Discussion in 'Homework Help' started by ppppp, Apr 29, 2011.
how oscillator gives output without input.plz give me ans frnds
Here is an experiment - http://www.allaboutcircuits.com/vol_6/chpt_6/7.html
Notice that the circuit is supplied with power. In the case of the 555, a comparator controls the charge/discharge of the timing capacitor.
Another kind is a phase shift oscillator - http://en.wikipedia.org/wiki/Phase-shift_oscillator
All electronic osciilators are powered, and most have a condition controlled by charge on a capacitor that governs the start up of the oscillation.
In the case of many oscillators, simple random noise gets amplified repeatedly and fed back to the input until the frequency determining system takes over and allows only amplification of the desired frequency.
Essentially, oscillators depend on some kind of amplifying system provided with positive feedback, but this more obvious with some types than others.
At one extreme we have simple oscillators using a single amplifying device with a frequency-selective positive feedback path. In this case it is easy to visualise random noise leading to an increasing signal building up around the feedback loop, assuming that (at least initially) the loop gain is more than one at some frequency where the signal comes back around the loop in phase with itself.
RC oscillators like the 555 are usually analysed in terms of large-signal processes where capacitors charge until a threshold is reached, triggering some event that leads to the capacitor discharging again. That said, the comparators or other trigger devices underlying this process can be considered to be rather specialised amplifiers.
Other oscillators are based on devices which present a negative incremental resistance or trigger characteristic between two terminals. These include tunnel diodes, some gas discharge tubes (including thyratrons) and uni-junction transistors. Here the amplification is an implicit process, a consequence of the negative incremental resistance.
As already told to you there are a lot of oscillator types.
These pages from the EDUCYPEDIA will give you a lot of links on the different types: