http://www.tedpavlic.com/teaching/osu/ece327/lab5_adc/lab5_adc_pnp_ramp_generator.pdf

http://www-personal.engin.umd.umich.edu/~fmeral/ELECTRONICS%20II/05%25c3-Discrete%20BJT/Voltage%20Ramp.pdf

http://www.simple-electronics.com/2011/10/triangular-wave-generator-using.html

http://www-personal.engin.umd.umich.edu/~fmeral/ELECTRONICS%20II/10%25c3-Relaxation%20(triangle)Osc./TriangleGeneratorProject.pdf

http://faculty.kfupm.edu.sa/EE/mkulaib/publications/a%20low-cost%20dual.pdf

http://www.simplecircuitdiagram.com/2010/10/15/sawtooth-generator-oscillator-with-transistors/

http://dspace.uta.edu/bitstream/handle/10106/1083/umi-uta-2196.pdf?sequence=1