Low frequency function generator. Relaxation generator of sawtooth voltage, signal, saw

The principle of operation of the relaxation generator is based on the fact that the capacitor is charged to a certain voltage through a resistor. When the required voltage is reached, the control element opens. The capacitor is discharged through another resistor to a voltage at which the control element closes. So the voltage on the capacitor increases according to an exponential law, then decreases according to an exponential law.

You can read more about how a capacitor is charged and discharged through a resistor by following the link.

Here is a selection of materials: The use of transistor analogues of a dinistor in relaxation generators is typical, since strictly defined parameters of the dinistor are required for the calculation and accurate operation of this generator. Some of these parameters for industrial dinistors either have a large technological spread or are not standardized at all. And making an analogue with strictly specified parameters is not difficult. Ramp voltage generator circuit The relaxation generator looks like this: (A1)- relaxation generator based on a diode thyristor (dinistor), (A2)- in circuit A1 the dinistor is replaced with a transistor analogue. You can calculate the parameters of the transistor analog depending on the transistors used and resistor values. Resistor R5 selected small (20 - 30 Ohms). It is designed to limit the current through the dinistor or transistors at the moment they open. In the calculations, we will neglect the influence of this resistor and assume that the voltage across it practically does not drop, and the capacitor through it is discharged instantly. The dinistor parameters used in the calculations are described in the article Volt-ampere characteristics of the dinistor. [Minimum output voltage, V] = [Maximum output voltage, V] = Calculation of the resistance of resistor R4 For resistor R4, two relationships must be met: [Resistance R4, kOhm] > 1.1 * ([Supply voltage, V] - [Dinistor turn-off voltage, V]) / [Holding current, mA] This is necessary so that the dinistor or its analogue is securely locked when the capacitor is discharged. [Resistance R4, kOhm] Supply voltage, V] - [ Dinistor unlocking voltage, V]) / (1.1 * [Unlocking current, mA]) This is necessary so that the capacitor can be charged to the voltage required to unlock the dinistor or its equivalent. The coefficient of 1.1 was chosen conditionally out of the desire to get a 10% reserve. If these two conditions conflict with each other, then this means that the circuit supply voltage for this thyristor is selected too low. Calculation of the relaxation oscillator frequency The frequency of the generator can be approximately estimated from the following considerations. The oscillation period is equal to the sum of the capacitor charging time to the dinistor unlocking voltage and the discharge time. We agreed to assume that the capacitor discharges instantly. So we need to estimate the charging time. Second option: R1- 1 kOhm, R2, R3- 200 Ohm, R4- trimmer 3 kOhm (set to 2.5 kOhm), Supply voltage- 12 V. Transistors- KT502, KT503. Generator Load Requirements The above relaxation generators can operate with a load that has a high input resistance so that the output current does not affect the charging and discharging process of the capacitor. [Load resistance, kOhm] >> [Resistor R4 resistance, kOhm]

Good afternoon, dear radio amateurs! Welcome to the website ““

We assemble a signal generator - a function generator. Part 1.

In this lesson Schools for beginner radio amateurs We will continue to fill our radio laboratory with the necessary measuring instruments. Today we will start collecting function generator. This device is necessary in the practice of a radio amateur to configure various amateur radio circuits– amplifiers, digital devices, various filters and many other devices. For example, after we assemble this generator, we will take a short break during which we will make a simple light-music device. So, in order to correctly configure the frequency filters of the circuit, this device will be very useful to us.

Why is this device called a functional generator, and not just a generator (low frequency generator, high frequency generator). The device that we will manufacture generates three different signals at its outputs: sinusoidal, rectangular and sawtooth. As a basis for the design, we will take S. Andreev’s diagram, which is published on the website in the section: Circuits – Generators.

First, we need to carefully study the circuit, understand the principle of its operation and collect the necessary parts. Thanks to the use of a specialized microcircuit in the circuit ICL8038 which is precisely intended for building a function generator, the design turns out to be quite simple.

Of course, the price of the product depends on the manufacturer, the capabilities of the store, and many other factors, but in this case we are pursuing one goal: to find the necessary radio component that would be of acceptable quality and, most importantly, affordable. You probably noticed that the price of a microcircuit greatly depends on its marking (AC, BC and SS). The cheaper the chip, the worse its performance. I would recommend choosing the “BC” chip. Its characteristics are not very different from “AS”, but much better than those of “SS”. But in principle, of course, this microcircuit will also work.

