Constant Voltage Generating Circuit

This invention relates to a temperature-compensated constant voltage generating circuit.

A conventional circuit of this type is as shown in FIG. 1. In FIG. 1, reference numeral 1 designates a resistor; 2, a series circuit of m diodes; 3, a resistor; and 4, a voltage supply terminal. These elements 1, 2, 3 and 4 provide a voltage level V.sub.1. Further in FIG. 1, reference numeral 5 designates a level down circuit for shifting down the voltage level V.sub.1 by a voltage which is represented by the sum of n (n being an integer) times the base-emitter voltage of a transistor or the anode-cathode voltage of a diode, i.e., a p-n junction voltage, and a predetermined voltage; reference numeral 6 designates the input terminal of the circuit 5; reference numeral 7 designates the output terminal of the circuit 5; and reference character V.sub.2 designates the voltage level at the output terminal 7. One example of the aforementioned level down circuit is as shown in FIG. 2. In FIG. 2, reference numerals 4, 6 and 7 designate elements denoted by the same reference numerals in FIG. 1; 21, 22 and 23 are NPN transistors; 24 is a diode; 25 is a resistor; and 26, 27 and 28 are current sources. With n=4, the voltage drop across the resistor 25 corresponds to the above-described predetermined voltage.

The operation of the circuit will now be described.

The voltage levels V.sub.1 and V.sub.2 are represented by the following expressions (1) and (2) respectively: ##EQU1##

where V.sub.BE is the base-emitter voltage of the transistor or the anode-cathode voltage of the diode, R.sub.1 is the resistance of the resistor 1, R.sub.2 is the resistance of the resistor 3, V.sub.cc is the supply voltage, and V.sub.O is the voltage drop across the resistor 25.

If A is inserted for R.sub.1 /R.sub.2 is expression (2), then expression (2) can be rewritten as follows: ##EQU2## If the values V.sub.cc, V.sub.O and R.sub.1 /R.sub.2 are constant irrespective of temperature variation, then the second term in expression (3) is constant irrespective of any temperature variation. Therefore, in order to maintain V.sub.2 unchanged despite a temperature variation, the first term should be equal to zero. Therefore, the condition for making the value of V.sub.2 independent of temperature is:

If B is used in place of R.sub.2 /R.sub.1, expression (4) can be rewritten as follows:

When expression (5) holds true, V.sub.2 is: ##EQU3##

Where the circuit shown in FIG. 1 is used practically, V.sub.2, V.sub.O, V.sub.cc and n are given so that B (=R.sub.2 /R.sub.1) and m are determined from expressions (5) and (6). In this case, the following two problems are involved:

1. The value m must be an integer. Therefore, as is apparent from expression (5), the variation of V.sub.2 due to temperature variation can be made zero only when n.multidot.R.sub.2 /R.sub.1 is an integer.

2. If, even when n.multidot.R.sub.2 /R.sub.1 is an integer, n or R.sub.2 /R.sub.1 is large, then m becomes considerably large. In practice, it is impossible to realize such a circuit.

In conclusion, it is, in general, impossible to make the variation of V.sub.2 due to temperature variation equal to zero with the circuit shown in FIG. 1.

As is apparent from the above description, the conventional circuit is deficient in that, in general, it is impossible to completely compensate for the variation of the output voltage level due to temperature variation.

Accordingly, an object of this invention is to provide a circuit which can in all cases completely compensate for the variation of an output voltage level due to temperature variations.

FIG. 1 is a circuit diagram illustrating a conventional temperature-compensated constant voltage generating circuit;

FIG. 2 is a circuit diagram showing one example of a level down circuit used in FIG. 1;

FIG. 3 is a circuit diagram illustrating a first embodiment of the invention;

FIG. 4 is a circuit diagram depicting a second embodiment of the invention; and

FIG. 5 is a circuit diagram showing one example of a level up circuit used in FIG. 4.

