Sawtooth Waveform Generator

The circuit

The following circuit belongs to the family of relaxation oscillators. Complementary bipolar transistors establish a DC characteristic similar to that of a gaseous discharge tube. At the beginning of the cycle, both transistors are in the non-conducting state. C₁ is charged via R₅ with an exponentially increasing voltage. When the firing voltage is reached, a regenerative process starts by which the capacitor is discharged until the current drops below a holding threshold. Thereafter, a new operating cycle begins.

The input file (sawtooth_waveform_generator.cir)

sawtooth waveform generator

* fo = 1 / (c1 r5 ln(1 + r2 / r1))

.control
tran 5us 1ms uic
plot v(2) xlabel t[s] ylabel output[V] title 'TRAN analysis'
plot v(5) xlabel t[s] ylabel v(5)[V] title 'TRAN analysis'
.endc

v0 1 0 dc 15V

r1 1 3 1.5k
r2 3 0 3.3k
r3 4 0 39k
r4 5 0 100
r5 1 2 100k

c1 2 0 2.2nF ic=0

q1 4 3 2 noname1
q2 3 4 5 noname2

.model noname1 pnp cjc=10pF rb=100 is=1p
.model noname2 npn cjc=10pF rb=100 is=1p

.end

The results