Assembling a simple function generator for the laboratory of a novice radio amateur

Good day, dear radio amateurs! Today we will continue to collect our function generator. So that you don’t jump around the pages of the site, I’ll post it again schematic diagram function generator, which we are assembling:

I’m also posting the datasheet ( technical description) ICL8038 and KR140UD806 microcircuits:

(151.5 KiB, 6,245 hits)

(130.7 KiB, 3,611 hits)

I have already collected the necessary parts to assemble the generator (I had some - constant resistances and polar capacitors, the rest were purchased at a radio parts store):

The most expensive parts were the ICL8038 microcircuit - 145 rubles and switches for 5 and 3 positions - 150 rubles. In total, you will have to spend about 500 rubles on this scheme. As you can see in the photo, the five-position switch is two-section (there was no one-section), but that’s not scary, more is better than less, especially since we may need the second section. By the way, these switches are absolutely identical, and the number of positions is determined by a special stopper, which you can set to the required number of positions yourself. In the photo I have two output connectors, although in theory there should be three: common, 1:1 and 1:10. But you can install a small switch (one output, two inputs) and switch the desired output to one connector. In addition, I want to draw attention to the constant resistor R6. There is no rating of 7.72 MOhm in the line of megaohm resistances; the closest rating is 7.5 MOhm. In order to get the desired value, you will have to use a second 220 kOhm resistor, connecting them in series.

I would also like to draw your attention to the fact that we will not finish assembling and adjusting this circuit to assemble a function generator. To work comfortably with the generator, we must know what frequency is being generated at the moment of operation, or we may need to set a certain frequency. In order not to use additional devices for these purposes, we will equip our generator with a simple frequency meter.

In the second part of the lesson, we will study another method of manufacturing printed circuit boards - the LUT (laser-iron) method. We will create the board itself in a popular amateur radio program for creating printed circuit boards – SPRINT LAYOUT.

I won’t explain to you how to work with this program yet. In the next lesson, in a video file, I will show you how to create our printed circuit board in this program, as well as the entire process of making a board using the LUT method.

Frame scan. The sawtooth voltage master generator (Fig. 11.4) is assembled using transistors VT1 And VT2. When the supply voltage is turned on, the capacitors C1 And C2 are charging. Currents flow through the base circuits of the transistors, which drive the transistors into saturation mode. After some time, the charging current of the capacitors will decrease and reach a value at which one of the transistors will come out of saturation. Change in voltage in the transistor collector circuit VT1 will close the transistor VT2. As a result, capacitor C1, included in the OOS circuit, will slowly discharge through the collector circuit of transistor VT1. Since the negatively charged capacitor plate C1 connected to the base of the transistor VT1, when the capacitor is discharged, the base current decreases and as a result, a ratio between the collector and base currents is automatically established that is exactly equal to the transistor current transfer coefficient. During the entire discharge time of the capacitor, the base current and voltage at the base change slightly. Current through resistors R1 And R2 remains constant and does not depend on the processes occurring in the device. Thus, during forward running, the generator has a deep OOS, which maintains a constant capacitor discharge current C1, and, consequently, high linearity of the sawtooth voltage. Since the current transfer coefficient of the transistor changes depending on the applied voltage (at the initial moment by 1 - 2%), the nonlinearity of the signal will be characterized by the same value. The capacitor discharge process stops at such voltages on the collector that require a significant increase in the base current to control the collector current. The transistor current transfer coefficient drops sharply. In this case, based on a transistor VT2 The closing signal is significantly reduced. Transistor VT2 opens. A positive voltage appears in its collector, opening the transistor. An avalanche-like process occurs. Both transistors are open. The work cycle repeats.

Rice. 11.4

The values ​​of the elements shown in the diagram form an output signal with an amplitude of more than 10 V and a frequency of 50 Hz. Resistors are used to regulate the amplitude of the output signal and its linearity. R7 And R8 respectively. Resistor R1 changes the frequency of the master oscillator.

Bipolar sawtooth signal generator. The ramp generator with adjustable slope (Fig. 11.5) consists of two integrating chains R5,C1 And R2,C2 and a threshold element built on transistors VT1 And VT2. Transistor based power up VT2 a 10 V signal appears. As the capacitor charges C1 tension decreases. At this time, the voltage at the base of the transistor VT1 increases. There are signals with different edges at different ends of the potentiometer. When the voltage at the transistor bases VT1 And VT2 becomes equal, they open and the capacitors discharge. After this, a new cycle of generator operation will begin. The slope of the output ramp signal can be adjusted over a wide range using a potentiometer.

