constant voltage circuit, an improved version of the circuit in FIG. 2, for providing a small input-output voltage difference;

FIG. 4 shows a constant voltage circuit according to another embodiment of this invention in which a negative voltage is supplied;

FIGS. 5 and 6 show constant voltage circuit adapted to respectively supply output voltages two and three times the output voltage of the constant voltage circuit in FIG. 1;

FIG. 7 shows a constant voltage circuit according to another embodiment of this invention, the circuit providing a small power dissipation and a small input-output voltage difference;

FIG. 8 shows a constant voltage circuit, an improved version of the circuit in FIG. 7, for suppressing an output voltage variation; and

FIGS. 9 to 11, respectively, show modified forms of the embodiments in FIGS. 6 to 8.

FIG. 1 shows a constant voltage circuit having four pnp transistors TR.sub.1, TR.sub.2, TR.sub.3 and TR.sub.4. The transistor TR.sub.1 has a collector connected through a resistor R.sub.1 to an output terminal 1. A current source 2 is connected to output terminal 1 and adapted to be driven by a power supply E and supplies a current. The emitter of the transistor TR.sub.1 is connected to the collector of the transistor TR.sub.2, and the transistor TR.sub.2 has a grounded emitter and a base connected to the collector of the transistor TR.sub.1. The base of the transistor TR.sub.1 is connected to a base of the transistor TR.sub.3. The transistor TR.sub.3 has an emitter connected through a resistor R.sub.2 to the emitter of the transistor TR.sub.1 and a collector connected through a resistor R.sub.3 to the output terminal 1. A resistor R.sub.4 is connected between output terminal 1 and a junction of the base of the transistors TR.sub.1 and TR.sub.3, and a resistor R.sub.5 is connected between ground and the junction of the bases of the transistors TR.sub.1 and TR.sub.3, the resistors R.sub.4 and R.sub.5 providing a bias circuit for applying a bias voltage to the bases of the transistors TR.sub.1 and TR.sub.3. The transistor TR.sub.4 has a base connected to the collector of the transistor TR.sub.3, a collector connected to the output terminal 1 and an emitter connected through a resistor R.sub.6 to ground.

The operation of the constant voltage circuit in FIG. 1 will now be explained below.

During the operation, electric current flows through the collector of the transistor TR.sub.1 by way of the resistor R.sub.1 and a collector voltage of the transistor TR.sub.1 is applied to the base of the transistor TR.sub.2. If transistors having a sufficiently large current amplification factor are used as the transistors TR.sub.1 and TR.sub.2, the collector voltage of the transistor TR.sub.1 is determined by the base emitter path of the transistor TR.sub.2. With I.sub.1 representing a current flowing through the collector-emitter path of the transistor TR.sub.1, and V.sub.0 a voltage on the output terminal 1, the current I.sub.1 will be given by

the base-emitter voltage V.sub.BE1 of the transistor TR.sub.1 is automatically adjusted to a value large enough to permit a flow of the current I.sub.1. Since the bases of the transistors TR.sub.1 and TR.sub.3 are connected to each other and the emitter of the transistor TR.sub.3 is connected through the resistor R.sub.2 to the emitter of the transistor TR.sub.1, an emitter current I.sub.2 flowing through the transistor TR.sub.3 becomes smaller than the emitter current I.sub.1 flowing through the transistor TR.sub.1 and in consequence a voltage increasing with a rise in temperature is developed across the resistor R.sub.2. The resistor R.sub.3 is connected to the collector of the transistor TR.sub.3, and a voltage having a value obtained by multiplication of a voltage across the resistor R.sub.2 by a ratio R.sub.3 /R.sub.2 of the resistance of the resistor R.sub.3 to the resistance of the resistor R.sub.2 is generated across the resistor R.sub.3. As a result, a voltage across the resistor R.sub. 3 is increased with a rise in temperature. On the other hand, a voltage decreasing with a rise in temperature is developed across the base-emitter path of the transistor TR.sub.4. The values of the resistors R.sub.2 and R.sub.3, if properly selected, permit compensation of voltage changes caused by the temperature variation in the base-emitter voltage of the transistor TR.sub.4 and the voltage across the resistor R.sub.3, making it possible to maintain the voltage V.sub.0 constant. Suppose that a voltage V.sub.0 on the output terminal has increased for some reason or other. Then, the collector voltage of the transistor TR.sub.3 is increased and in consequence the collector current of the transistor TR.sub.4 is increased. Since, however, electric current from the current source 2 is virtually constant, the voltage on the output terminal is decreased. As a result, the voltage V.sub.0 on the output terminal is maintained substantially constant.

