Sine wave generators are traditionally considered rather capricious devices: achieving stable amplitude and low distortion usually requires a more complex circuit. This is especially noticeable over a wide range of supply voltages, where most simple solutions either become unstable or require careful tuning.

Simplicity in a circuit is typically poorly matched with high operational stability. However, this design manages to combine both qualities thanks to the unusual connection of the MOSFET transistor and the ingenious use of a frequency-setting LC circuit.

The generator produces a sine wave output with a maximum frequency of 650 kHz. It contains only five components, is quick to assemble, and requires no tuning at the specified values. The circuit operates over a supply voltage range of 5 to 60 V with a current consumption of approximately 1–5 mA, depending on the supply voltage and the resistance of resistor R1. If necessary, the output power can be increased by adding just a few additional components—the generator's parameters will be virtually independent of the connected load. Another feature of the circuit is its high efficiency, which is maintained across the entire supply voltage range.

The schematic diagram of the generator is shown in Figure 1. Unlike conventional circuits, the supply voltage Up is applied not to the drain of the MOSFET transistor Q1, but to its gate, thereby creating a constant bias relative to the source. Initially, the drain-source junction is open, and current begins to flow through inductor L1. Due to the accumulation of energy in the magnetic field, the inductor maintains the voltage at the gate-source junction for some time, but it gradually decreases to a level at which the transistor turns off. After the transistor turns off, the energy stored in the LC circuit begins to flow back, causing the reverse process. After some time, the gate voltage again reaches the transistor's turn-on threshold, and the cycle repeats. This creates stable oscillations in the circuit.

Inductance L1, together with capacitors C1 and C2 and the internal capacitance of transistor Q1, forms a pi-shaped oscillatory circuit C1-L1-C2. This circuit determines the sinusoidal shape of the output signal and ensures high frequency stability. The device's operating frequency is calculated using classic formulas for a pi-shaped circuit, where the equivalent capacitance is determined by the series connection of C1 and C2, taking into account the parasitic capacitance of the MOSFET transistor.

Oscillograms of the generator operation are shown in Figures 3 and 4 with a supply voltage Up of 12 volts and R1 equal to 4 kOhm. The values ​​of C1 and C2 here are 1.2 nF, and L1 is 550 uH. Figure 3 shows the signal at the OUT output, and Figure 4 additionally displays the signal at the drain of transistor Q1 (measuring point 1). This oscilloscope trace clearly shows the process of energy accumulation and return in the LC circuit, as well as the switching moment of the MOSFET transistor, which maintains oscillation.

The output power of the base oscillator is determined primarily by the resistance of R1, so the circuit is designed accordingly.Primarily designed for operation with high-impedance loads. However, adding a few additional components allows for the implementation of a source follower on transistor Q2, significantly increasing the device's load capacity. The circuit diagram of the oscillator with a power amplifier is shown in Figure 2. In this embodiment, the output signal is taken from inductor L2, which, if necessary, can be implemented as a transformer to match virtually any load. Due to the high input impedance of the follower, the amplifier stage has virtually no effect on the operation of the main LC oscillator. This maintains frequency stability and a good sine waveform even with a significant increase in output power.

Figure 5 shows the circuit's operation with an additional power amplifier. The yellow beam indicates the voltage across capacitor C2, and the blue beam indicates the signal at the OUT output. The oscilloscope traces indicate that connecting the output stage has virtually no effect on the sine waveform or the operating mode of the main oscillator. This maintains frequency stability and low distortion even when a load is applied.

Particular attention should be paid to the selection of transistor Q1. For stable generation and a high-quality sine wave, it is advisable to use a MOSFET with a minimum input capacitance C_GS or C_iss, especially when operating at frequencies above 100 kHz. One of the most successful options turned out to be the IRF840 transistor, which is well suited for both the main oscillator and the amplifier stage on Q2 [1]. In this case, transistor Q2 has less stringent input capacitance requirements, so virtually any suitable MOSFET can be used in this unit. In some cases, even a bipolar NPN transistor can be used instead of Q2.

The resistance of resistor R1 is selected depending on the supply voltage Up. For the 5-12 V range, a resistance of approximately 3-5 kOhm is usually suitable, while for 50-60 V, approximately 15-20 kOhm. The resistor's power should also correspond to the operating voltage: for low-voltage applications, 0.125 W is sufficient, while for 50-60 V, a resistor with a power of at least 0.5 W is desirable. The remaining resistors in the circuit (Fig. 2) can have a power rating of 0.125 W or higher.

It is advisable to use capacitors with minimal dielectric loss, although standard film capacitors also work well here. Inductor L1 can be used as a ready-made choke, however, for precise frequency adjustment, it is preferable to wind the coil yourself and use an adjustable core. For L2 in the amplifier stage, a standard factory choke is suitable. The method for calculating the ratings and selecting the components will be discussed below.

With the values ​​shown in Diagram 1 and using an IRF840 transistor, the generator's resonant frequency is approximately 550 kHz. It is calculated using formulas for a U-shaped circuit, taking into account the input and output capacitances of the MOSFET. If we assume that C1 = C2 = C, then for other values, the resonant frequency of such a circuit is determined by the expression: \[\tag{1} f_r = {1 \over 2 \pi \sqrt{L_1 C_e}} \] where \(L_1\) is the inductance of coil L1, and \(C_e\) is the equivalent capacitance of the circuit: \[\tag{2} C_e = {(C + C_{oss}) (C + C_{iss}) \over 2C + C_{oss} + C_{iss}} \] Here \(C_{iss}\) and \(C_{oss}\) are the input and output capacitances of transistor Q1, which are taken from the reference data [1]. All quantities in the given formulas are substituted in the SI system.

It is recommended that the capacitances of capacitors C1 and C2 not be less than 500 pF if the input capacitance of transistor Q1 exceeds 1000 pF. Otherwise, the parasitic capacitances of the MOSFET will have an excessively strong influence on the parameters of the pi-circuit, complicating the calculations and reducing the circuit's repeatability. For this reason, the recommended resonant frequency for such transistors should not exceed approximately 650 kHz. The lower frequency limit is fundamentally unlimited and is determined only by the selected values ​​of L1, C1, and C2, as well as the practical dimensions of the coil and capacitors.

An important parameter of the oscillatory circuit is its characteristic impedance: \[\tag{3} Z = \sqrt{L_1 \over C_e} \] The amplitude of the sinusoidal signal at the generator output largely depends on this value. The optimal value of \(Z\) is approximately in the range of 500...1200 Ohm: the higher the characteristic impedance of the circuit, theA larger amplitude can be obtained at the circuit output.

For the amplifier stage on transistor Q2, it is also necessary to select the minimum inductance of inductor L2. It is determined from the condition: \[\tag{4} L_2 \gt {1500 \over 2\pi f_r} \] If we substitute expression (1) instead of frequency \(f_r\), the same condition can be written using the parameters of the main circuit: \[\tag{5} L_2 \gt 1500 \sqrt{L_1 C_e} \] In practice, it is better to select the inductance of L2 with a small margin relative to the calculated minimum so that the amplifier stage places less load on the oscillator and does not degrade the shape of the sinusoidal signal.