Rice. 11.5

Rice. 11.6

Controllable generator. The sawtooth signal generator (Fig. 11.6, a) is built according to an integrator circuit with a large time constant, which is determined by the expression t = h 21 E C 1 R 4 where h 21e is the transistor current transfer coefficient VT1. Transistor VT1 opens slowly: capacitor C1 included in the OOS circuit. The voltage in the collector circuit decreases. At some point the diode opens VD2 and shunts the transistor input VT2. Transistor VT2 closes. To speed up the closing process, a dynamic load is included in its collector - a transistor VT3. Through the emitter of the transistor VT3 capacitor C1 charges quickly. As a result, the reverse motion of the sawtooth signal is minimized. Its duration is less than 5 x. The duration of the ramp signal can be adjusted using the base current of the transistor VT1(Fig. 11.6,6).

Ramp signal generator on the integrator. The generator (Fig. 11.7) is based on a transistor integrator. The K122UD1 integrated circuit is used as a threshold and amplification element. The response threshold of the microcircuit, equal to 3 V, is set by a divider Rl, R2. When the power is turned on, the voltage in the collector of the transistor cannot change abruptly. Negative feedback through a capacitor it generates a linearly increasing signal at the output. The time constant is equal to t=h 21E R 3 C 2, where h 21E is the current transfer coefficient of the transistor. When the collector voltage reaches 3V, the IC will switch. The positive voltage at pin 5 will pass through the diode and turn on the transistor. The capacitor will discharge C2. The collector will return to zero potential.

Rice. 11.7

The circuit will begin a new cycle of operation. A circuit with the indicated element values ​​generates an output signal with an amplitude of 3 V, a repetition rate of 100 Hz and a trailing edge duration of 0.1 ms.

Triggered bipolar signal generator. To obtain a high-voltage sawtooth signal in the generator (Fig. 11.8), two cascades are used, at the outputs of which falling and rising signals are formed. Each stage consists of two transistors. Transistors VT2 And VT4 are discarding,a VT1 And VT3- active elements in whose collectors output signals are generated. After turning on the power, the voltage at the transistor collector VT3 cannot change abruptly. This is prevented by OOS through a capacitor C2. The voltage at the collector will slowly increase. The rate of increase in voltage is determined by the time constant t=L 2 1E Cz(Ru-(-+Rt), where hziE- transistor current transfer coefficient. Resistor R7 is limiting. In another cascade, at the first moment a voltage of 100 V appears. Then the voltage decreases and tends to zero. Transistor Collector Voltage Reset VT1 occurs at the moment when the input pulse arrives. At this time the transistor opens VT4. Pulse signal from capacitor C4 goes to the base of the transistor VT2 and opens it. Simultaneous capacitor reset occurs C1 And C2.

Rice. 11.8

Ramp generator with adjustable linearity. The generator (Fig. 11.9) is based on the principle of charging a capacitor C2 stabilized current. The current stabilizer is built on a transistor VT2. Signal from capacitor C2 goes to the input of the emitter follower. When a sawtooth signal is formed, the voltage across the capacitor increases. Simultaneously with the increase in voltage across the capacitor, the base current of the transistor increases VT3. As a result, the capacitor is charged not with a constant current, as required by a linear increase in voltage, but with a current that decreases over time. The charge on the capacitor is affected by the input impedance of the emitter follower. To obtain a sawtooth voltage, it is necessary to compensate the base current of the transistor. This can be achieved by an OS circuit connecting the emitters of the transistors VT2 And VT3. As the output signal of the emitter follower increases, the emitter current of the transistor increases VT2. Changing the resistance of the resistor R9 in the os circuit, we can achieve an increasing or decreasing output waveform.

Rice. 11.9

To discharge the capacitor, the circuit uses a blocking generator. While the capacitor is charging, the diode is closed by the supply voltage. When the transistor VT1 open, capacitor C2 discharges through a diode VD1. The amplitude of the output signal is controlled by a resistor R5, and the frequency is a resistor R1. The maximum amplitude is 15 V.

