Low distortion, state variable 2-phase oscillation circuit

Function of the circuit

Amplifiers used in audio equipment require low-distortion signal sources when testing. Recently, there are many products of low-distortion OP amplifiers for audio circuits. Oscillators have distortion, so measurements are limited. State variable circuits are the basic components of active filters. If positive feedback is added, it can become an oscillator.

The integrator is composed of a two-stage inverting amplifier. The high-frequency distortion generated by the integrator itself is attenuated by an average of 6DB/OCT per stage. This circuit is used in a low-distortion oscillation circuit. The phase difference between the input and output of the integrator is 90 degrees, and a two-phase oscillation output can be obtained.

How the Circuit Works

If there is no positive feedback, the OP amplifier A1 only acts as an inverting amplifier, and the integrators A2 and A3 each have a 90-degree phase shift, and the entire circuit will produce a 360-degree phase lag. When the loop gain is greater than 1, the circuit oscillates, and the oscillation frequency is FO=1/πCO.RO. In order to stabilize the oscillation, R5 is connected in series with the drain-source resistance TDS of the J-FET.

The CR oscillator circuit that can directly generate a sine wave requires an amplitude stabilization circuit, which is an important part of the oscillator circuit. The feedback voltage output by A2 and divided is connected to the non-inverting input of the OP amplifier A1 to control the gain. When the output increases, the divided voltage increases and the oscillation output amplitude decreases.

Diodes D1 and D2 are full-wave rectifier circuits to detect the oscillation output level. Zener diode D4 generates a -5V reference voltage. The series-connected grounding tube D3 is used to offset the temperature coefficient of the forward voltage drop of D1 and D2. If there is no DS, the oscillation level will drop when the ambient temperature rises.

OP amplifier A4 performs comparison, error amplification and integration. In order to stabilize the AGC loop, the average current through resistors R9 and R10 and the DC current flowing out of R11 must be equal. Because the current flowing through R11 is constant, once the oscillation output level drops, A4 will integrate the negative current, making the threshold voltage of TR1 close to zero potential, increasing the positive feedback amount, and raising the output level.

The capacitor C1 in the A4 feedback circuit is an integral capacitor, which determines the response speed of the AGC loop. The Zener diode D5 prevents the integrator from saturating. The voltage range is +0.6V~-5V. The integrator is a primary lag element, so it takes a long time to stabilize the loop. R4 is an advance compensation resistor added to shorten the stabilization time. If the resistance value is too large, the pulsation generated in the rectifier circuit cannot be removed, and the distortion rate will increase.

Use of components

OP amplifiers A1~A3 are related to distortion rate, so low distortion products must be selected. The stability of oscillation frequency depends on the temperature coefficient of capacitor CO. If styrene capacitors are used, a stability of +150PPM/℃ can be obtained. After D1~D2 are thermally balanced, the amplitude stability depends on the stability of the Zener voltage. If high stability is required, IC can be used to generate the reference voltage.