I was searching the web for schematic which can generate a trapezoidal waveform. None of the results was suitable for my needs, so i started to experiment in LTspice (still learning it), and I designed an interesting circuit, that has great flexibility and it's not very complex.
The left side of the circuit is standard square wave oscillator made with U1 opamp. It can be replaced with 555 oscillator.
The next stage consist of two identical constant current sources (CCS), one with PNP transistor and the other with NPN transistor which alternatively charge and discharge a capacitor. The upper CCS starts to charge the capacitor C1 when the square wave goes down and the lower CCS starts to discharge the capacitor when the square wave goes up. The up slope angle is determined by the R2 and C1 and the down slope angle - by R4 and C1. The two slopes can have different angles (here the duty cycle is also adjusted):
If we increase the two slopes enough or increase the frequency, the output signal will become triangle wave:
The third stage is just a buffer.
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