Frame scan. The sawtooth voltage master generator (Fig. 11.4) is assembled using transistors VT1 And VT2. When the supply voltage is turned on, the capacitors C1 And C2 are charging. Currents flow through the base circuits of the transistors, which drive the transistors into saturation mode. After some time, the charging current of the capacitors will decrease and reach a value at which one of the transistors will come out of saturation. Change in voltage in the transistor collector circuit VT1 will close the transistor VT2. As a result, capacitor C1, included in the OOS circuit, will slowly discharge through the collector circuit of transistor VT1. Since the negatively charged capacitor plate C1 connected to the base of the transistor VT1, when the capacitor is discharged, the base current decreases and, as a result, a ratio between the collector and base currents is automatically established that is exactly equal to the transistor current transfer coefficient. During the entire discharge time of the capacitor, the base current and voltage at the base change slightly. Current through resistors R1 And R2 remains constant and does not depend on the processes occurring in the device. Thus, during forward running, the generator has a deep OOS, which maintains a constant capacitor discharge current C1, and, consequently, high linearity of the sawtooth voltage. Since the current transfer coefficient of the transistor changes depending on the applied voltage (at the initial moment by 1 - 2%), the nonlinearity of the signal will be characterized by the same value. The capacitor discharge process stops at such voltages on the collector that require a significant increase in the base current to control the collector current. The transistor current transfer coefficient drops sharply. In this case, based on a transistor VT2 The closing signal is significantly reduced. Transistor VT2 opens. A positive voltage appears in its collector, opening the transistor. An avalanche-like process occurs. Both transistors are open. The work cycle repeats.

Rice. 11.4

The values ​​of the elements shown in the diagram form an output signal with an amplitude of more than 10 V and a frequency of 50 Hz. Resistors are used to regulate the amplitude of the output signal and its linearity. R7 And R8 respectively. Resistor R1 changes the frequency of the master oscillator.

Bipolar sawtooth signal generator. The ramp generator with adjustable slope (Fig. 11.5) consists of two integrating chains R5, C1 And R2, C2 and a threshold element built on transistors VT1 And VT2. Transistor based power up VT2 a 10 V signal appears. As the capacitor charges C1 tension decreases. At this time, the voltage at the base of the transistor VT1 increases. There are signals with different edges at different ends of the potentiometer. When the voltage at the transistor bases VT1 And VT2 becomes equal, they open and the capacitors discharge. After this, a new cycle of generator operation will begin. The slope of the output ramp signal can be adjusted over a wide range using a potentiometer.

Rice. 11.5

Rice. 11.6

Controllable generator. The sawtooth signal generator (Fig. 11.6, a) is built according to an integrator circuit with a large time constant, which is determined by the expression t = h 21 E C 1 R 4 where h 21e is the transistor current transfer coefficient VT1. Transistor VT1 opens slowly: capacitor C1 included in the OOS circuit. The voltage in the collector circuit decreases. At some point the diode opens VD2 and shunts the transistor input VT2. Transistor VT2 closes. To speed up the closing process, a dynamic load is included in its collector - a transistor VT3. Through the emitter of the transistor VT3 capacitor C1 charges quickly. As a result, the reverse motion of the sawtooth signal is minimized. Its duration is less than 5 x. The duration of the ramp signal can be adjusted using the base current of the transistor VT1(Fig. 11.6,6).

Ramp signal generator on the integrator. The generator (Fig. 11.7) is based on a transistor integrator. The K122UD1 integrated circuit is used as a threshold and amplification element. The response threshold of the microcircuit, equal to 3 V, is set by a divider Rl, R2. When the power is turned on, the voltage in the collector of the transistor cannot change abruptly. Negative feedback through a capacitor generates a linearly increasing signal at the output. The time constant is equal to t=h 21E R 3 C 2, where h 21E is the current transfer coefficient of the transistor. When the collector voltage reaches 3V, the IC will switch. The positive voltage at pin 5 will pass through the diode and turn on the transistor. The capacitor will discharge C2. The collector will return to zero potential.

Rice. 11.7

The circuit will begin a new cycle of operation. A circuit with the indicated element values ​​generates an output signal with an amplitude of 3 V, a repetition rate of 100 Hz and a trailing edge duration of 0.1 ms.

