JPH0423963B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a circuit structure of a current controlled oscillator.

(Prior Art) A current-controlled oscillator used in a voltage-controlled oscillator (VCO), etc. usually has two constant-current circuits whose operation is controlled by an input signal voltage, and two constant-current circuits and one capacitor. The circuit is basically configured with a current switching circuit that alternately switches and connects between the two. That is, an oscillation output is obtained through a periodic operation in which one capacitor is charged through one current of a constant current circuit, and then the other constant current circuit is switched to draw and discharge the charged charge. Usually, a current switching circuit consists of a CMOS inverter inserted between two constant current circuits, and one capacitor is connected between the inverter output terminal and one of the power supply terminals (e.g. earth terminal). . Also,
A signal obtained by converting the voltage between the terminals of the capacitor, which changes due to charging and discharging, into a binary value is fed back to the gate electrode of this inverter, and the inverter is usually constructed as a so-called free-running oscillation circuit. Typically, this signal conversion involves a Schmitt
A trigger circuit is used. Transmission frequency in this case
If _psc is written using a general formula, it is as follows.

1/ _psc = 2C/I ₀ (V _SH −V _SL ) + 4t _d However, I ₀ ... Constant current value of constant current circuit C ... Capacitance size V _SH , V _SL ... Hysteresis voltage value of Schmitt trigger circuit t _d ...Operation delay time of the Schmitt circuit and inverter (problem to be solved by the invention) As already mentioned, this oscillator is a free-running oscillator, so the current switching circuit of the CMOS inverter and the Schmitt trigger circuit that controls its operation are required. and are interrelated in terms of circuit operation. Therefore, if this loop circuit ceases to operate normally, the oscillation mode will be disturbed and the oscillation frequency _psc will change. In a normal circuit, CMOS
The threshold value of the inverter is designed to be 1/2 of the power supply voltage, and the reference voltage of the Schmitt trigger circuit is also set to the same voltage. However, in general, the channel length (gate length) of a MOS field effect transistor tends to vary during the manufacturing process, and the resulting variation in gate threshold value can reach as much as ±20% from lot to lot. On the other hand, the output voltage characteristics of a Schmitt trigger circuit generally exhibit slow rising and falling waveforms near the oscillation limit of the entire circuit. Unless the output voltage characteristic itself is adjusted to a square waveform or the output voltage characteristics themselves are improved to a square waveform, the current switching operation timing of the inverter will vary. In other words, it becomes impossible to output the specified oscillation frequency. As described above, the current controlled oscillator with the conventional circuit configuration has the disadvantage that it can only deal with variations in the gate threshold value of the CMOS inverter that constitutes the current switching circuit in a complicated manner, and is extremely difficult to mass-produce.

In view of the above circumstances, an object of the present invention is to provide a current controlled oscillator including a current switching circuit in which current switching timing is not affected by variations in gate threshold values in field effect transistors.

(Means for Solving the Problems) The current controlled oscillator of the present invention has two constants each having either the input side or the output side connected to one of the power supply terminals and whose operation is controlled by the input signal voltage. The constant current circuit is composed of a current circuit and two field effect transistor differential amplifiers of different polarities connected in parallel between the two constant current circuits, each in response to either one of the input binary signals. a current switching circuit that alternately switches and connects between each of the circuits and a common differential output terminal; and a current switching circuit that is connected between the common differential output terminal and one of the power supply terminals and alternates each time the current switching circuit switches. A capacitor that is repeatedly charged and discharged, and a level difference between the voltage across the capacitor and a reference voltage set equal to the reference voltage of the two field effect transistor differential amplifiers are binarized and the two field effect transistor differential amplifiers are connected to each other. and a signal conversion circuit which is fed back to the current switching circuit as an input signal of the current switching circuit.

That is, according to the present invention, a current switching circuit that performs a switching operation based on a binary signal from a signal conversion circuit includes field effect transistors having the same polarity as each transistor of a conventional CMOS inverter, and It is constructed by connecting in parallel two field effect transistor differential amplifiers whose sources are commonly connected. At this time, the reference voltages of the two differential amplifiers and the reference voltage of the signal conversion circuit are set to be equal to each other (for example, 1/2 of the power supply voltage). Here, it is desirable that the signal conversion circuit exhibit hysteresis characteristics with respect to input, such as a Schmitt trigger circuit. However, any substitute can be used instead.

(Function) According to the present invention, a parallel connection circuit of two field effect transistor differential amplifiers performs a switching operation using a binary signal of a signal conversion circuit using a set reference voltage as a threshold. Therefore, even if there is manufacturing variation in the gate threshold voltage of the field effect transistor, the switching operation between the two constant current circuits for the capacitance can always be performed in synchronization with the binary input signal.

(Example) The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a connection circuit diagram showing one embodiment of the present invention. In this example, transistors Q ₁ , Q ₂ , Q ₃ and Q ₄ constitute two constant current circuits, and transistors Q ₅ , Q ₆ and Q constitute two field effect transistor differential amplifiers of a current switching circuit. ₇ , _Q8
, a capacitor C ₁ connected between the common differential output terminal P and the earth terminal GND, the voltage across the capacitor C ₁ to the positive phase input terminal, and 1/2 voltage of the power supply potential V _DD to the positive phase input terminal. A Schmitt trigger circuit S is connected to each of the negative phase input terminals and feeds back its output to the gate terminals of transistors _Q5 and _Q7 . Here, the reference voltages of the two differential amplifiers are Schmitts
It is set equal to the reference voltage V _DD /2 of the trigger circuit S and is applied to the common gate connection terminal of transistors Q ₆ and Q ₈ . Further, V _IN is an input signal.

