RC Oscillator Circuits Basic and Typical Diagram

The RC oscillator circuit is common that used to generate stable oscillation signals. It consists of a resistor (R) and a capacitor (C) and achieves oscillation by establishing a feedback loop between them. Here we will introduce the basics of it and give specific examples of the Wien bridge oscillator.

RC Oscillator Circuit Basic

The combination of capacitor and resistor in the RC oscillation circuit forms a feedback loop with negative feedback, so that charge can flow periodically, thereby generating an oscillation signal. And it is generally used to generate low-frequency signals in the range of 1Hz~1MHz.

1. Working Principle

1) Initial state: Assume that the capacitor has no charge initially, and then an external excitation signal is applied.

2) Charge process: When an external excitation signal is applied to the circuit, the capacitor begins to charge. Current flows from the power source to the capacitor and the voltage gradually increases.

3) Discharge process: When the capacitor is fully charged, the resistor begins to discharge. Current flows from the capacitor to the resistor and the voltage gradually decreases.

4) Repeated cycles: After the capacitor is discharged, it is recharged and the process is repeated.

Due to the feedback loop, the capacitor continuously switches between charge and discharge processes, producing a periodic oscillating signal.

2. Oscillating Conditions

In order for the RC oscillation circuit to work normally, certain starting conditions need to be met. These conditions determine the stability of the oscillation frequency and the appropriate range of amplitude.

The main vibration starting conditions are as follows:

1) Positive feedback gain is greater than or equal to 1: In the RC oscillator circuit, the feedback loop must provide sufficient positive feedback so that the output signal can be maintained or amplified to a certain extent. If it is less than 1, the circuit cannot oscillate.

2) Phase shift is 0 or 360°: The feedback signal should be delivered to the input with zero phase or a complete 360° phase to maintain positive feedback gain.

3) The total phase gain is 1: The total phase gain of the oscillator circuit must be 1, that is, the phase delay and gain exactly cancel out in the entire frequency range.

The above three conditions jointly determine the starting frequency and amplitude of the RC oscillator circuit. In actual design, the values of capacitors and resistors can be adjusted to meet these starting conditions and obtain the required signal.

To sum up, the RC circuit generates a stable oscillation signal through the charging and discharging process of the capacitor and resistor in the feedback loop, and the starting conditions determine the stability of the oscillation frequency and amplitude. This simple and common circuit structure is widely used in many electronic devices and communication systems.

3. Applications

The RC oscillator circuit has a wide range of uses. For example, when used as a vibrator, it is used to generate waveform output, such as sine waves, triangle waves, etc. If the parameters of R and C are selected well, a pulse waveform with a very narrow bandwidth can be generated. In addition, it is also used as a filter LPF/HPF, differentiator, integrator, etc. with an integrated operational amplifier.

The sine wave generated by the commonly used LC oscillation circuit has a higher frequency. To generate a lower frequency sine wave, the oscillator circuit must have a larger inductance and capacitance. This will not only make the components large, bulky, and inconvenient to install, but also difficult to manufacture. Therefore, sinusoidal oscillating circuits below 200kHz generally use RC circuits with lower oscillation frequencies.

4. Classification

Common RC oscillator circuits include RC phase shift oscillator circuit and RC bridge oscillator circuit.

1) RC phase-shifted oscillator

It has the advantages of simple structure and low cost, but it also has the disadvantages of poor output waveform quality and difficulty in adjustment. And it is mainly used in situations with low requirements.

Figure (a) shows a typical leading circuit. VT is a common emitter amplifier, R3 and R4 are the bias resistors of VT, C1~3 and R1~R3 form a three-level phase-shifting network. The maximum phase shift of the first-level C phase shift circuit is 90°, so a third-level RC phase shift circuit is required to phase-shift the voltage output by the VT with 180° and add it to the base of the VT as a positive feedback voltage to achieve oscillation.

Figure (b) shows a hysteretic circuit. In order to reduce the influence of the input resistance of amplifying V72 on the frequency characteristics of the RC phase shift circuit, this type of oscillator adds a common collector amplifier composed of VI1 between the phase shift network and V12 as a buffer level.

2) RC bridge oscillator

It is also called the Wien bridge oscillator. With the advantages of easy starting, good output waveform, large output power, and convenient frequency adjustment, so it is widely used. The typical circuit is as follows.

Where, VT1 and VT2 form a common emitter coupling amplifier; R1, C1, R2 and C2 form a series and parallel frequency selection network; R6 and R11 are series negative feedback resistors; C7 is the output voltage feedback capacitor; R3, R4 and R7, R8 are the bias resistors of amplifier tubes VT1 and VT2 respectively.

5. Commonly Used Connection Methods

1) The simplest oscillator

The characteristics of this oscillator are: T≈(1.4~2.3)R*C.

Power supply fluctuations will make the frequency unstable, suitable for low-frequency oscillations less than 100KHz.

2) Add compensation resistor for oscillation

T≈(1.4～2.2)R*C, the influence of power supply on frequency is reduced, and the frequency stability can be controlled at 5%.

3) Ring oscillator

Using TTL inverter, the frequency can reach 50MHz.