Triggered bipolar signal generator. To obtain a high-voltage sawtooth signal in the generator (Fig. 11.8), two cascades are used, at the outputs of which falling and rising signals are formed. Each stage consists of two transistors. Transistors VT2 And VT4 are discarding, a VT1 And VT3- active elements in whose collectors output signals are generated. After turning on the power, the voltage at the transistor collector VT3 cannot change abruptly. This is prevented by OOS through a capacitor C2. The voltage at the collector will slowly increase. The rate of increase in voltage is determined by the time constant t=L 2 1E Cz(Ru-(-+Rt), where hzi E- transistor current transfer coefficient. Resistor R7 is limiting. In another cascade, at the first moment a voltage of 100 V appears. Then the voltage decreases and tends to zero. Transistor Collector Voltage Reset VT1 occurs at the moment when the input pulse arrives. At this time the transistor opens VT4. Pulse signal from capacitor C4 goes to the base of the transistor VT2 and opens it. Simultaneous capacitor reset occurs C1 And C2.

Rice. 11.8

Ramp generator with adjustable linearity. The generator (Fig. 11.9) is based on the principle of charging a capacitor C2 stabilized current. The current stabilizer is built on a transistor VT2. Signal from capacitor C2 goes to the input of the emitter follower. When a sawtooth signal is formed, the voltage across the capacitor increases. Simultaneously with the increase in voltage across the capacitor, the base current of the transistor increases VT3. As a result, the capacitor is not charged DC, as required by a linear increase in voltage, but by a current that decreases over time. The charge on the capacitor is affected by the input impedance of the emitter follower. To obtain a sawtooth voltage, it is necessary to compensate the base current of the transistor. This can be achieved by an OS circuit connecting the emitters of the transistors VT2 And VT3. As the output signal of the emitter follower increases, the emitter current of the transistor increases VT2. Changing the resistance of the resistor R9 in the os circuit, we can achieve an increasing or decreasing output waveform.

Rice. 11.9

To discharge the capacitor, the circuit uses a blocking generator. While the capacitor is charging, the diode is closed by the supply voltage. When the transistor VT1 open, capacitor C2 discharges through a diode VD1. The amplitude of the output signal is controlled by a resistor R5, and the frequency is a resistor R1. The maximum amplitude is 15 V.

CONTROLLED GENERATORS

Generator based on a field-effect transistor. The generator (Fig. 11.10) is based on a capacitor charge - direct current, which is set field effect transistor VT4. The rate of charge of the capacitor is determined by the resistor R10. The increasing voltage is applied to the base of the emitter follower transistor, the output of which is connected to the trigger - transistors VT1 And VT2. The trigger output signal goes to the base of the transistor VT3 to relieve voltage on the capacitor.

In the initial state, transistors VT2 And VT3 closed. As soon as the voltage across the capacitor reaches 6 V, the trigger is triggered and the transistor opens VT3. The capacitor is discharged through the open transistor. When the voltage across the capacitor decreases to 1 V, the trigger returns to its original state. A new capacitor charging cycle begins.

The element values ​​shown in the diagram allow you to adjust the output signal frequency from 15 to 30 kHz. If you install a capacitor with a capacity of 0.033 μF, then the frequency of the output signal is 1 kHz.

Rice. 11.10 Fig. 11.11

Triangular signal generator based on op-amp. In the diagram of Fig. 11.11 on the capacitor WITH a triangular signal with an amplitude of 0.6 V is generated. The capacitor is charged and discharged by the output signal of the op-amp, which automatically changes at the moment when the voltage on the capacitor reaches the opening threshold. The opening threshold is set by a divider R2 And R3. The repetition rate of the output signal is determined by the expression f=l/4R 1 C. A resistor is used to equalize the slopes of the leading edge and falling edge of the output signal. R6.

Triangle signal shaper. Figure shaper 11.12 allows you to get a triangular-shaped signal at the output. The signal amplitude reaches 90% of the supply voltage with a fairly high linearity of the fronts.

The shaper is based on the principle of charging and discharging a capacitor through current generators built on transistors. The collector currents of the transistors are determined by the reference voltages of the zener diodes and emitter resistors. In the absence input signal equal currents must flow through the transistors. If current equality is not satisfied due to variations in the values ​​of zener diodes and resistors, then the resistor should be adjusted R4. The appearance of an input signal with an amplitude greater than the breakdown voltage of the zener diodes will cause an imbalance in the collector currents. The positive half-wave of the input signal will reduce the transistor current VT2. Transistor current VT1 will remain unchanged. The difference collector current will charge the capacitor. With the arrival of the negative half-wave, the collector current of the transistor will decrease VT1. Transistor current VT2 will be set to nominal. The capacitor will be discharged by the current of the transistor VT2. If the amplitude of the input signal is less than the supply voltage, then there is a direct relationship between the amplitudes of the input and output signals, and if it is greater than the supply voltage, then the amplitude of the output signal is constant.