Suppose now that in the constant voltage circuit shown in FIG. 1 transistors having a sufficiently great current amplification factor are used as transistors TR.sub.1, TR.sub.2 and TR.sub.3. With I.sub.1, I.sub.2 and I.sub.3 representing currents flowing through the collector-emitter paths of the transistors TR.sub.1, TR.sub.3 and TR.sub.4, respectively, and V.sub.c3 a collector voltage of the transistor TR.sub.3, the following equations are established: ##EQU1## where

q : electron charges

K : Bolzmann's constant (K/q = 8.66 .times. 10.sup.-5 V/.degree.K)

.alpha. : constant

n : constant as determined by a transistor manufacturing method (for example, n = 1.5 for a double diffusion silicon transistor)

V.sub.g0 : voltage corresponding to the band gap energy of a semiconductor substrate used (for example, V.sub.g0 = 1.218V for silicon)

T : absolute temperature

Differentiating the equation (2) by the absolute temperature T and rearranging it yields ##EQU2## Differentiating both the terms of the equation (4) by the absolute temperature T gives ##EQU3## Substituting the equation (7) in the equation (6) and rearranging it gives ##EQU4## Differentiating the equation (3) by the absolute temperature T and rearranging it gives ##EQU5## by sybstituting the equation (8) into the equation (9), ##EQU6## By differentiating the equation (5) by the absolute temperature, ##EQU7## By substituting in the equation (11) a conditional equation ##EQU8## under which the voltage V.sub.0 on the output terminal 1 is not varied due to a temperature variation, ##EQU9## since I.sub.3 .noteq. 0.

If, as derived from the equation (12), I.sub.3 R.sub.6 is a value independent of the absolute temperature, that is, if the equations (10) and (13) are equal to each other with a sufficiently great gain of the transistor TR.sub.4, a temperature compensation in the constant voltage circuit is satisfactorily made. An output voltage V.sub.0 substantially unaffected by the temperature variation is found from the following equation: ##EQU10## With V.sub.g0 = 1.218V, I.sub.3 R.sub.6 = 0.060V, T = 298.degree. K, n = 1.5, V.sub.c3 = 0.730V, V.sub.BE2 = 0.720V, R.sub.2 R.sub.3 = 0.1 and I.sub.2 /I.sub.1 = 0.1, then

in this case, if the fourth term on the right side of the equation (14) is small enough to be negligible and

the output voltage V.sub.0 is obtained which is not varied due to the temperature variation. That is, when the amplification operation of transistors which are connected in the succeeding stage is taken into consideration, it will be found that, in order to effect temperature compensation of the constant voltage circuit, the constant voltage circuit should be designed so that I.sub.3 in the equation (11) may vary in proportion to the absolute temperature. In other words, it is necessary that the following equation be satisfied. ##EQU11## Substituting the equation (16) into the equation (11) yields ##EQU12## The output voltage V.sub.0 is found, like the equation (15), from the equation (17), ##EQU13##

If each value of the resistors R.sub.1 and R.sub.6 is so properly set as to fulfill the equation (15) in a case where the transistor TR.sub.4 has a high amplification factor and the equation (18) in a case where the transistor TR.sub.4 has a low amplification factor, a constant voltage circuit with low voltage can be obtained which is unaffected by the temperature variation. It is to be noted that the resistor R.sub.6 can have a value of zero.

FIG. 2 is a constant voltage circuit according to another embodiment of this invention, in which the amplification degree of the transistor TR.sub.4 is enhanced. In this Figure, like reference numerals are employed to designate elements corresponding to those shown in FIG. 1 and further explanation will be therefore omitted.

