Improved Astable Multivibrator
  The conventional two transistor astable multivibrator has been modified to produce better waveform which are more steeply rising and falling than those from the conventional astable. The improvement in the waveform is achieved by using a diode pair in each collector branch of the transistors. This restricts the direction of current flow for the charging and discharging of the timing capacitors. This article also derives the time period of oscillations for the modified circuit and discusses the limits on source and sink currents at the transistor collectors. 
Raju Baddi
physics.pop-ph physics.class-ph