Experts explain oscillation circuits, oscillation conditions and commonly used oscillators

A circuit that can automatically convert DC power into an AC signal with a certain amplitude and frequency without an external signal is called an oscillation circuit or oscillator. This phenomenon is also called self-excited oscillation. In other words, a circuit that can generate an AC signal is called an oscillation circuit.

An oscillator must include three parts: an amplifier, a positive feedback circuit, and a frequency selection network. The amplifier can amplify the input signal applied to the input end of the oscillator so that the output signal maintains a constant value. The positive feedback circuit ensures that the feedback signal provided to the input end of the oscillator is in the same phase, only in this way can the oscillation be maintained. The frequency selection network only allows a specific frequency f 0 to pass through, so that the oscillator produces a single frequency output.

Whether the oscillator can oscillate and maintain a stable output is determined by the following two conditions: one is that the feedback voltage uf and the input voltage U i must be equal, which is the amplitude balance condition. The second is that uf and ui must have the same phase, which is the phase balance condition, that is, it must be positive feedback. Generally speaking, the amplitude balance condition is often easy to achieve, so when judging whether an oscillation circuit can oscillate, it mainly depends on whether its phase balance condition is met.

Oscillators can be divided into several types according to the oscillation frequency, such as ultra-low frequency (below 20 Hz), low frequency (20 Hz to 200 kHz), high frequency (200 kHz to 30 MHz) and ultra-high frequency (10 MHz to 350 MHz). Oscillators can be divided into two types according to the oscillation waveform, namely sinusoidal oscillation and non-sinusoidal oscillation.

Sine wave oscillators can be divided into three types according to the components used in the frequency selection network: LC oscillators, RC oscillators and quartz crystal oscillators. Quartz crystal oscillators have high frequency stability and are only used in situations with high requirements. In general household appliances, various LC oscillators and RC oscillators are widely used.

LC Oscillator

The frequency selection network of the LC oscillator is the LC resonant circuit. Their oscillation frequencies are relatively high, and there are three common circuits.

(1) Transformer feedback LC oscillation circuit

Figure 1 (a) is a transformer feedback LC oscillation circuit. Transistor VT is a common emitter amplifier. The primary of transformer T is an LC resonant circuit that plays a frequency selection role, and the secondary of transformer T provides a positive feedback signal to the amplifier input. When the power is turned on, a weak transient current appears in the LC loop, but only the current with the same frequency as the loop resonant frequency f0 can generate a higher voltage at both ends of the loop. This voltage is sent back to the base of transistor V through the coupling of the transformer primary and secondary L1 and L2. As can be seen from Figure 1 (b), as long as the connection method is correct, the feedback signal voltage is the same phase as the input signal voltage, that is, it is positive feedback. Therefore, the oscillation of the circuit is rapidly strengthened and finally stabilized.

The characteristics of the transformer feedback LC oscillation circuit are: wide frequency range, easy to start oscillation, but low frequency stability. Its oscillation frequency is: f0 = 1/2πLC. It is often used to generate sine wave signals from tens of kilohertz to tens of megahertz.

(2) Inductor three-point oscillation circuit

Figure 2 (a) is another commonly used inductor three-point oscillation circuit. In the figure, inductors L1, L2 and capacitor C form a resonant circuit that plays a frequency selection role. The feedback voltage is taken from L2 and added to the base of the transistor VT. From Figure 2 (b), it can be seen that the input voltage and feedback voltage of the transistor are in phase, satisfying the phase balance condition, so the circuit can oscillate. Since the three poles of the transistor are connected to the three points of the inductor respectively, it is called an inductor three-point oscillation circuit.

The characteristics of the inductor three-point oscillator circuit are: wide frequency range, easy to start oscillation, but the output contains more high-order modulation waves and the waveform is poor. Its oscillation frequency is: f0 = 1/2π LC, where L = L1 + L2 + 2M. It is often used to generate sine wave signals below tens of megahertz.


