Generators based on the OS series KR1446. Encyclopedia of radio electronics and electrical engineering

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A feature of the op-amp of the KR1446 series is the allowable input and output voltage range extended to the power supply voltage. This allows them to be used in a variety of low-voltage applications where maximum utilization of the source voltage is important.

The generator circuit, which produces two output signals - triangular and rectangular, is shown in fig. 1.

At the junction of resistors R1 and R2, an artificial midpoint is formed, the voltage at which determines the mode of both op-amps. An integrator is built on the op-amp DA1.1, resistor R4 and capacitor C1, and a Schmitt trigger is built on the op-amp DA1.2 with resistors R3 and R5. A feature of the trigger is a very wide hysteresis loop Uhist = Upit R3 / R5 and accurate and stable switching thresholds. A wide loop allows you to get the maximum amplitude of the triangular voltage at the output of the integrator, since the amplitude of this signal corresponds to the width of the loop.

The generation frequency can be calculated with sufficient accuracy for practice by the formula fr = R5/4C1 R3 - R4. With the ratings of the elements indicated in the diagram, the generation frequency is 265 Hz, and with a change in the supply voltage from 2,5 to 7 V, its deviation from the specified value did not exceed 1%.

The described device can easily be turned into a voltage-controlled generator (VCO) if the non-inverting input of the DA1.1 op-amp is disconnected from the artificial midpoint of the power source and a control voltage is applied to this input. The dependence of the generation frequency on the control voltage Ucontrol is illustrated by the lower curve in fig. 2. It was taken at a supply voltage of 5 V, the capacitance of the capacitor C1 is 430 pF and DA1 - KR1446UD5.

The initial section of the curve - from 5 to 500 mV - has a good linearity, the frequency varies proportionally from 10 to 1000 Hz. If it is necessary to increase the length of the linear section, the integrator can be made asymmetric (Fig. 3). At a low voltage level at the output of the op-amp DA1.2, i.e., at the output of the generator, a relatively slow integration process occurs, the rate of rise of the voltage at the output of the op-amp DA1.1 is determined by the voltage Uynp and the time constant of the C1R1 circuit. The full swing of the sawtooth voltage at the output of the op-amp DA1.1, as in the generator according to the circuit in Fig. 1, is equal to the width of the hysteresis loop of the Schmitt trigger, assembled on the op amp DA1.2, so the duration of the forward stroke tnp= Upit R1 R3 C1 / Uynp R6.

When at the output of the op-amp DA1.2 the low voltage level is replaced by a high one, the VD1 diode opens, R1 is connected in parallel with the resistor R2 and the integration speed increases significantly, and the duration of the reverse stroke decreases accordingly. Therefore, with a high degree of accuracy, we can assume that the period of the generated signal determines the duration of the forward stroke, and the frequency is equal to: fr = Uynp·R6/Upit·С1·R2·R3.

On fig. Figure 2 shows the experimentally obtained dependence of the frequency of the output pulses (upper curve) on the control voltage for the generator according to the circuit of fig. 3. The section of the linear dependence has become six times longer - up to a control voltage value of 3 V (up to an output frequency of 6 kHz).

The dependence of the generation frequency on the control voltage near zero largely depends on the zero shift of a particular instance of the used op-amp. In practice, the frequency of the output pulses can be equal to zero both with a small positive voltage Ucontrol and with a negative one.

In the practical application of the generator, in cases where it is possible to exceed the control voltage over the supply voltage, a resistor with a resistance of several tens of kilo-ohms should be included in the break in the control circuit (pin 3 of the op-amp DA1.1).

On fig. 4 shows a VCO circuit in which zero fine tuning is possible.

At a low voltage at the output of the op-amp DA1.2 (generator output), the transistor VT1 is closed and the voltage at the output of the integrator at the op-amp DA1.1 gradually decreases at a rate proportional to the control voltage. When it drops to the lower switching threshold of the Schmitt trigger on the op-amp DA1.2, a high level will appear at its output, which will open the transistor VT1. Since a voltage of about 1.1 mV is applied to the non-inverting input of the DA4 op-amp from the divider R5R100, this op-amp will go into switching mode and the voltage at its output will begin to rise at a rate determined by the maximum output current of the operational amplifier and the capacitance of the capacitor C1. When the voltage at the output of the op-amp DA1.1 reaches the upper switching threshold of the Schmitt trigger on the op-amp DA1.2, it will switch, the transistor VT1 will close, the process will repeat.

Since the duration of the reverse stroke is much shorter than the forward stroke, the frequency depends on the control voltage quite linearly.

Without resistors R1 and R2, the frequency is zero when the input voltage is equal to the voltage across resistor R5. The R1R2 circuit allows you to make sure that the frequency is zero at zero control voltage.

The width of the hysteresis loop of the Schmitt trigger on the op amp DA1.2 at R7=R8 is Uhyst = Upit/(1+2R9/R8), and the generated frequency can be determined by the formula fr= Uynp (1+2R9/R8) /Upit/ C1 C3.

On fig. 5 shows the experimentally taken dependence of the frequency of signal output by the operational amplifier DA1.2, which fully provides the op-amp of the series in question. It is also important that the amplitude of the signal at the output of the op-amp DA1.2 was less than the signal limiting levels at the output of the op-amp DA1.1.

In the mentioned prototype, this was achieved by a reduced supply voltage of the op-amp DA1.2 compared to the op-amp DA1.1. A similar effect in the generator according to the circuit in Fig. 6 is achieved by transferring the op-amp DA1.2 to the amplification-limiting mode, as a result of which trapezoidal oscillations with a reduced level of harmonics are formed at its output compared to a rectangular signal generated by the corresponding op-amp in the prototype.

When indicated in Fig. With 6 ratings of elements and a supply voltage of 5 V, resistor R5 can tune the output signal frequency within 1600 ... 5800 Hz, however, at a frequency of 3000 Hz or more, the shape of the generated sinusoid was noticeably distorted, and the amplitude dropped to 60% of the value at low frequency. At C1=C2=0,047 µF, the tuning band is equal to 170...640 Hz with a good signal shape, and the amplitude non-uniformity over the band did not exceed 10%.

Author: S. Biryukov, Moscow

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