The collector of the transistor TR.sub.4 is connected to the base of a transistor TR.sub.8 and through a resistor R.sub.10 to a constant current souce comprising pnp transistors TR.sub.5 and TR.sub.6, npn transistor TR.sub.7 and resistors R.sub.7, R.sub.8 and R.sub.9. The base of the transistor TR.sub.6 is connected to the base of the transistor TR.sub.5, the base of the latter transistor being connected to its own collector. The emitters of the transistors TR.sub.5 and TR.sub.6 are connected respectively through the resistors R.sub.8 and R.sub.9 to the positive terminal of a power supply E. The transistor TR.sub.7 has a base connected to the base of the transistor TR.sub.2, a collector connected to the base of the transistor TR.sub.5 and an emitter connected to ground through the resistor R.sub.7. The emitter of the transistor TR.sub.8 and collector of the transistor TR.sub.6 are connected to the base of a first stage transistor TR.sub.9 which constitutes a Darlington circuit together with a transistor TR.sub.10. The emitter of the succeeding stage transistor TR.sub.10 in the Darlington circuit is connected to an output terminal, and the collectors of the transistors TR.sub.9 and TR.sub.10 are connected to the positive terminal of the power supply E.

During the operation of the constant voltage circuit a given electric current flows through the collector of the transistor TR.sub.7 to cause a bias to be applied to the base of the transistor TR.sub.6 and a constant current is supplied to the collectors of the transistors TR.sub.4 and TR.sub.8 through the emitter-collector path of the transistor TR.sub.6. That is, a circuit including the transistors TR.sub.4 and TR.sub.8 is equivalently coupled to a very high load resistance and the input impedance of the Darlington circuit is very high. In consequence, a very high voltage amplification can be obtained and the variation of the output voltage V.sub.0 is restricted to a minimum as in the case of the constant voltage circuit in FIG. 1. A capacitor C.sub.0 is a capacitor for preventing oscillation and it is connected between the collector and the base of the transistor TR.sub.4 so as to make the operation of the constant voltage circuit stable. A starting circuit, comprising a transistor TR.sub.11 and a resistor 11, makes it easy to start the constant voltage circuit. In the absence of the starting circuit an output voltage on the output terminal 1 is not increased even when the power supply is turned ON. That is, since no electric current flows through the transistor TR.sub.7 before the turning ON of the power supply, even when the power supply is turned ON, no electric current flows through the transistor TR.sub.6 and in consequence the output on the voltage is maintained to be zero. The starting circuit permits electric current to flow through the resistor R.sub.11 by way of the transistor TR.sub.11 during the turning ON of the power supply. Since a bias voltage is applied through the resistor R.sub.11 to the base of the transistor TR.sub.6, the constant voltage circuit is immediately started. The starting circuit may be constructed of a resistor circuit and in this case the output voltage is greatly varied. If in this case the collector current of the transistor TR.sub.7 is made great and electric current flowing through the starting circuit or the substitute resistor circuit is set to be a lowest possible value necessary to turn the transistor TR.sub.11 ON, the output voltage variation can be restricted to minimum. In this embodiment, a resistor R.sub.12 and diode-connected npn transistor TR.sub.12 are employed in place of the resistor R.sub.5 in the constant voltage circuit in FIG. 1. The transistor TR.sub.12 has an emitter and collector connected to ground and a base connected to the resistor R.sub.12. Since the base emitter voltage of the transistor TR.sub.12 is decreased with a rise in temperature, a direction in which the base-biased voltage of transistors TR.sub.1 and TR.sub.3 is varied due to the temperature variation is the same as a direction in which the base-emitter voltage of the transistor TR.sub.12 is varied due to the temperature direction. As a result, a stable operation is assured over a wide temperature variation range.

In the constant voltage circuit in FIG. 2 the output voltage is about 1.6 volts lower than the input voltage due to the presence of the transistor TR.sub.6 and the darlington circuit comprising the transistors TR.sub.9 and TR.sub.10. In consequence, if an output voltage of, for example, 1.3V is to be obtained, an input voltage of 2.9 volts is required with the resultant poor efficiency.

A constant voltage circuit in FIG. 3 is designed to overcome such defects. In FIG. 3 an npn transistor TR.sub.13 having a base connected to a junction between the collectors of transistors TR.sub.4 and TR.sub.6. The collector of the transistor TR.sub.13 is connected to the base of a pnp transistor TR.sub.14 whose emitter is connected to the power supply E. The collector of the transistor TR.sub.14 is connected to an npn transistor TR.sub.15 having a collector connected to its base and to the output terminal 1. The emitters of the transistors TR.sub.15 and TR.sub.13 are connected to each other and to a constant current source comprising an npn transistor TR.sub.16 and resistor R.sub.13. The transistor TR.sub.16 has a collector connected to a junction between the emitters of the transistors TR.sub.13 and TR.sub.15, a base connected to collector of the transistor TR.sub.1 and an emitter connected through a resistor R.sub.13 to ground.

