astable flip flop 02 PUBLIC

The classic 2 transistor astable multivibrator with and without anti-saturation clamps.

The classic 2 transistor astable multivibrator with and without anti-saturation clamps. SW1 and SW2 switch D1 & D2 in and out to show the effect of collector-base anti-saturation Schottky clamping diodes. The diodes prevent the transistors from saturating by applying local negative feedback gated by the astable switching action. As the collector voltage falls the diode is forward biased and so pulls the base voltage down, preventing the transistor being turned hard on and saturating. For this technique to work, the drop across the diodes has to be less than Vbe+Vce_sat of the transistors.

This technique works best in ICs and was the basis of the increased speed of the LSTTL over the TTL family, which did not have the Schottky clamps. Note the slight change in period between the two variants.

The unequal base resistors help it start up asymmetrically.