astable multivibrator


Posted on Feb 7, 2014

The astable multivibrator I found in wikipedia and also my college material are the same, which is on the left hand side of the picture. While for the schematic at the right hand side, I found it from the example of LTSpice. I would say because it`s drawn wrongly, the correct circuit is the one on the left. It may perhaps work, but it`s not a go


astable multivibrator
Click here to download the full size of the above Circuit.

od example, and it`s not something I would consider. The circuit on the left is prone to not starting to oscillate, especially in simulations. The circuit on the right has DC negative feedback, which keeps the transistors out of saturation except right after they switch ON. This allows oscillation to start more readily, especially in simulations. There are ways of getting the circuit on the left to start in simulation. In practice, it will generally start, due to component mismatches and the startup transients introduced by the power supply turn-on step. I agree that the circuit on the right is not as useful as the other one. Since the transistors don`t saturate, you won`t get good logic "0" levels, although there are relatively simple ways to get around this. The wire on the capacitor that connects to a transistor`s base is driven to a negative voltage when the other transistor conducts. Then the capacitor discharges into R2. Since the max reverse voltage on a silicon transistor`s emitter-base junction is only about 5V then a small supply voltage is needed or protection diodes should be used in series with the emitters. When C1 is fully charged, Q1 is turned on. C1 acts as a battery and its -ve side is connected to the base of Q2, that`s why the potential at the base of Q2 is -ve when C1 starts discharging, right Then R3 charges or discharges C2 enough to turn on Q1 a little and the negative end of C1 drives the base of Q2...




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