Monolithic integrated circuit for an RC oscillator

A monolithic integrated circuit for an RC oscillator includes a differential amplifier acting as a switching stage for comparing a threshold voltage corresponding to the maximum oscillator voltage with the capacitor voltage of the RC circuit. An output of the differential amplifier is connected to the S input of an R-S flip-flop which is set when the capacitor voltage reaches the threshold voltage. A switching transistor has its collector and emitter connected across the capacitor and a base connected to the output of the R-S flip-flop for discharging the capacitor when the flip-flop is set. A second transistor has its base-emitter junction connected across the base-collector junction of the switching transistors so that when the switching transistor has discharged the capacitor to a point that only its collector-to-emitter saturation voltage dropped across it, the second transistor is turned on to provide a signal to the reset input of the flip-flop, thereby inhibiting the discharge of the capacitor and allowing the capacitor to recharge through a current source.

BACKGROUND OF THE INVENTION 
The present invention relates to a monolithic integrated circuit for an RC 
oscillator as is known in principle from German Offenlegungsschrift No. 
1,921,035. 
In the known RC oscillator, the capacitor is charged and discharged between 
the two voltage values corresponding to the maximum and the minimum 
oscillator voltage. One terminal of the capacitor is grounded, while the 
other is connected to a terminal of the integrated circuit and, via a 
current source, to the live terminal of the supply-voltage source. The 
known RC oscillator includes a switching stage with a threshold 
corresponding to the maximum oscillator voltage. Connected in parallel 
with the capacitor is a discharge-current source which may comprise at 
least one transistor. In the simplest case, the collector-emitter junction 
of a switching transistor is connected across the capacitor, which 
switching transistor is turned on when the voltage across the capacitor 
reaches the threshold of the switching stage. 
In the known RC oscillator, however, the aforementioned switching stage 
with a threshold corresponding to the maximum oscillator voltage is 
designed as a threshold circuit with two switching thresholds. The 
switching stage connects the discharge circuit when the capacitor voltage 
reaches the upper threshold of response, and disconnects the discharge 
circuit when the capacitor voltage reaches the lower threshold of 
response. In an embodiment of the known RC oscillator, the threshold 
circuit comprises a voltage divider determining the voltage of the upper 
threshold of response, with one divider resistor being shunted, at the 
instant the threshold circuit responds, by an additional resistor which is 
disconnected again when the capacitor voltage reaches the lower switching 
threshold so obtained. 
Such threshold circuits, whose thresholds are preset by means of a voltage 
divider, usually consist of a differential amplifier having the 
voltage-divider tap connected to one of its inputs. The lower limit of the 
lower threshold of response is determined by the base-to-emitter threshold 
voltage of the differential-amplifier transistor connected to the tap. 
Especially in the case of circuits having only one supply-voltage source, 
however, the minimum threshold of response is higher because of the 
emitter resistor common to both transistors of the differential amplifier. 
With the known circuit, it is therefore impossible to utilize the full 
supply-voltage range for the oscillator voltage because the minimum 
oscillator voltage cannot be lowered close enough to ground potential. 
The smaller the supply voltage available, the more significant this 
disadvantage becomes. For example, in battery-operated equipment, the 
ratio of the attainable swing of the oscillator voltage to the battery 
voltage deteriorates noticeably towards smaller battery voltages. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a monolithic integrated 
circuit for an RC oscillator which is not subject to the above-mentioned 
limitation regarding the lower threshold determining the minimum 
oscillator voltage, so that, by suitable choice of the upper threshold, as 
large an oscillator-voltage swing as possible can be selected. 
According to a broad aspect of the invention, there is provided a 
monolithic integrated circuit for an RC oscillator whose capacitor is 
charged and discharged between two voltage values corresponding to the 
maximum and the minimum oscillator voltage when one terminal of said 
capacitor is coupled to ground and the other terminal of said capacitor is 
coupled to a terminal of an integrated circuit and, via a current source, 
to the live terminal of a source of supply voltage comprising: a switching 
stage having a threshold corresponding to the maximum oscillator voltage; 
a switching transistor having base, emitter and collector terminals for 
discharging said capacitor when the voltage across said capacitor reaches 
the threshold voltage of said switching stage, the saturation voltage of 
said switching transistor serving as the threshold corresponding to the 
minimum oscillator voltage; control means coupled to said switching stage 
and said switching transistor for turning said switching transistor on and 
off; and means for inhibiting said control means allowing said capacitor 
to be charged.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The RC oscillator consists of a frequency-determining capacitor 1, one 
terminal of which is grounded, and of a current source 2, which may be 
implemented by a resistor 3, for example. Via the current source 2, which 
has one terminal connected to the live terminal of the supply-voltage 
source U.sub.B, and the other to the ungrounded terminal of the capacitor 
1, the latter is charged. 
The common terminal of capacitor 1 and current source 2 is connected to one 
of the external terminals 4 of the monolithic integrated circuit. Via this 
terminal 4, the collector-emitter junction of the switching transistor 5 
is connected in parallel with the capacitor 1, coupled to the integrated 
circuit from outside. When the transistor 5 is on, the capacitor 1 is 
discharged through this transistor. 
