Design Ideas

Op-amp oscillators simplify RF designs

Charles Kitchin, Analog Devices, Wilmington, MA

Figure 1 shows a novel circuit that uses a low-cost, high-speed op amp as a crystal-controlled RF sine-wave oscillator. An op-amp oscillator offers several advantages. Because an op amp is a complete amplifier with a high-impedance input and a low-impedance output, it considerably simplifies the circuit design by replacing the traditional multiple-transistor circuit with its interstage transformer and numerous other components. This simple op-amp circuit is much smaller, uses less supply current, and directly drives low-impedance loads.

The AD811AN is a low-cost, 140-MHz video op amp with a 100-mA output-current capability. Its output directly drives 50V coaxial cables, and, if you properly match it to the output load, it can provide 1W or more of output power. The crystal connects to a high-impedance input, so the crystal current is practically zero. The circuit can use commonly available microprocessor crystals (costing less than $1) that operate over a wide range of frequencies. The entire circuit should cost less than $10 to build.

Resistors R₁ and R₂ bias the op amp to midsupply. Oscillation occurs because of positive feedback from the op amp's output through the crystal and capacitor C₇ to the positive op-amp input. The negative-feedback loop comprising R₃ and C₂ maintains dc stability. Capacitor C₂ rolls off the higher frequencies, and the op amp operates at unity dc gain. You should choose the value of C₂ such that its reactance is approximately 200V at the operating frequency.

The oscillator's output waveform is a rough sine wave, which undergoes lowpass filtering by the pi output network comprising capacitors C₅ and C₆ and inductor L₁. The output filter greatly reduces harmonics and matches the output impedance of the op amp to the load. Resistor R₄ isolates the op-amp output from the lowpass-filter network; without this resistor, the circuit can break into secondary oscillations. The output impedance of the op amp, in series with R₄, is approximately 30V. Capacitors C₅ and C₆ should be of equal value for driving 50V load impedances.

The component values in the Table in Figure 1 provide approximately 5V p-p into a 50V load at two common microprocessor frequencies using a 9V supply and approximately 7.5V p-p with a 12V supply. You can extend the output-voltage (and output-power) drive beyond these figures by using a 15V or higher supply (to a maximum of 36V), by increasing R₄ above 10V (to maintain stability), and by adding a heat sink to protect the chip from overheating. Inductor L₁ can be a commercial inductor or an RF choke. You can easily hand-wind the inductor onto a 1-in.-diameter plastic-film can or pill bottle, using 20-gauge insulated hookup wire. Six turns closely wound onto a film provide approximately 1.4 mH; four turns provide approximately 0.7 mH.

Figure 2 shows how you can connect the op-amp oscillator to directly drive the base of an RF power transistor. You can vary the ratio of capacitors C₅ and C₆ to match transistor-base impedances lower or higher than 50V. However, you should maintain the same overall series-capacitance value of C₅ and C₆. L₂ is an RF choke that provides a low dc resistance between the transistor base and ground, and it does not add to the RF loading on the op amp. You should select the value of L₂ such that it does not resonate with other circuit components at a harmonic of the oscillator's fundamental operating frequency. (DI #1929)