In the constant voltage circuit shown in FIG. 3 the output signal of the transistor TR.sub.4 is amplified by the transistors TR.sub.13 and TR.sub.14 and it is 100% negatively fed by the transistor TR.sub.15 back to the emitter of the transistor TR.sub.13. In consequence, the input signal to the base of the transistor TR.sub.13 and the output signal are in phase with and substantially equal in magnitude to each other. The transistors TR.sub.13, TR.sub.14 and TR.sub.15 constitute a voltage follower circuit having a small input-output voltage difference. Since the transistor TR.sub.14 is operated even in such a state that the collector-emitter voltage is lowered sufficiently to about a saturation voltage, a voltage between the positive terminal E and the output terminal 1 can be restricted to about 0.3V. If, therefore, an output voltage of, for example, 1.3V is to be obtained, an input voltage of about 1.6V may be used. When, for example, an Mn dry battery is used as a power supply, even if the power supply voltage is dropped to 1.6V, a constant voltage of 1.3V is obtained as an output voltage, providing a prominent improvement from the standpoint of economy. The same result is also obtained if a diode is used in place of the transistor TR.sub.15 or if the collector of the transistor TR.sub.15 is connected to the power supply E.

FIG. 4 shows a constant voltage circuit according to another embodiment of this invention, in which a power supply voltage for supplying a negative voltage may have a wide range. In this embodiment, like reference numerals are employed to designate elements corresponding to those shown in FIG. 3 and further explanation is therefore omitted.

The collector of the transistor TR.sub.4 is connected to the collector of a transistor TR.sub.17 which constitutes one element of a constant current source. The collector voltage of the transistor TR.sub.4 is fed, through an emitter follower circuit comprising an npn transistor TR.sub.18 and pnp transistor TR.sub.19, to that amplification circuit, comprising a pnp transistor TR.sub.20 and npn transistor TR.sub.21, where it is reversed in its phase and amplified. The transistor TR.sub.18 has a base connected to the collectors of the transistors TR.sub.4 and TR.sub.17, a collector connected to ground and an emitter connected to the base of the transistor TR.sub.19. The emitter of the transistor TR.sub.19 is connected to the base of the transistor TR.sub.20 whose emitter is connected to ground. The collector of the transistor 20 is connected to the base of the transistor TR.sub.21 whose collector is connected to ground.

A resistor R.sub.14 and npn transistor TR.sub.22 and TR.sub.23 constitute a constant current source. A pnp transistor TR.sub.24 and npn transistors TR.sub.25 and TR.sub.26 perform the same operation as the transistors TR.sub.13, TR.sub.14 and TR.sub.15 in FIG. 3 and constitute a voltage follower circuit having a small input-output voltage difference. That is, the transistor TR.sub.22 has a base connected to its own collector and an emitter connected through the resistor R.sub.14 to the negative terminal of the power supply E and the transistor TR.sub.23 has an emitter connected to the negative terminal of the power supply E, a base connected to the base of the transistor TR.sub.22 and a collector connected to the base of the transistor TR.sub.24 and to the emitter of the transistor TR.sub.21. The transistor TR.sub.24 has a collector connected to the base of the transistor TR.sub.25 and an emitter connected to the collector of the transistor TR.sub.26, and the transistor TR.sub.25 has an emitter connected to the negative terminal of the power supply E and a collector connected to the emitter of the transistor TR.sub.26, collector of the transistor TR.sub.19 and output terminal 1. The transistor TR.sub.26 has a base connected to its own collector.

An npn transistor TR.sub.27 and resistor 15 constitutes a constant current source for supplying a constant electric current to the transistor TR.sub.18. The transistor TR.sub.27 has an emitter connected through the resistor 15 to the output terminal 1, a base connected to the base of a transistor TR.sub.2 and a collector connected to the emitter of the transistor TR.sub.18.

pnp transistors TR.sub.28, TR.sub.17, TR.sub.29 and TR.sub.30 are connected respectively to resistors R.sub.16, R.sub.17, R.sub.18 and R.sub.19, constituting constant current sources. The emitters of the transistors TR.sub.28, TR.sub.17, TR.sub.29 and TR.sub.30 are connected respectively through resistors R.sub.16, R.sub.17, R.sub.18 and R.sub.19 to ground, and the bases of the transistors TR.sub.28, TR.sub.17, TR.sub.29 and TR.sub.30 are connected to the drain of a field effect transistor TR.sub.11. The transistor TR.sub.28 has a collector connected to its own base and to the output terminal 1 through a resistor R.sub.20, the transistor TR.sub.29 has a collector connected to the emitter of the transistor TR.sub.24, and the transistor TR.sub.30 has a collector connected to the collector of the transistor TR.sub.22.