The capacitance of the capacitor is calculated by the formula C = 10 3 I/2fU m ax (μF), where I is the transistor current; f is the frequency of the input signal; U max - amplitude of the output signal.

Rice. 11.12 Fig. 11.13 Fig. 11.14

Rice. 11.15

Wide-range triangle waveform generator. The triangular signal generator (Fig. 11.13) allows you to obtain a frequency from 0.01 Hz to 0.1 MHz. The 20 V output signal is generated on the capacitor C4 collector currents of transistors VT4, VT6. When the capacitor is charged, the transistors VT4 And VT5 open and transistors VT3 And VT6 closed. When the voltage across the capacitor increases to the level determined by the divider R1 - R3 transistor VT1 will open. Transistors will open after it VT3 And VT6, which close the transistors VT4 And VT5 The process of discharging the capacitor through the transistor will begin VT6 When the lower level is reached, the transistor will open VT2. This process returns the circuit to its original state. The capacitor charges again. The output frequency can be varied linearly using a resistor R5 with an overlap of 20 times. For a capacitor with a capacity of 1 nF and with R5 = 510 kOhm, the frequency is 001 Hz

Step signal generator. In the initial state (Fig. 11-14), the capacitor is charged to the supply voltage. All transistors are closed. An input pulse of positive polarity opens the transistor VT1. A current flows through this transistor which discharges the capacitor. The voltage across the capacitor decreases. The second input pulse will also discharge the capacitor to a discrete voltage value. As a result of this, each pulse will reduce the voltage on the capacitor in steps. As soon as the voltage on the capacitor is equal to the voltage on the divider R4, R5, transistor opens VT2 and the relaxation process begins in the compound cascade. Transistors VT2 And VT3 open. The process of charging the capacitor occurs. After this, a new cycle of discharging the capacitor begins.

Keystone signal generator with adjustable rise time. The generator (Fig. 11.15) is based on a multivibrator that controls the operation of current-setting transistors VT3 And VT4. When the transistor VT2 open, via transistor VT3 capacitor charging current flows NW. The rate of rise of the voltage across the capacitor (or the edge of the output signal) depends on the charging current, which is regulated by a resistor R12 The maximum voltage across the capacitor is limited by the zener diode VD2. When the multivibrator transistors are switched to another state, the process of discharging the capacitor begins. Transistor VT3 closes and the transistor VT4 opens. Transistor discharge current VT4 adjustable with resistor R15. The value of this current determines the decay of the output signal. The frequency and duty cycle of the output signal are regulated by resistors R2 And R4. The generator can operate in a wide frequency range, up to 1 MHz. With large changes in the frequency of the output signal, it is necessary to change the capacitor values C1 And C2.

OP-AMP GENERATORS

Controllable sawtooth signal generator. The generator (Fig. 11.16) consists of a threshold device and an integrator. Output voltage negative polarity of a threshold device built on an op-amp DA1, is supplied to the integrator input. Capacitor C, included in the OOS circuit, is gradually charged. At the output of the op-amp DA2 a linearly increasing signal is generated. When at the non-inverting input of the op-amp DA1 there will be zero potential, it will switch. The positive output signal passes through the diode and discharges the capacitor. When the capacitor is completely discharged, the op-amp DA1 will return to its original state and a new cycle of generating the output signal will begin. The repetition rate of the output signal is determined by the expression f = 3/C(R 3 + R 4).

Generator based on OU K153UD1. The triangular pulse generator (Fig. 11.17, a) is built on two op-amps. The first op-amp performs the functions of an integrator, and the second is a threshold element. Op-amp output voltage DA1 increases (decreases) linearly. When it becomes equal in absolute value to the output voltage of the op-amp DA2, the second op-amp will switch and on the divider R5, R6 The polarity of the voltage will change. In this case, the op-amp output signal DA1 will decrease (increase) linearly. At a later moment, the op-amp output signal will be compared DA1 with op-amp closing threshold DA2. Secondary switching of the op-amp will occur DA2. Dependence of the period of a triangular signal on the transmission coefficient of the op-amp DA2 shown in Fig. 11.17.6.

