Circuit Diagram 24-Analysis of Signal Generator Circuit Principle

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Circuit Diagram 24-Analysis of Signal Generator Circuit Principle [Copy link]

The low-frequency signal generator (function signal generator, commonly known as signal source) can output sine waves, square waves, sawtooth waves, etc. This signal source is widely used in scientific research, production, and teaching experiments.

1. Sine wave signal generator

The circuit composition of the sine wave signal generator is shown in the figure below. The sinusoidal oscillation circuit consists of the following and parts.

Amplification part - the amplitude stabilization link composed of uA741 operational amplifier, RP, and R1 constitutes the basic amplification circuit. The frequency selection network is composed of R3 and C1 in series and R2 and C2 in parallel.

Positive feedback network - formed by R3 and C1 connected across the input and output terminals of the operational amplifier. R3C1 and R2C2 are respectively connected in series and then in parallel, and then combined with the amplifier to form a positive feedback with frequency selection characteristics. From the characteristics of the frequency selection network, we can know that the natural frequency of the network is 344699. At this frequency, the output voltage of the operational amplifier has the same phase as the voltage sent back to the in-phase input terminal, which satisfies the positive feedback and has the maximum positive feedback coefficient.

According to the self-oscillation condition of the sinusoidal oscillation circuit, the frequency must meet the requirements of self-oscillation.

However, due to the large open-loop gain of the operational amplifier,

The output voltage waveform produces serious distortion (approximately a square wave), so a negative feedback composed of R1 and RP is added to the circuit to eliminate the distortion. In this way, the basic amplifier circuit becomes an amplifier with a negative feedback network and a common-mode input.

Adjusting RP changes the voltage gain of the basic amplifier circuit. According to the sine oscillation circuit, the balanced condition is replicated.

2. Square wave generator

The square wave generator circuit is shown in the figure below. As soon as the power is turned on, since the bases of the two transistors are connected to the power supply through the base resistors, both transistors tend to conduct electricity. However, the currents in the two transistors cannot be completely equal. This is because the transistor characteristics are not strictly symmetrical, the parasitic capacitance is not exactly the same, and there is always a transistor with stronger conductivity. If it is VT1, the collector current of VT1 increases, and the collector voltage of VT1 will decrease. Since the voltage across capacitor C1 cannot suddenly change, the change in the collector voltage of VT1 is all added. To the base of VT2, the base potential of VT2 decreases, thereby reducing the base current. The decrease in the base current of VT2 leads to a decrease in the collector current of VT2 and an increase in the collector voltage. This voltage change is coupled by C2 and is added to the base of VT1, increasing the base current of VT1, so that the collector current of VT1 becomes larger and larger, forming a positive feedback process.

This process continues until VT2 is cut off and VT1 is saturated. Due to the alternating charge and discharge of coupling capacitors C1 and C2, VT1 and VT2 are alternately saturated and cut off. In order to stabilize the output pulse, the output of VT2 is drawn from the emitter.

In addition, VT3 is an inverter and VT4 is an emitter follower. The output rectangular pulse frequency is 10KHz and the amplitude is 1V. R2, R7, and R10 are the stabilizing resistors of VT1, VT2, and VT3 respectively, and R8 is the current limiting resistor of VT3. By adjusting the potentiometer RP1, the output pulse frequency can be changed, and by adjusting RP2, the amplitude of the output pulse can be changed.

3. Sawtooth Wave Generator

The following figure a) is a sawtooth wave generator composed of complementary circuits. In the figure, R3 is the current limiting resistor of the base of VT1.

When it is turned on, it can prevent excessive base current from flowing through the base of transistor VT1; R4 is the discharge resistor of the collector leakage current of transistor VT2 to ensure that VT1 is reliably turned off when VT2 is turned off. In general, the power supply U2 is greater than U1.

When the power supplies U1 and U2 are just turned on, since there is no charge in the capacitor C, the voltage across the two ends is zero. At this time, for transistor VT2, the emitter potential is zero, and the base potential is the potential of point A, so Ua is greater than Ue. Transistor VT2 is turned off, and there is no current in the base of VT1, so it is in the cut-off state. Capacitor C is charged by the power supply through resistor R5. As the charging time increases, the potential at point E continues to rise. When the potential at point E exceeds the potential at point A, VT2 enters the amplification region, and its collector current flows to the base of VT1 through resistor R3. VT1 also leaves the cutoff and enters the amplification region. The collector current of VT2 is amplified by VT1 and then fed back to the base of VT2. In this way, the positive feedback results in VT1 and VT2 being forced to be saturated. Capacitor C discharges through saturated transistors VT1 and VT2.

As the discharge time increases, the voltage of capacitor C continues to decrease. When the voltage at point E drops to a certain value, if the parameters are selected appropriately, the base current of transistor VT1 is insufficient to maintain the saturation of VT1, and the circuit will turn to cutoff. Through positive feedback, VT2 is also cutoff, and the circuit enters the second cycle. The output waveform is shown in Figure b above.