Ramp voltage generator. Ramp pulse generator circuit

An electronic generator is a device for generating continuous electrical oscillations of various shapes, frequencies and powers. Very often, generators are made on the basis of op-amps.

Multivibrator

Multivibrator called a voltage generator with a shape close to rectangular. Its name reflects the fact that such a voltage, when expanded into a Fourier series, appears as a series containing many higher harmonics (multi - a lot of).

According to the characteristics of the op-amp (see Fig. 2.13, b) it can be seen that the output voltage of the amplifier depends linearly on the input voltage only in a very narrow range - hundreds of microvolts. If the input voltage is outside this range, then the output signal can take only two values: +UВь1Х (≈ +12 V) and -UВь1Х (≈ -12 V). The principle of forming a rectangular multivibrator voltage is based on this feature of the operational amplifier (Fig. 2.20, A).

Rice. 2.20. Multivibrator(A) and graphs explaining its operation (b)

Let us assume that at the moment of switching on there is a small (a few millivolts) negative potential difference between the inputs of the amplifier. In this case, a voltage + UOUT will be generated at the output, and to the non-inverting input from the divider R 1, R 2 positive potential will be applied +U n. The capacitor will begin to charge along the “Uout–R3–C–case” circuit, trying to reach the potential + Uout. The potential at the inverting input will begin to rise until it exceeds the potential at the non-inverting input +U D. At this moment, the amplifier will generate a negative voltage at the output -U out and will create a negative potential at the non-inverting input -U D. The capacitor will now begin to recharge as it reaches its potential -U vyx. However, as soon as the potential at the inverting input becomes lower than the potential at the non-inverting input -U D, the amplifier will generate a positive voltage at the output +U vyx. Such an abrupt process of changing the output voltage from + U off until -U output and back will be repeated until the supply voltage is removed from the operational amplifier. Graphs demonstrating the described processes are shown in Fig. 2.20, b. The period of G-oscillations is determined by the capacitor charging time constant τ = R 3C, as well as the extent to which the potential generated by the divider R 1, R 2, less voltage Uout.

Ramp voltage generator

The voltage across the capacitor increases linearly if it is charged with a direct current that does not depend on the voltage across it, and the load resistance is prevented from influencing this current, i.e. condition must be met R n >>R. Integrating over time the expression

Condition I c = const in the circuit of a sawtooth voltage generator (RVG) based on an op-amp (Fig. 2.21, A) provided by constant voltage Uin. While the transistor is locked, for a period of time t n the capacitor is charged and the voltage across it increases in a straight line. The amplifier, trying to make the potential difference at its inputs close to zero, generates an output voltage that repeats the voltage on the capacitor. When a pulse Udischarge is applied, the transistor opens and the capacitor quickly discharges through it in a time t razr, after which the charging process is repeated. The output voltage of the circuit takes on a sawtooth shape, which is maintained as long as the voltage value is within the range from -Uout to +Uout.

Sawtooth is a voltage that increases in proportion to time and decreases abruptly. In Fig. 46, A shows an ideal sawtooth voltage having a rise time t nar and time of decline t sp, equal to zero. It is obvious that the period of such tension T equal to the rise time. Real sawtooth voltage generators have a not quite linearly increasing voltage and a non-zero decay time (Fig. 46, b).

Ramp voltage is used to scan an electron beam in electron beam devices.

Rice. 46. Curves of changes in ideal (a) and real (b) sawtooth voltage

Let's consider the operation of a controlled transistor sawtooth voltage generator with capacitive feedback (Fig. 47).

Rice. 47. Sawtooth voltage generator circuit

The generator is controlled by pulses of negative polarity through a diode VDI. In the initial state, the transistor VT1 locked by positive voltage supplied from the emf source. E bee through a resistor R 2,diode VDI and resistor R 1.Capacitor WITH charges via R K , R 1,VDI And R 2 approximately to voltage E ke.When a control pulse is applied, the diode VD1 locked. Transistor VTI opens, since voltage is now supplied to its base through a resistor R. The discharge of the capacitor begins through the open transistor. The base and collector potentials decrease abruptly at the moment the transistor is unlocked. Capacitive feedback between the collector and base keeps the capacitor discharge current almost constant.

At the end of the control pulse, the diode is unlocked and the transistor is closed by the voltage of the emf source. E bee, and the capacitor starts charging WITH.

To ensure complete discharge of the capacitor and obtain the maximum amplitude of the sawtooth voltage, the duration of the control pulses is selected based on the ratio

τ = (1,1 – 1,2)t size

Where t size- capacitor discharge time.

The frequency of the sawtooth voltage is determined by the parameters of the discharge circuit and is limited by the frequency properties of the transistor.

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.

The sawtooth signal shown above is shown. The recovery time is always less than the sweep time. A sawtooth signal is produced when the return time becomes zero. The sweep speed of sawtooth waves depends on the capacitor used in the circuit. The sweep speed is controlled by a resistor placed in the circuit.

The charging and discharging of the capacitor generates the signal shown in the figure below. The transistor provides a low resistance through which the capacitor becomes a discharge. Instantaneous voltage and supply voltage are measured in volts, time is measured in the latter, resistance is measured in ohms, and a capacitor is measured in farads.

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.

The term "sawtooth" refers to the waveform and can therefore have any rise or fall time as long as the waveform maintains the basic saw blade shape. Pilot generator. is a circuit that generates a saw blade signal either from an external input or from self-oscillations, as in a relaxation oscillator. A circuit designed to produce a sawtooth function will have a very slow linear ramp that rises from a steady state to a peak. When the peak voltage of the ramp is reached, the voltage will return to the initial level very quickly.

