General Engineering Terms
"A" "B" "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
"N", "O", "P" "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"

Triangle Wave Oscillator

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 pulse to the limits of the voltage supplied by the Vcc lines. The LM107 is configured as the classic operational amplifier Integrator circuit.

Triangle Wave Oscillator circuit
Triangle Wave Oscillator

Oscillator Description

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.

Component Selection

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.

Note the LM101 & LM107 Op Amps may be discontinued or out-dated, as are the other Temperature Range Variations, LM201, LM301, LM207 & LM307.
Refer to the list of Companies making Operational Amplifier ICs.

Circuit Recommendation

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.

Triangle Wave Generator

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.

LM3900 Triangle Wave Oscillator circuit
Triangle Wave Generator

The frequency is again set by the resistor and capacitor across the integrator R1 and C1 [R2 should be set to 2x of R1]. The equation for the frequency follows: f = [Vcc - Vb] / [2R1C1 x Vo].
Vcc is the supply voltage.
Vo is the difference between the trip voltages of the hysteresis.
Vb is the DC voltage at the minus input.

The resistors of the threshold detector set the high and low points of the triangle wave, and the resistors and capacitor of the integrator sets the time it takes to reach those points [the frequency].

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