After retirement, while reviving my old lab, I found some decades old forgotten RCA 3080 OTAs. I used them in my 2Hz to 1KHz variable frequency voltage source project, which I use as a reference source in other projects.

 

I summarized and adapted the basic oscillator idea for my needs from the Analog Databook [1],

the theory of sine wave oscillators is from Design of sine wave oscillators [2], 

and the idea of ​​a 3-phase oscillator is from Elektor 7-8/99 [3].

 

The basis of the circuit is a buffered phase-shift oscillator, which consists of three voltage-controlled low-pass filters, each of which turns the phase by 60°, and a voltage-controlled amplifier, which turns the phase by an additional 180°, and ensures appropriate amplification of the feedback loop so that the output of the first filter is also of the correct sinusoidal shape and is not distorted. Each filter at the output also amplifies the signal by a factor of 2, increasing the filter frequency by the same factor, which reduces the need for feedback loop amplification. The gain is set by a simple PI regulator, the measured value is provided by a rectifier with a filter, a change in the reference value of the regulator changes the output voltage in the range between approximately 2 and 6 Vpp. The frequency is set via an integrator and a voltage limiter in the range of about three decades. The task of the desired frequency integrator is to limit rapid frequency changes. The limiter ensures that the voltage difference between the V- and bais inputs of the OTA is at least 0.65V, it sets the lower frequency limit of the oscillator. The upper frequency limit is determined by the resistors on the OTA's bais inputs, limiting the bais current to about 1 mA. Trimmers are built into the limiting resistors to compensate for the amplitude error in the area of ​​minimum frequencies. Apart from the trimmer for setting the bais input, there are three more trimmers in the circuit for setting the output phase voltages and an additional trimmer for the initial setting of the gain of the oscillator feedback loop.

 

The frequency range is changed by filter capacitors, for values ​​of C = 2.2 nF the range is between 5 Hz and 21 KHz, one turn of the potentiometer covers the entire tonal range. For values ​​of C = 47nF, the range is 2 Hz to 1.1 KHz, and for values ​​of C = 470nF, the range is 2 Hz to 110 Hz. The frequency and output phase voltages are internally adjusted with two potentiometers or via jumpers from external voltage sources. The VCO input is in the range of ±10 V, and the amplitude input is 0 to -10 V. The supply voltage of the oscillator is ±4.5V.

 

The circuit is not fully optimized but works reliably. But it also has drawbacks. Calibration is demanding, the signal amplitude is around 3% despite the regulation. Operation at low frequencies requires larger values ​​of rectifier filter capacitors, which means larger time constants and problems with output voltage regulation dynamics. When operating above 30 KHz, it is necessary to reduce the output voltage values ​​due to distortions. I did not develop the circuit in this direction.

 

[1] National Semiconductor Operational Amplifiers Databook, OTA LM13700, 1995,

[2] Texas Instruments, Analog Applications Journal 2000,

[3] Elektor 7-8/99, Burr-Brown application UAF42