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.

Operation of a unipolar transistor circuit

The falling time is much shorter than the rising time, but is not instantaneous, although it looks the same compared to the rising time. Fall time is also referred to as flyback when the signal is used as a sweep generator. The circuit functions as an oscillator and switches off the charging and discharging of the capacitor. Of course, you can also make the frequency variable by adding a trimmer as the current setting. The top side of the trimmer remains connected to the supply voltage. While the other end of the trimmer remains unconnected as in the configuration.

[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.

This charging time is the increasing ramp of the sawtooth shaft, as well as the sweep time in specific applications. The ramp time depends on the resistor and capacitor values. Fall time is the time required for the capacitor to discharge through the transistor. The vacuum tube circuit on the right is another example of a circuit that outputs a sawtooth waveform. This circuit was used as a sweep generator in an oscilloscope or other display. The ramp or sweep portion of the output is used to move the electron beam from left to right across the display, while the retrace or flyback portion returns the beam to its starting point.

[Resistance R4, kOhm] Supply voltage, V] - [ Dinistor unlocking voltage, V]) / (1.1 * [Release 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% margin.

If these two conditions conflict with each other, then this means that the circuit supply voltage for this thyristor is selected too low.

This circuit is used as an example to show the vacuum tube used as a sawtooth generator and the second method of changing the sweep time. A switch is used to change the sweep time, just as a variable resistor is used in the circuit above it.

This is a measure of time based on the amount of voltage change. Another important consideration is the use of the linear part of the rise time of the capacitors. Only the first time the constant is a linear ramp or some linear one. As the capacitor is able to charge further, the charging time slows down more and more. Of course, the saw ramp is linear in its rise time. The same applies to the capacitor discharge time. The longer the discharge time, the smaller the linear discharge will be.

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.

Could you show me how to make a variable frequency sawtooth oscillator? A sawtooth wave is characterized by a positive linear voltage reversal accompanied by a sharp drop to zero. One way to generate a sawtooth surface is to slowly charge a capacitor through a DC source, and then quickly discharge the capacitor, shorting it.

By repeating this process, a sawtooth wave is created. But DC supplies can be tricky, especially if you want to customize it. Instead of a constant current source, a fixed resistor is often used to limit the cap's charging current. However, the voltage across the charging capacitor using a fixed resistor is not linear. But by choosing a section of the curve that is more or less linear, as shown by the red dotted lines, we can create a pseudopilos. The 555 timer is an astable oscillator that uses charging and discharging a capacitor.

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.

Not perfect, but good enough for most electronics. The waveform is then buffered and conditioned. The frequency bank changes the frequency, and the waveform control adjusts the wave so that the top and bottom of the waveform are not clipped.

A more linear ramp wave can be generated using a digital counter with weighted outputs. Look at the sawtooth generator in Figure 3. Does it look like number 3? These currents are summed at the non-inverting op-amp and output node as a voltage.

[Load resistance, kOhm] >> [Resistor R4 resistance, kOhm]

RAMP VOLTAGE GENERATOR- generator of linearly varying voltage (current), an electronic device that generates periodic voltage (current) fluctuations in a sawtooth shape. Basic The purpose of gpn is to control the time sweep of the beam in devices using cathode ray tubes. G.p.n. They are also used in devices for comparing voltages, time delays and pulse expansion. To obtain a sawtooth voltage, the process of charging (discharging) a capacitor in a circuit with a large time constant is used. The simplest G. p.n. (Fig. 1, a) consists of RC integrating circuit and a transistor that performs the functions of a periodically controlled switch. impulses. In the absence of pulses, the transistor is saturated (open) and has a low resistance of the collector - emitter, capacitor section WITH discharged (Fig. 1, b). When a switching pulse is applied, the transistor is turned off and the capacitor is charged from a power source with voltage - E k- direct (working) stroke. Output voltage G.p.n., removed from the capacitor WITH, changes by law. At the end of the switching pulse, the transistor is unlocked and the capacitor WITH quickly discharges (reverse) through low resistance emitter - collector. Basic characteristics of G.p.n.: amplitude of sawtooth voltage, coefficient. nonlinearity and coefficient using power supply voltage. When in this scheme

Duration of forward stroke T p and the frequency of the sawtooth voltage are determined by the duration and frequency of the switching pulses.

The disadvantage of the simplest G. p.n. is small k E at low The required e values are in the range of 0.0140.1, with the smallest values being for the comparison and delay devices. The nonlinearity of the sawtooth voltage during forward stroke occurs due to a decrease in the charging current due to a decrease in the voltage difference. Approximate constancy of the charging current is achieved by including a nonlinear current-stabilizing two-terminal network (containing a transistor or vacuum tube) in the charging circuit. In such G. p.n. And . In G. p.n. with positive By voltage feedback, the output sawtooth voltage is supplied to the charging circuit as a compensating emf. In this case, the charging current is almost constant, which provides values of 1 and = 0.0140.02. G.p.n. used for scanning in cathode ray tubes with electric magnets. beam deflection. To obtain a linear deflection, a linear change in the current in the deflection coils is necessary. For a simplified equivalent coil circuit (Fig. 2, a), the current linearity condition is satisfied when a trapezoidal voltage is applied to the coil terminals. This trapezoidal stress (Fig. 2, b) can be obtained from the State University of Science. when connected to the charging circuit it will supplement. resistance R d (shown in Fig. 1, A dotted line). The deflection coils consume large currents, so the trapezoidal voltage generator is supplemented with a power amplifier.

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.