4) Oscillator composed of Schmitt trigger

5) An oscillator composed of two transistors

Where, R5=R8, R7=R6, C5=C6

Wien Bridge Oscillator Circuit

1. Working Principle

The Wien bridge oscillator is widely used to generate variable frequency oscillators in the frequency range from several Hz to hundreds of kHz. It mainly consists of two parts:

① RC series-parallel frequency selection network with positive feedback => to meet the phase balance conditions

② Non-inverting amplifier with negative feedback => to meet the amplitude balance condition

Its working principle is: when the circuit is first powered on, it will contain disturbance components with rich frequencies. Different frequency components will be amplified by the amplifier, and then reduced by the feedback network (RC frequency selection network), and the cycle will be repeated. Only the component of a certain frequency can oscillate stably. That is to say, the component of frequency f0 will neither cause saturation distortion due to the continuous amplification of the amplifier, nor will it eventually disappear due to too strong attenuation.

The typical circuit model is shown in the figure below, in which R1, C1, R2, and C2 form an RC series-parallel frequency selection network. Usually R1=R2=R, C1=C2=C; R3 and Rf form the feedback network of the in-phase proportional amplifier, and it is voltage series negative feedback.

2. How to satisfy the phase balance condition?

The Wien bridge oscillator uses a non-inverting amplifier, that is, φA=0. In order to meet the phase balance condition φA +φF=2nπ (n=0, 1, 2, …), the phase angle of the feedback network is required to be φF=0, to keep oscillating stably. This frequency is the resonant frequency f0 of the RC frequency selection network. We need to find the specific expression of this frequency point. In order to simplify the analysis, the RC series-parallel frequency selection network in the Wien bridge is drawn separately in the figure below:

1) The impedance of the RC series circuit is:

2) The impedance of the RC parallel circuit is:

3) The feedback coefficient of the oscillation circuit is:

Let the imaginary part of the denominator of the above equation= 0, and the solution is ω0 = 1/RC. Where ω0 is the resonant angular frequency of the circuit, and the resonant frequency f0 = 1/2πRC can also be obtained. Then the above formula can be rewritten as:

4) Amplitude-frequency characteristics:

5) Phase-frequency characteristics:

Make the amplitude-frequency characteristic curve and phase-frequency characteristic curve of the RC series-parallel frequency selection network as shown in the figure below (the abscissa is ω/ω0). It can be seen that the phase angle range of the frequency selection network is (π/2,-π/2). When the circuit resonates, the phase angle φF is exactly 0, so the phase balance condition is met. At this time, the feedback coefficient F reaches the maximum value 1/3 (this value is an important basis for meeting the amplitude balance condition).

3. Amplitude balance and vibration starting conditions

When f= f0, F=1/3, according to the amplitude balance condition |A·F|=1, the solution is A=3, that is, the amplification factor of the non-phase amplifier is 3 when the oscillator is balanced. If the amplifier is a non-inverting proportional amplifier as shown in the above typical circuit model, then according to the relationship of voltage series negative feedback, we can get A=1+Rf/R3, and the solution is Rf=2R3. This shows that in order to balance the oscillator, Rf and R3 must satisfy the relationship between them. But there is another condition that needs to be met: |A·F|> 1, that is, Rf>2R3.

These two conditions seem contradictory, but they can actually be achieved by changing characteristics of the thermistor instead of ordinary resistors. Let Rf be the thermistor with a negative temperature coefficient. When the oscillation starts, the current in the branch is small and the temperature is low, Rf>2R3. After a while, the output amplitude gradually increases, the feedback signal at both ends of the negative feedback branch also increases, so the current in the circuit increases and the temperature rises. At this time, the resistance of Rf decreases, so the negative feedback strengthens, thereby increasing the resistance of the oscillation amplitude. On the contrary, when the oscillation amplitude weakens, the resistance of Rf increases, causing the negative feedback to reduce, limiting the reduction in amplitude. In this way, when the oscillator starts to vibrate, the oscillator can stabilize at the equilibrium condition |A·F|=1, at which time Rf=2R3.

Another way to achieve the transformation from start-up to equilibrium conditions is to make Rf>2R3, and connect a branch circuit consisting of a pair of reverse diodes and resistor R4 in series at both ends of Rf. The circuit connection is as shown in the figure below.

When the circuit is powered on and starts working, since Rf>2R3, the circuit can start to oscillate normally. As the output amplitude gradually increases, the forward resistance r of the diode will gradually decrease until it turns on. At this time, A=1+ R3/Rf//(R4+r), the amplification factor of the amplifier is reduced, thereby limiting the increase in oscillation amplitude. However, the oscillation amplitude will continue to increase. Under this situation, make the forward resistance r of the diode to reduce, which just meets the relationship: Rf//(R4+r)=2R3, then the circuit will reach a balanced state and be stable in the balanced state. (Because the circuit satisfies the amplitude stability and phase stability conditions, detailed explanation will not be given here). Since the forward resistance r is generally very small when the diode is turned on, we can make the value of Rf//R4 slightly less than 2R3, that is, Rf//R4=18.75Ω<2R3=20Ω.

The output waveform is shown in the figure below. It can be seen that the circuit has obtained a stable and undistorted waveform. The amplification factor of the circuit is A=3, and the oscillation period T≈2.955ms can be read. The actual oscillation frequency f=1\T= 338.409Hz , which is very close to the theoretical oscillation frequency f0=1\2πRC=338.637Hz.

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