(3) Capacitor three-point oscillator circuit

Another commonly used oscillation circuit is the capacitor three-point oscillation circuit, as shown in Figure 3 (a). In the figure, the inductor L and the capacitors C1 and C2 form a resonant circuit that plays a frequency selection role. The feedback voltage is taken from the capacitor C2 and added to the base of the transistor VT. As can be seen from Figure 3 (b), the input voltage and feedback voltage of the transistor are in phase, satisfying the phase balance condition, so the circuit can oscillate. Since the three poles of the transistor in the circuit are connected to the three points of the capacitors C1 and C2 respectively, it is called a capacitor three-point oscillation circuit.

The characteristics of the capacitor three-point oscillation circuit are: high frequency stability, good output waveform, and frequency up to 100 MHz or more, but the frequency adjustment range is small, so it is suitable for fixed frequency oscillators. Its oscillation frequency is: f 0 =1/2π LC, where C= C 1 C 2 C 1 +C 2.

The amplifiers in the above three oscillation circuits all use common emitter circuits. The oscillator with common emitter connection has higher gain and is easy to start. The amplifier in the oscillation circuit can also be connected in the form of a common base circuit. The oscillation frequency of the oscillator with common base connection is relatively high and the frequency stability is good.

RC Oscillator

The frequency selection network of the RC oscillator is an RC circuit, and its oscillation frequency is relatively low. There are two commonly used circuits.

(1) RC phase shift oscillator circuit

Figure 4 (a) is an RC phase shift oscillator circuit. The three RC networks in the circuit play the role of frequency selection and positive feedback at the same time. From the AC equivalent circuit of Figure 4 (b), we can see that because it is a single-stage common emitter amplifier circuit, the output voltage Uo of the transistor VT is 180° out of phase with the output voltage Ui. When the output voltage passes through the RC network and becomes the feedback voltage Uf and is sent to the input end, since the RC network only produces a 180° phase shift for the voltage of a certain frequency f0, only the signal voltage with a frequency of f0 is positive feedback and causes the circuit to oscillate. It can be seen that the RC network is both a frequency selection network and a part of the positive feedback circuit.

The characteristics of the RC phase-shift oscillator circuit are: the circuit is simple and economical, but the stability is not high and the adjustment is not convenient. It is generally used as a fixed frequency oscillator and in occasions where the requirements are not too high. Its oscillation frequency is: when the parameters of the three RC networks are the same: f 0 = 1 2π 6RC. The frequency is generally tens of kilohertz.

(2) RC bridge oscillator circuit

Figure 5 (a) is a common RC bridge oscillation circuit. The series-parallel circuit of R1C1 and R2C2 on the left side of the figure is its frequency selection network. This frequency selection network is also part of the positive feedback circuit. This frequency selection network has no phase shift (phase shift is 0°) for a signal voltage with a specific frequency of f0, and voltages of other frequencies have phase shifts of varying sizes. Since the amplifier has two stages, the feedback voltage Uf taken from the output end of V2 is in phase with the input voltage of the amplifier (2-stage phase shift 360°=0°). Therefore, when the feedback voltage is sent back to the input end of VT1 through the frequency selection network, only a voltage with a specific frequency of f0 can meet the phase balance condition and oscillate. It can be seen that the RC series-parallel circuit plays the role of frequency selection and positive feedback at the same time.

In fact, in order to improve the working quality of the oscillator, a series voltage negative feedback circuit composed of Rt and RE1 is added to the circuit. Among them, Rt is a thermistor with a negative temperature coefficient, which can stabilize the oscillation amplitude and reduce nonlinear distortion of the circuit. From the equivalent circuit of Figure 5 (b), it can be seen that this oscillation circuit is a bridge circuit. R1C1, R2C2, Rt and RE1 are the four arms of the bridge respectively. The input and output of the amplifier are connected to the two diagonals of the bridge respectively, so it is called an RC bridge oscillation circuit.

The performance of the RC bridge oscillator circuit is better than that of the RC phase shift oscillator circuit. It has high stability, small nonlinear distortion, and convenient frequency adjustment. Its oscillation frequency is: when R1=R2=R, C1=C2=C, f0=12πRC. Its frequency range is from 1 Hz to 1 MHz.