The voltage across the capacitor 1 and, hence, at the terminal 4 can 
increase until it has reached virtually the same amplitude as the voltage 
at the tap of the voltage divider connected across the supply-voltage 
source and consisting of resistors 6 and 7. This tap and the terminal 4 
are coupled to the two inputs of a differential amplifier consisting of 
transistors 9 and 10 and acting as a switching stage 8. The base of the 
transistor 9 is connected to the capacitor 1 and to the terminal 4, while 
the base of the transistor 10 is coupled to the tap of the voltage 
divider. The transistors 9 and 10 are provided with collector resistors 11 
and 12 and with a common emitter resistor 13, which may all be replaced, 
in known manner, by constant current sources. 
The output of the switching stage 8, which output is identical with the 
collector of the transistor 9, is coupled to the S input of an R-S 
flip-flop 14, which is thus set when the voltage across the capacitor 1 
reaches or exceeds the threshold set with the voltage divider 6, 7. 
The output associated with the S input of the R-S flip-flop 14 is coupled 
to the base of the switching transistor 5, so the latter is turned on by 
the signal applied to it as a result of the R-S flip-flop 14 being set 
and, consequently, discharges the capacitor 1, as mentioned above. 
Connected across the base-collector junction of the switching transistor 5 
is the base-emitter junction of the transistor 15, which is turned off 
when the switching transistor 5 is in the saturation state. The collector 
of the transistor 15 is connected to the supply voltage U.sub.B via a 
resistor 16. When the switching transistor 5 has discharged the capacitor 
1 to such a point that only its collector-to-emitter saturation voltage 
drops across it, the transistor 15 is turned on and resets the R-S 
flip-flop 14 via the latter's R input, whereby the switching transistor 5 
is cut off again. Now, the charging of the capacitor 1 through the current 
source 2 can start anew. According to the invention, the saturation 
voltage of the switching transistor 5 thus serves as the threshold 
corresponding to the minimum oscillator voltage. 
There are several ways to cause the transistor 15 to conduct when the 
switching transistor 5 is driven into saturation. For instance, through a 
resistor 17 connected to the two bases and to that output of the R-S 
flip-flop 14 associated with the S input, a current driving the switching 
transistor 5 into the heavily conducting state may be injected into the 
base of this transistor, thereby increasing the base-to-emitter threshold 
voltage of this transistor to such a value that the transistor 15 is 
definitely turned on. On the other hand, the base-to-emitter threshold 
voltage of the transistor 15 may be lowered in comparison with the 
base-to-emitter threshold voltage of the switching transistor 5 by making 
the base-emitter pn junction of transistor 15 larger, preferably by one 
order of magnitude, than that of the switching transistor 5. By these 
alternative measures, which are appropriate especially in the case of npn 
transistors as are shown in the figure for the transistors 5, 9, 10, 15, 
the base-to-emitter threshold voltage of the transistor 15 can be made 
smaller than the base-to-emitter threshold voltage of the switching 
transistor 5, which is reduced by the saturation voltage of this 
transistor, so the transistor 15 will be on when the switching transistor 
5 is operated in the saturation region. 
If the transistors 5, 15 are pnp transistors, it will be particularly 
advantageous to realize both transistors as a single, special 
two-collector lateral transistor, with the collector of the transistor 15 
having to be "behind" the collector of the switching transistor 5 with 
respect to the current flow. Preferably, the collector of the switching 
transistor 5 is enclosed by the collector of the transistor 15. Such a 
lateral transistor structure has the property that, when the transistor 
belonging to the enclosed collector is driven into saturation, this 
collector acts as the emitter of the transistor belonging to the enclosing 
collector, i.e., that the transistor 15 begins to conduct when the 
switching transistor 5 is driven into saturation. In this case, the 
collector resistor 16 of the transistor 15 may be dispensed with, and the 
R input of the R-S flip-flop 14 may be energized directly from the 
enclosing collector. 
If the transistors 5, 15 are npn transistors, they may be combined into a 
double-emitter transistor whose first emitter is grounded, and whose 
second emitter is connected to the collector resistor 16, which must now, 
of course, be called the emitter resistor, and to the R input of the R-S 
flip-flop 14. As soon as the partial transistor corresponding to the 
switching transistor 5 is driven on, the second emitter acts as the 
collector of an inversely operated partial transistor, which energizes the 
R-input. 
It is also advantageous to implement the R-S flip-flop 14 using the 
so-called I.sup.2 L technology, while the remaining components are made 
using conventional bipolar technology. "I.sup.2 L" is the English 
abbreviation for "Integrated Injection Logic" and refers to a special 
bipolar logic circuit fabrication technique, cf. "Elektronikpraxis", 
October 1975, pp. 7 to 10, and "Philips Technical Review", 1973, pp. 76 to 
85. 
The monolithic integrated circuit according to the invention shows freqency 
stability which, in particular, is independent of supply-voltage 
variations. The circuit already works at operating voltages of as little 
as 0.8 V, so it can be readily used as the tone generator of a 
battery-operated alarm clock. Since such alarm clocks are commonly 
operated with batteries having a rated voltage of 1.5 V which decreases to 
about IV during the battery life, alarm clocks equipped with the 
integrated circuit of the invention are capable of oscillating until the 
end of the battery life.