Generator based on a unijunction transistor with an amplifier. Ramp signal generator (Fig. 11.18, A) built on an op-amp, which performs the functions of an integrator. The slew rate of the output signal depends on the input voltage. When the voltage at the output of the op-amp reaches 8 V, the unijunction transistor opens. Positive pulse on resistor R2 passes through the diode and the integrating capacitor is discharged. The dependence of the output signal frequency on the input voltage is shown in Fig. 11.18, b.

Rice. 11.16 Fig. 11.17

Generator with double POS. Generator (Fig. 11.19, A) consists of an integrator made on an op-amp DA2. When op-amp DA2 switches, the PIC voltage is applied to its non-inverting input, which determines the circuit’s response threshold. From potentiometer R4 to the non-inverting input of the op-amp DA1 the second POS is in effect. If the value of this connection is less than the op-amp opening threshold DA2, then the leading edge of the pulse signal at the op-amp output DA1 will pass through the capacitor C1 to its inverting input. From this moment the process of charging capacitor C1 begins. Op-amp output voltage DA1 slowly increases. When it reaches the op-amp opening threshold DA2, the op amp switches DA2. The process of discharging the capacitor begins C1. The pulse repetition rate of the output signal is determined by the expression f=K 2 /4RC(K 1 -K 2);

Rice. 11.18

Rice. 11.19

Rice. 11.20

K 1 = R 2 /(R 2 +R 3); K 2 = R" 4 /(R" 4 + R" 4). Depending on the level of the PIC signal in the op-amp DA1 You can adjust the output signal step. The maximum value, DE is determined by the voltage at the divider R2, R3. In Fig. 11.19.6 shows voltage diagrams in circuit races.

Triggered signal generator. Output voltage (Fig. 11.20, a) generated on the capacitor NW, equals U 3 = = (t/C 3)I 2. The capacitor is charged by a linearly increasing current I 2 = U 2 /R 5 of the transistor VT2. Transistor collector current control VT2 carried out by voltage across the capacitor C2 (U 2 = (t/C 2)I 3). This voltage depends on the transistor current VT3 (l 3 =U B /R 4). As a result, U 3 = U b t 2 / C 2 C 3 R 4 R 5 . For the element ratings indicated on the diagram, the output signal frequency is 5 kHz. Resetting capacitors C2 And NW carried out by an external signal through transistors VT4 And VT1. In Fig. 11.20.6 shows voltage diagrams at different points of the circuit.

sec signal conditioner x . Function Formation secx carried out from the input harmonic signal. The circuit (Fig. 11.21, a) can operate from a few hertz to hundreds of kilohertz. In the first transistor, the input signal is limited with an amplitude of 2.5 V. The second transistor increases the slope of the edges of the rectangular signal and changes its phase. Signal at the collector of the transistor VT2 summed with the input signal across a resistor R6. The output signal is selected at a specific point on the potentiometer so that a specific value for the trough depth of the sec function can be set. It should be noted that this generation scheme may produce an error of up to 10% at some points. As the amplitudes of the meander and harmonic signals increase, the error decreases. To increase the accuracy of the formation of the sec a function; You can install a diode limiting circuit at the input (Fig. 11.21.6). The role of this circuit is to smooth out the peaks of the harmonic signal. With the help of an additional circuit, the simulation accuracy can be increased to 5%.

Rice. 11.21

COMPLEX SIGNAL GENERATORS

Diode generator of complex signals. Signals of complex shapes are formed (Fig. 11.22) as a result of changing the gain of the differential amplifier. At small input signals all diodes are closed. Gain determined by resistors R2, R3 And R11, R12, close to unity. As the input signal level increases, the diodes in the emitter circuits of the transistors begin to conduct. This leads to an increase in gain. The output signal becomes steeper. Three levels of gain variation are used for both positive and negative input signal polarities. Each circuit, consisting of diodes and a potentiometer, determines a different opening threshold. The exact shape of the output signal is adjusted by the appropriate potentiometer.