AM and Detection Circuits

Broadcasting and radio communication use modulation technology to add low-frequency sound signals to high-frequency signals for transmission. The process of restoring the signal in the receiver is called demodulation. The low-frequency signal is called the modulating signal, and the high-frequency signal is called the carrier. Common continuous wave modulation methods include amplitude modulation and frequency modulation, and the corresponding demodulation methods are called detection and frequency discrimination.

Next, we will introduce the amplitude modulation and detection circuits.

(1) AM circuit

Amplitude modulation is to make the amplitude of the carrier signal change with the amplitude of the modulating signal, while the frequency of the carrier remains unchanged. The circuit that can complete the amplitude modulation function is called an amplitude modulation circuit or amplitude modulator.

Amplitude modulation is a nonlinear frequency conversion process, so its key is to use nonlinear devices such as diodes and transistors. According to which circuit the modulation process is carried out in, the transistor amplitude modulation circuit can be divided into three types: collector amplitude modulation, base amplitude modulation and emitter amplitude modulation. The following takes the collector amplitude modulation circuit as an example.

Figure 6 is a collector amplitude modulation circuit. The equal-amplitude carrier generated by the high-frequency carrier oscillator is added to the transistor base through T1. The low-frequency modulation signal is coupled to the collector through T3. C1, C2, and C3 are high-frequency bypass capacitors, and R1 and R2 are bias resistors. The LC parallel circuit of the collector resonates at the carrier frequency. If the static operating point of the transistor is selected at the curved part of the characteristic curve, the transistor is a nonlinear device. Because the collector current of the transistor changes with the modulation voltage, the two signals in the collector are amplitude modulated due to nonlinear effects. Since the LC resonant circuit is tuned to the base frequency of the carrier, the amplitude modulated wave output can be obtained at the secondary of T2.

(2) Detection circuit

The function of the detection circuit or detector is to extract the low-frequency signal from the amplitude modulated wave. Its working process is just the opposite of amplitude modulation. The detection process is also a frequency conversion process, and nonlinear components must be used. Commonly used ones are diodes and triodes. In addition, in order to extract the low-frequency useful signal, a filter must be used to filter out the high-frequency component, so the detection circuit usually contains two parts: nonlinear components and filters. The following takes the diode detector as an example to illustrate its work.

Figure 7 is a diode detection circuit. VD is the detection element, C and R are low-pass filters. When the input modulated wave signal is large, the diode VD works intermittently. In the positive half cycle, the diode is turned on and charges C; in the negative half cycle and when the input voltage is small, the diode is turned off and C discharges to R. The voltage obtained at both ends of R contains many frequency components. The high-frequency part is filtered out by capacitor C, and then the DC blocking capacitor C0 blocks the DC, and the restored low-frequency signal can be obtained at the output end.

Frequency modulation and frequency discrimination circuits

Frequency modulation is to change the carrier frequency with the amplitude of the modulating signal, while the amplitude remains unchanged. Frequency demodulation is to demodulate the original low-frequency signal from the FM wave, and its process is exactly the opposite of FM.

(1) Frequency modulation circuit

A circuit that can perform frequency modulation is called a frequency modulator or frequency modulation circuit. The commonly used frequency modulation method is direct frequency modulation, which is a method of directly changing the frequency of the carrier oscillator with a modulation signal. Figure 8 shows its general idea, in which a variable reactance element is connected in parallel to the resonant circuit. The low-frequency modulation signal is used to control the change of the variable reactance element parameters, so that the frequency of the carrier oscillator changes.

(2) Frequency discrimination circuit

The circuit that can perform the frequency discrimination function is called a frequency discriminator or frequency discrimination circuit, sometimes also called a frequency detector. The frequency discrimination method is usually divided into two steps. The first step is to convert the constant amplitude FM wave into a FM wave whose amplitude varies with the frequency. The second step is to use a general detector to detect the amplitude change and restore it to a low-frequency signal. Commonly used frequency discriminators include phase discriminators and proportional discriminators.