In this circuit connection, the current switching operation itself is performed by the conventional transistors _Q5 and _Q7 , respectively. That is, the level difference between the voltage across the capacitor C ₁ and the reference voltage V _DD /2 is converted into a binary signal of "H" or "L" by the Schmitt trigger circuit S, which gates the transistors Q ₅ and Q ₇ , respectively. Control. Here, when the Schmitt trigger circuit S outputs "L", the transistor _Q5
conducts the constant current I ₀ from the transistor Q ₅
C ₁ and outputs "H", transistor Q ₇ becomes conductive and a constant current I ₀ flows from capacitor C ₁ to transistor Q ₂ . However, since transistors Q ₅ and Q ₇ constitute a differential amplifier with a reference voltage of V _DD /2 between transistors Q ₆ and Q ₈ , the operation of transistors Q ₅ and Q ₇ , respectively, is similar to this. Regulated by reference voltage. Therefore, transistors Q ₅ and Q ₇ operate with their gate threshold voltages equivalently V _DD /2, charging and discharging capacitor C ₁ , so that the transistors Q 5 and Q 7 operate as free-running transistors that eliminate the influence of variations in gate threshold voltage. An oscillator may be configured.

FIG. 2 is a diagram of output waveforms of various parts in the oscillation circuit of the present invention, and shows that variations in the gate threshold values of transistors _Q5 and _Q7 have no effect. here,
a, b, and c respectively show a Schmitt trigger circuit output waveform diagram, an operation waveform diagram of transistor _Q7 , and a charging/discharging voltage waveform diagram of capacitor _C1 .

3 and 4 respectively show a connection circuit diagram of a conventional circuit configuration and an output waveform diagram of each part thereof, and are shown for the purpose of facilitating comparison with the present invention. Therefore, the same reference numerals are used for parts common to FIGS. 1 and 2. Fourth
The figure shows a case where the threshold value of transistor Q ₇ becomes higher than the set value V _DD /2. When the output of the Schmitt trigger circuit reaches the threshold shown in waveform a, transistor _Q7 turns on for the first time as shown in waveform b, and the voltage across capacitor _C1 decreases as shown in waveform c. As a result, The periods of waveforms a and b are shortened, that is,
It is shown that the oscillation frequency of the entire circuit increases. In this circuit connection, the independent operation of each of transistors Q ₅ and Q ₇ forming the CMOS inverter governs the switching timing characteristics of the direct current switching circuit. Therefore, variations in the gate threshold voltages of these transistors directly affect and cause variations in the oscillation frequency.

〔Effect of the invention〕

As explained in detail above, according to the present invention,
By simply adding a simple circuit, it is possible to completely eliminate the deviation of the oscillation frequency from the specified value due to variations in the gate threshold value, which has been a problem in the past, and the output voltage of each part can be adjusted to the bias point (for example, V _DD /2 ), it is possible to easily operate the oscillation circuit to its characteristic limits. Furthermore, the output characteristics of the signal conversion circuit can be set independently of the current switching circuit, allowing a large degree of freedom in design.
For example, even if there is a large delay in the rise and fall of the output characteristic, the phenomenon of duty ratio fluctuation of the oscillation output, which has been a problem with conventional circuit configurations, does not occur. Furthermore, since the minimum voltage required to switch a field effect transistor differential amplifier is 0.1V, it is sufficient to set the output voltage of a Schmitt trigger circuit to V _DD /2±0.1V, which is extremely It is possible to design circuits suitable for mass production.

[Brief explanation of drawings]

FIG. 1 is a connection circuit diagram showing an embodiment of the present invention;
FIG. 2 is an output waveform diagram of each part in the oscillation circuit of the present invention, and FIGS. 3 and 4 are connection circuit diagrams and output waveform diagrams of each part of the conventional circuit configuration. Q ₁ , Q ₂ , Q ₃ , Q ₄ ... Constant current circuit transistor, Q ₅ , Q ₆ , Q ₇ , Q ₈ ... Current switching circuit transistor, I ₀ ... Constant current, C ₁ ... Capacity, S...Schmitt trigger circuit, P...common differential output terminal,
V _IN ...Input signal voltage, V _DD ...Power supply voltage, GND
...Earth, a... Schmitt trigger circuit output voltage waveform, b... Operation waveform of transistor _Q7 , C
...Charging and discharging voltage waveform of capacitor _C1 , V _SH , V _SL ... Hysteresis voltage of Schmitt trigger circuit.

Claims (1)

[Claims] 1. Between two constant current circuits whose input or output sides are each connected to one of the power supply terminals and whose operation is controlled by the input signal voltage, and the two constant current circuits. between each of the constant current circuits and a common differential output terminal in response to either one of the input binary signals. a current switching circuit which is alternately switched and connected; a capacitor which is connected between the common differential output terminal and one of the power supply terminals and which is alternately charged and discharged every switching operation of the current switching circuit; and both ends of the capacitor. Signal conversion that binarizes the level difference between the voltage and a reference voltage set equal to the reference voltage of the two field effect transistor differential amplifiers and returns it to the current switching circuit as an input signal of the two field effect transistor differential amplifiers. A current controlled oscillator comprising a circuit. 2. The current controlled oscillator according to claim 1, wherein the reference voltage is set to 1/2 of the power supply voltage. 3. The current controlled oscillator according to claim 1, wherein the signal conversion circuit is comprised of a Schmitt trigger circuit.