One embodiment of the invention is as shown in FIG. 3. In FIG. 3, reference numerals 4, 5, 6 and 7 designate elements denoted by like reference numerals in FIG. 1; 8 and 9 are resistors for dividing a supply voltage to obtain a reference voltage level V.sub.B ; 10 is an NPN transistor, 11 is a current source; 12 is a series circuit of m' diodes; and 13 is a current source. The circuit elements 10, 11, 12 and 13 form a first circuit 30 for shifting up the reference voltage level V.sub.B to a first voltage level V.sub.1. The level down circuit 5 represents a second circuit which receives the first voltage level V.sub.1 and shifts down the latter by a voltage which is the sum of an integer times a p-n junction voltage and a predetermined voltage, to provide an output voltage level.

The operation of the circuitry in FIG. 3 will be described by using the same reference symbols as those in the description of FIG. 1. ##EQU4##

where R.sub.1' is the resistance of the resistor 8, and R.sub.2' is the resistance of the resistor 9.

From expressions (7) and (8), ##EQU5## If V.sub.cc, V.sub.O and R.sub.1' /R.sub.2' are constant irrespective of temperature variation, then the second term in expression (9) is constant irrespective of any temperature variation. Thus, the condition for making the value V.sub.2 independent of temperature change is:

or

When expression (11) holds true, then ##EQU6## Therefore, when V.sub.2, V.sub.O, V.sub.cc and n are given, it is always possible to determine R.sub.1' /R.sub.2' and m' from expressions (11) and (12). Thus, it is possible to realize a circuit which can render the variation of V.sub.2 due to temperature variation equal to zero in all cases.

A second embodiment of the invention is as shown in FIG. 4. As is apparent from a comparison of FIG. 4 with FIG. 3, in the second embodiment, instead of the level down circuit 5 (FIG. 3) a level up circuit is employed. In FIG. 4, reference numeral 31 designates a voltage supply terminal; 32 and 33 are resistors for dividing a supply voltage to obtain a reference voltage level V.sub.B ; 34 is a PNP transistor; 35 is a current source; 36 is a series circuit of m' diodes; and 37 is a current source. The circuit elements 34, 35, 36 and 37 form a first circuit 50 for shifting down the reference voltage level V.sub.B to a first voltage level V.sub.1. Further in FIG. 4, reference numeral 38 designates a second circuit which shifts up the first voltage level V.sub.1 by a voltage which is the sum of n (n being an integer) times the base-emitter voltage of a transistor or the anode-cathode voltage of a diode, i.e., a p-n junction voltage, and a predetermined voltage; reference numeral 39 designates the input terminal of the second circuit 38; reference numeral 40 designates the output terminal of the second circuit 38; and reference character V.sub.2 designates the second voltage level at the output terminal 40. One example of the level up circuit 38 is as shown in FIG. 5. In FIG. 5, reference numeral 31, 39 and 40 designate elements designated by the same reference numerals in FIG. 4; 41, 42 and 43 are PNP transistors; 44 is a resistor; 45 is a diode; and 46, 47 and 48 are current sources. With n=4, a voltage drop across the resistor 44 corresponds to the aforementioned predetermined voltage. The principle of operation of the second embodiment is similar to that of the first embodiment of FIG. 3.

Thus, a constant voltage generating circuit in which the variation of the output voltage level due to temperature variations may be completely compensated in all cases can be realized according to the invention. The invention has been described on the assumption that V.sub.cc, V.sub.O and R.sub.1' /R.sub.2' are not affected by temperature variation. V.sub.cc is originally constant, and therefore there is no problem in maintaining this parameter constant. In addition, in the case of an integrated circuit, R.sub.1' /R.sub.2' can readily be maintained unchanged irrespective of a temperature variation. Even in the case where the voltage drop V.sub.O is affected by a temperature variation, the employment of the invention is more effective in minimizing the variation of the output level V.sub.2 due to temperature variation than the prior art.