The emitter follower circuit constituted by the npn transistor TR.sub.18 and pnp transistor TR.sub.19 is so incorporated into the constant voltage circuit that by increasing the input impedance of the emitter follower circuit the amplification factor of the transistor TR.sub.4 is made great so as to restrict the output voltage variation of the constant voltage circuit to a minimum and that even if a capacitor C.sub.0 has a small value a sufficient oscillation preventing effect is obtained. With the capacitor C.sub.0 incorporated into the constant voltage circuit an amplification G.sub.V is given, as well known in the art, by ##EQU14## where R.sub.s denotes a signal source impedance R.sub.s as viewed toward the signal source from the base of the transistor TR.sub.4 and A denotes a voltage amplification factor before the capacitor is incorporated into the circuit comprising the transistor TR.sub.4.

As will be evident from the equation (17) the capacitor C.sub.0 is equivalent to a capacitor having (1+A) times its capacitance. As a result, a sufficiently low cut-off frequency and oscillation preventing effect are obtained. In this embodiment a sufficient oscillation preventing effect is obtained using a capacitor of, for example 20pF. The 20pF capacitor can be easily incorporated into an integrated voltage circuit.

For a -1.5V input voltage, a -1.3V output voltage unaffected by the temperature change is obtained in the constant voltage circuit shown in FIG. 4.

Although in the above-mentioned embodiment explanation is directed to the obtainment of a stable .+-.1.3V output voltage with a .+-.1.5V power supply as an input voltage, it is possible to obtain an output voltage having an integral multiple of .+-.1.3V.

FIG. 5 shows a constant voltage circuit as obtained by adding npn transistors TR.sub.31 and TR.sub.32, resistor R.sub.20 and capacitor C.sub.0 to the constant voltage circuit shown in FIG. 1. In this Figure, like reference numerals are employed to designate elements corresponding to those shown in FIG. 1 and further explanation is therefore omitted.

The transistor TR.sub.31 has an emitter connected to the base of the transistor TR.sub.4 and to ground through the resistor R.sub.20 and a base connected to the collector of the transistor TR.sub.3. The capacitor C.sub.0 is connected to the collectors of the transistors TR.sub.3 and TR.sub.4 and the transistor TR.sub.32 has a collector connected to the positive terminal of a power supply E, a base connected to the collector of the transistor TR.sub.4 and an emitter connected to the output terminal 1 and to the collector of the transistor TR.sub.31.

In this embodiment the emitter-base voltage of the transistors TR.sub.4 and TR.sub.31 is decreased with a rise in temperature. The decreased emitter-base voltage of the transistors TR.sub.4 and TR.sub.31 is compensated by a voltage across the resistor R.sub.3 which is increased by the temperature increase. The constant voltage circuit provides a 2.6V stable output voltage corresponding to two times the output voltage of the constant voltage circuit in FIG. 1.

FIG. 6 shows another embodiment of this invention. This embodiment is the same as the embodiment in FIG. 5 except that an npn transistor TR.sub.33 has an emitter connected to the resistor R.sub.3 and a collector and base connected to the output terminal 1. Since the base-emitter paths of the transistors TR.sub.4, TR.sub.31 and TR.sub.33 are connected in series with one another, this embodiment provides a 3.9V output voltage corresponding to three times the output voltage of the circuit in FIG. 1.

With the constant voltage circuit in FIGS. 1 to 4 as a reference voltage circuit it is possible to design a constant voltage circuit for generating a desired voltage.

FIG. 7 shows another embodiment of this invnetion. In this embodiment, like reference numerals are employed to designate elements corresponding to those shown in the above-mentioned embodiment and further explanation is therefore omitted.

