Circuit Diagram Analysis and Detailed Explanation of Switched Capacitor Wien Oscillator

Switched Capacitor Wien Oscillator

The RC sinusoidal oscillator composed of operational amplifiers, resistors and capacitors has many forms, but in summary, there are mainly phase-shift oscillators, double-T frequency-selective network oscillators, and Wien oscillators. The Chinese oscillators The RC oscillator can generate a sinusoidal signal with a wide frequency range and small distortion. Therefore, a Wen's oscillator is usually used in an RC oscillator. Due to the large resistance and capacitance values of this kind of Wien Oscillator, it is very difficult to make a monolithic integrated circuit. If the resistance R in the Wien Oscillator is replaced with a switched capacitor (Figure 5.3-1). It becomes a MOS integrated switched capacitor oscillator. In addition to introducing the switched capacitor Wien Oscillator, this section will also introduce the switched capacitor sine oscillator. It is implemented using a double second-order switched capacitor filter (see Chapter 5, Section 3). Its advantages are amplitude, The frequency is stable and the distortion is small. Now they are introduced separately as follows.

The RC Wien Oscillator is shown in Figure 6.2-1a. It is easy to prove that its oscillation frequency and oscillation conditions are respectively

Where A1 is the closed-loop gain of the op amp.

If the resistor R in the figure is replaced by a parallel switched capacitor circuit and a series switched capacitor circuit in Figure 5.3-1, a switched capacitor Wien Oscillator is obtained. The circuit form is shown in Figure 6.2-10. In the figure, ф1 and ф2 represent two-phase clock signals (see Figure 5.3-1c) to control the on and off of the switch. The resistance R in figure (a) and the capacitance CR in figure (b) have the following relationship:

Substituting equation (6.2-3) into equation (6.2-1), the oscillation frequency of the switched capacitor Wien Oscillator is obtained as

The above formula shows that the oscillation frequency fo of the switched capacitor Wien Oscillator is determined by the ratio of the clock signal frequency fcx and the capacitance CR/C. After the CR/C ratio is determined, the sine signal frequency is only determined by the clock signal frequency fcx. If CR/C=0.314, then

The main disadvantage of the Wien Oscillator is that its output amplitude is more difficult to control. If the closed-loop gain of the op amp is A1>3, the output sine signal becomes a square wave, and the shape is severely distorted; if A1<3, the oscillator stops oscillating. To obtain a sinusoidal signal with small distortion and stable amplitude, it is necessary to introduce a non-linear element in the negative feedback loop to control the closed-loop gain of the op amp.The amplitude control circuit is shown in Figure 6.2-2. The depletion type MOS tube in the picture is used as a non-linear element to control the operational amplifierloop gain.Inside the dashed box is the absolute value detector circuit (see Section 4 for its working principle), which changes the output sinusoidal signal of the oscillator into a negative DC voltage. This voltage is applied to the gate of M1 to change the on-resistance of M1 to adjust the closed-loop gain of A1 op amp.When the oscillator just starts to work, the M1 grid potential is zero,The resistance is small, the closed-loop gain of A1 op amp,The output amplitude of the oscillator increases rapidly. The signal passes through the absolute value detection circuit to make the M1 grid potential drop,the resistance increases,decrease. When the output amplitude of the oscillator is a certain amplitude,equal to 3, achieving dynamic balance. In this way, the absolute value detection circuit and the M1 tube keep the output amplitude of the oscillator at a constant value.

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