Triangle Wave Oscillator

Description: This oscillator circuit is designed to generate both a rectangular wave and a triangle waveform, operating within the frequency range of 1 kHz to 10 kHz. The first operational amplifier in the configuration produces the square wave, while the second operational amplifier is responsible for generating the triangle wave. The LM101 is configured as an oscillator that swings rail-to-rail, producing a square wave pulse that reaches the limits of the voltage supplied by the Vcc lines. The LM107 is set up as a classic operational amplifier integrator circuit. The LM101 functions as a threshold detector, utilizing the grounded negative input as a reference level. Due to feedback applied to the positive input from the output pin, the voltage comparator exhibits hysteresis. If an external voltage is applied to the positive pin, the circuit operates as a voltage threshold detector. The input signal is derived from a voltage divider formed by 10 kΩ and 8.2 kΩ resistors. The true input to the circuit is provided by the LM107 through the 8.2 kΩ resistor, while the input to the positive pin combines feedback from the output, thereby introducing hysteresis. The threshold trip point is adjustable via a 1 MΩ resistor. The LM107 functions as the integrator, producing the integration of the rectangular wave, resulting in the triangle waveform. The integration circuit is formed by a combination of a 0.1 µF capacitor, a 1.4 kΩ resistor, and a 140 kΩ adjustable resistor with the LM108. The oscillation frequency is determined by the product of the resistors and capacitors (RxC), with the frequency adjustable by the 140 kΩ resistor. Adjusting the 1 MΩ resistor also affects the trip point, influencing the frequency. The square wave output spans rail-to-rail, while the triangle output peaks at the trip point setting, which is lower than the maximum square wave value. To ensure linearity in the sides of the triangle wave, the maximum values must remain significantly lower than the peak voltage, as the voltage across a capacitor becomes non-linear as it approaches maximum charge. The operational amplifier integrated circuits (ICs) utilized in this circuit are general-purpose devices and can be substituted as necessary. The amplifiers depicted represent the original devices used in an application note that has been published for many years. In general, any operational amplifier could replace either device, including the LM741. However, many of the passive components are critical in setting the frequency of operation and should not be substituted unless a different output frequency is desired.

Operational Amplifiers: Both the LM101 and LM107 are general-purpose operational amplifiers. They are available in various package options, including an 8-pin or 16-pin plastic DIP package, a ceramic DIP package suitable for military and space applications, an 8-lead TO-5 case, or a surface mount flatpack package. Each operational amplifier requires one 0.01 µF bypass capacitor per voltage supply lead. It is recommended to use low inductance capacitors placed near the body of the device and close to the supply leads, with short traces connecting the device and capacitor. Depending on the operational amplifier used, an offset adjust pin may be available; however, alternative methods can be employed to shift the output voltage. For different techniques for operational amplifier offset null adjustment, refer to relevant resources. Another variation of the circuit employs a current mode LM3900 Norton operational amplifier. The LM3900 operates with a positive supply voltage (Vcc) and ground, differing from other operational amplifiers that utilize a differential supply. In this configuration, one of the inputs is tied to Vcc instead of ground, as seen in the previous circuit. Although the circuit is drawn in a similar configuration to the earlier one, variations do occur between the two. The component values also differ, primarily to demonstrate an alternative setup capable of producing a 1.8 ms pulse. The frequency is again determined by the resistor and capacitor across the integrator (R1 and C1).This oscillator circuit is designed to generate both a rectangular wave and a triangle waveform, around 1kHz to 10kHz. The first operational amplifier in the chain generates the square wave, while the second operational amplifier generates the triangle wave.

The LM101 is set up as an oscillator which swings rail-to-rail producing the square wave p ulse to the limits of the voltage supplied by the Vcc lines. The LM107 is configured as the classic operational amplifier Integrator circuit. The LM101 is configured as a threshold detector, with the grounded minus input being used as a reference level. However because of the feedback being applied to the plus input [from the output pin] the voltage comparator also exhibits hysteresis.

That is if the plus pin was just used had an external voltage applied than the circuit would just function as a voltage threshold detector. The signal being applied to the input is developed by the voltage divider form by the 10k and 8. 2k resistors. The true input to the circuit arrives from the LM107 through the 8. 2k resistor, but the input to the plus pin combines feedback from the output, which than adds the hysteresis.

The threshold trip point is adjusted by the 1Meg resistor The LM107 side of the circuit is the integrator and produces the integration of the rectangular wave, or the resultant triangle waveform. The 0. 1uF capacitor, 1. 4k resistor and 140k adjustable resistor combined with the LM108 form the integrator. The oscillation frequency is determined by the product of the resistors and capacitors [RxC]. The frequency of the oscillation is adjusted by the 140k resistor. Although changing the trip point, by adjusting the 1Meg resistor also effects the frequency. The square wave output will be from rail-to-rail while the triangle output will peak out at the trip point setting, which is lower than the maximum square wave value.

In fact for the sides of the triangle wave to be linear, the maximum values most be much lower than the peak voltage. As the voltage across a capacitor becomes non-linear as it approaches maximum charge. The operational amplifier ICs used in the circuit are both general purpose devices, and may be substituted as required.

The amplifiers shown represent the original devices used in an application note, which has been in printed for many years. In general any Op Amp could be used to replace either device, including an LM741. However many of the passive parts set the frequency of operation and should not be substituted, unless a new output frequency is required.

Operational Amplifiers: Both the LM101 and LM107 are general purpose operational amplifiers. The available package options include an 8-pin or 16-pin Plastic DIP Package, or Ceramic DIP Package [military and space applications], an 8-Lead TO-5 Case or a surface mount Flatpack Package. One 0. 01uF bypass capacitor per voltage supply lead of each Op Amp is used. Low inductance capacitors placed near the body of the device and close to the supply leads is recommended.

Short traces between the device and capacitor is also desired. Depending on the Op Amp used an off-set adjust pin may be offered. However other techniques may be used to shift the output up or down. Refer here for different methods for OpAmp Off-Set Null Adjustment. Another variation of the circuit uses a current mode LM3900, Norton operational amplifier. The LM3900 uses a positive supply voltage [Vcc] and ground, and does not uses a differential supply like the other Op Amps. So one of the inputs is tied to Vcc and not ground as the previous circuit. Also there has been an attempt to draw the circuit in the same configuration as the previous one, but variations between the two due occur.

The component values are also different, but really only to show another possible setup, which is a 1. 8mS pulse. The frequency is again set by the resistor and capacitor across the integrator R1 and C1 [R