Discrete signal generator of special forms. The generator (Fig. 11.23) is based on a multiphase multivibrator, which is triggered by a pulse of positive polarity. The transistors in the circuit will open one by one VT3. Only one transistor is in the open state. The transistor will go into a conducting state VT2, which is in the emitter of the transistor VT1 will direct the current determined by the resistor R5. If the resistor resistances change according to a certain law, then the amplitude of the output signal changes according to the same law. Using resistors R5 you can obtain any law of change in the output signal. The channel switching frequency is determined by the time constant R6C2.

Rice. 11.22 Fig. 11.23

Rice. 11.24

Function generator. A pulse signal of positive polarity is supplied to the generator input (Fig. 11.24). Logic circuit 2I - NOT of the K133LAZ integrated circuit is closed. At output 1 a signal of negative polarity appears with a duration equal to the duration of the input signal. This signal on the RC circuit is differentiated, and the positive pulse closes the second logic circuit. A pulse of negative polarity with a duration of 5 μs appears at the output of this circuit. All subsequent chains work in the same way. At outputs 1 - 7, pulse signals appear sequentially one after another. All these signals are summed through certain weighting resistors at the input of the op-amp. Depending on the sequence of accepted resistances of the weighting resistors at the output of the op-amp, a signal of any complexity can be generated. The amplitude of the output signal is determined by the resistance of the resistor R4. To balance the op-amp, the resistance of the resistor R3 is selected according to the total resistance of the weighing resistors.

Luca Bruno, Italy

Pulse width modulators often use analog ramp voltage generators. The low-cost oscillator circuit shown in Figure 1 can be used in low-power applications at frequencies up to 10 MHz. The circuit is characterized by good stroke linearity and frequency stability.

The circuit is made on a single inverter with an input Schmitt trigger, operating as a modified multivibrator. The output voltage is taken from the timing capacitor C T, the voltage on which varies from the lower to the upper thresholds of the inverter. R T C T is charged with a constant voltage, so the voltage on the capacitor increases according to an exponential law and can be approximated by a straight line only in the initial section of the exponential.

The simplest way to improve the linearity of the ramp voltage is to increase the supply voltage of the R T C T chain. To do this, a capacitor C 1 with a capacitance at least an order of magnitude greater than C T is added to the circuit, acting as a charge pump generator. During the falling edge of the sawtooth, when the inverter output is low, this capacitor is quickly charged through diode D 1 to a voltage V CC minus the forward voltage drop across the diode. At the same time, the capacitor C T is discharged through the diode D 2.

When the falling edge of the voltage on C T reaches the lower threshold V T - the Schmitt trigger, the inverter output will go to a high logic level. Capacitor C 1 will begin to charge, and the sum of the voltages at C 1 and at the inverter output will be established at the cathode of diode D 1. D 1 will close, and the R T C T circuit will begin to charge, trying to equal the voltage on capacitor C 1. At the moment when the voltage on C T rises to the upper threshold of V T + Schmitt trigger, the inverter output will return to “log”. 0" and the cycle will begin to repeat.

The linearity of the “saw” is proportional to the sum of the supply voltages V CC and V DD. Since V DD is +5 V and fixed, the only way to improve linearity is through V CC. The degree of nonlinearity of the working area of ​​the sawtooth voltage can be assessed using the following expression:

E NL % - nonlinearity error in percent,
M I - angle of inclination of the working area of ​​the “saw” in the initial section,
M F - angle of inclination of the working area at the final section,

V F - forward voltage drop across diode D 1.

The time constant R T C T determines the frequency of the ramp voltage F O . This frequency can be estimated, neglecting the discharge time C T and any discharge C 1, using the expression:

K is a constant determined from the following expression:

Simulation of the circuit with values ​​of C T =100 pF and R T =2.2 kOhm shows that the nonlinearity of the sawtooth voltage is equal to

• 28% at V CC = V DD = 5 V,
• 18% at V CC = 10 V and V DD = 5 V,
• 14% at V CC = 15 V and V DD = 5 V.

A circuit layout was assembled in which V DD =V CC =5 V, C T =100 pF and R T =2.2 kOhm. The inverter was a 74HC14 chip in a standard DIP package, which has a propagation delay of 15 ns (versus 4.4 ns for the SN74LVC1G14 at a supply voltage of 5 V). The measured frequency was approximately 12.7 MHz.

Any CMOS inverter with a Schmitt trigger at the input can be used as IC 1. However, to increase frequency stability, you should select microcircuits from the fastest families, with a short propagation delay time and a large output current. The produced one is quite suitable