In FIG. 7 the collector of the npn transistor TR.sub.33 is connected to a current mirror comprising transistors TR.sub.5, TR.sub.6 etc. This constant voltage circuit is lower in current dissipation then the constant voltage circuit in FIG. 3 additionally including a constant current source comprising the transistor TR.sub.7 etc. A constant current source comprising a transistor TR.sub.34 and resistor R.sub.21 is used in place of the resistor R.sub.4 in the constant voltage circuit in FIG. 1. Since the constant current source can supply a necessary bias current by using a resistor sufficiently lower than the resistor R.sub.4 in the constant voltage circuit in FIG. 3, where the constant voltage circuit is embodied as an integrated circuit, it is possible to advantageously reduce a chip size. A diode-coupled transistor TR.sub.35 is cooperated with the transistors TR.sub.33, TR.sub.31 and TR.sub.6 to generate an output voltage corresponding to substantially four times the voltage equivalent to the band gap energy of a semiconductor substrate used. A field effect transistor TR.sub.36 for drive has a gate connected to ground, and a source and drain respectively connected to the emitter and collector of the transistor TR.sub.33. Immediately after the power source is turned ON, while a voltage on the output terminal 1 is not increased, the emitter voltage of the transistor TR.sub.33 is maintained to a low value, the source voltage of the field effect transistor TR.sub.36 is at a low level and the bias of the transistor TR.sub.36 is shallow. In consequence, a large current flows through the transistor TR.sub.36 by way of the transistor TR.sub.35, resistor R.sub.1, transistor TR.sub.2 and variable resistor R.sub.22. After the constant voltage circuit is triggered, a deep bias is applied to the transistor TR.sub.36 due to an increased emitter voltage of the transistor TR.sub.33 and an electric current flowing through the transistor TR.sub.36 is decreased, minimizing the effect exerted by the transistor TR.sub.36 on the output voltage and improving the variation rate of the output voltage of the constant voltage circuit.

The resistor R.sub.22 is one used to compensate for the variation of an output voltage resulting from a manufacturing error in the manufacturing processes of integrating the constant voltage circuit. The provision of one extra terminal permits the resistor R.sub.22 to be adjusted from the outside. Transistors TR.sub.37 and TR.sub.38, respectively, serve to increase the input impedancce of transistor TR.sub.13 and TR.sub.15, increasing the amplification factor of the transistor TR.sub.4. A circuit comprising a resistor R.sub.23 and transistor TR.sub.39 allows a relatively small current varied by a temperature variation to be applied to the transistor TR.sub.31. The constant voltage circuit in FIG. 7 provides a small input-output voltage difference, a small current dissipation and a substantially constant output voltage against the temperature difference.

FIG. 8 shows a constant voltage circuit according to another embodiment of this invention. In this Figure, like reference numerals are employed to designate elements corresponding to those shown in FIG. 7 and further explanation is therefore omitted.

In FIG. 8, transistors TR.sub.4 and TR.sub.31 are driven using as a load a constant current source comprising transistors TR.sub.40, TR.sub.41 and TR.sub.42 and resistors R.sub.24, R.sub.25 and R.sub.26. The transistor TR.sub.40 is of an npn type and has an emitter connected through the resistor R.sub.24 to ground, a base connected to the corrector of the transistor TR.sub.1 and a collector connected to the collector and base of the npn transistor TR.sub.41. The transistor TR.sub.41 has an emitter connected through the resistor 25 to the output terminal, and the collector connected to its own base which is connected to the base of the npn transistor TR.sub.42. The transistor TR.sub.42 has a collector connected to the collector of the transistor TR.sub.4 and an emitter connected through a resistor R.sub.26 to the output terminal.

The collector voltage of the transistor TR.sub.4 is, after amplified by the transistor TR.sub.43, supplied to the output terminal 1 through an emitter follower circuit comprising transistors TR.sub.13, TR.sub.14 and TR.sub.15.

In the constant voltage circuit shown in FIG. 8 an electric current determined by the voltage of the output terminal 1 flows through the collectors of the transistors TR.sub.4 and TR.sub.31, and the transistors TR.sub.4 and TR.sub.31 provide a great amplification degree. As a result, there is provided a stable constant voltage circuit involving a small variation of an output voltage with respect to a power supply voltage and reducing the effect of the manufacturing variation of circuit elements to provide a small output variation.

FIGS. 9 to 11 show the modified forms of the embodiments shown in FIGS. 6 to 8, respectively. In the modification shown in FIGS. 9 to 11 the base of a transistor TR.sub.33 is connected through a variable resistor R.sub.27 to the output terminal (not connected directly to the output terminal) and through a resistor 28 to ground. An output voltage appearing on the output terminal 1 is controlled by adjusting the resistor R.sub.27. In the circuit shown in FIG. 9 the base of the transistor TR.sub.33 may be connected to the collector of the transistor 32, not to the output terminal 1.