IC555 can be used to generate PAM with one NPN transistor connected at output. The chip is configured in astable mode to generate pulse train to get samples of information (audio) signal. Its frequency should be at least twice that of audio signal. Usually it is 8 KHz (because audio signal is up to 3.4 KHz) but for better quality here I am keeping it 32 KHz. This pulse train output is fed to base of NPN transistor. The collector of transistor is coupled with low frequency audio (information) signal through positive clamper made up of capacitor C1 and diode D1. The positive clamper will shift the level of audio signal above 0 V. The output at the collector of transistor is PAM wave. The amplitude of pulses generated by IC555 varies in accordance with the instantaneous amplitude of information signal.
Pulse Width Modulation (PWM) using IC 555
If we apply variable voltage at control input (pin 5) of IC555 in astable mode then the width of output pulse will change accordingly. When we apply low frequency audio signal to the control input, internally the control voltage will change the reference voltage of comparator so charging time of capacitor changes every time and so width of o/p pulse also changes. So here as shown in figure the audio signal is applied at the control input through positive clamper circuit. The values of R1, R2 and C1 are chosen to generate pulse train of around 64 KHz. The width of o/p pulse will change in accordance with the amplitude of audio signal as shown in figure in Circuit Diagram 1tab.
Pulse Position Modulation (PPM) using IC 555
PPM is simply PWM + monostable multi-vibrator. Means if we fed the PWM output to trigger input of IC555 in monostable mode then the output of monostable will be PPM wave. So to generate PPM we require one astable multi-vibrator to generate PWM signal and one monostable multi-vibrator to get desire PPM. The final circuit and the waveforms are as shown in Circuit Diagram 2 tab.