Understand Oscillators.

[lord loh.]

I am having some difficulties understanding oscillators.

So here are a few questions that I am framing to help understand oscillators.

1. When a circuit is initially powered on, it's coltage raises from 0 to maximum. Should this vary fast change in voltage be considered for oscillator operation?(sustained waveforms)

My teacher told me that it need not be. But I differ.



2. If an inductor is given a changing EMF(0 to maximum), it shall generate a changing magnetic field. This shall cause a nearby magnetic needle to show instantanious deflection.

Suppose I shunt the inductor by a capacitor, will the needle still show deflection?

I guess not. The changing voltage shall be shunted via the capacitor and the indictor shall receive the current only when the voltage has reached maximum. I am doubtful of this (contradicting thoughts to my own explaination) the currewnt through the inductor should go from 0 to maximum any way.

 

[Optikon]

I am having some difficulties understanding oscillators.

So here are a few questions that I am framing to help understand oscillators.

1. When a circuit is initially powered on, it's coltage raises from 0 to maximum. Should this vary fast change in voltage be considered for oscillator operation?(sustained waveforms)

My teacher told me that it need not be. But I differ.



2. If an inductor is given a changing EMF(0 to maximum), it shall generate a changing magnetic field. This shall cause a nearby magnetic needle to show instantanious deflection.

Suppose I shunt the inductor by a capacitor, will the needle still show deflection?

I guess not. The changing voltage shall be shunted via the capacitor and the indictor shall receive the current only when the voltage has reached maximum. I am doubtful of this (contradicting thoughts to my own explaination) the currewnt through the inductor should go from 0 to maximum any way.

2) it depends on what your actual LC network looks like. The current through the inductor will depend on the NET impedance of the network.

Every real inductor already has a parasitic capacitor that shunts it but we can still cause needle movements from the field. The key is how big this capacitor is and what the final impedance looks like. Work it out from an impedance standpoint first and then look at the final inductor current. This final resulting current will drive the magnetic field formed. Then you can answer whether or not you will see a needle move.

If you are talking about a transient even, then liekly the discharged capacitor (short at DC through inductor) will have an inrush current when voltage is first applied. If this current flows through your inductor that it will respond with a field buildup and an opposing EMF which will tend to decreas this large current trying to flow..

 

[i_build_stuff]

1) Most LC oscillators, such as the Colpitts and others based on class-A gain stages, don't care about the initial transient. The thermal noise that occurs in resistors (there's an equation to predict the amount) is enough to start the oscillator.

Oscillators that are based on class-C stages, however, do need a turn-on transient to start, as that configuration will only amplify when the signal voltage reaches a certain level.

2) Any current (changing or not) in an inductor will generate a field, which could deflect a compass needle. Adding a parallel capacitor won't change this in the DC case. In the non-DC case, it still doesn't matter because you said you were applying a changing *voltage*--V sources (which ideally have zero impedance) wouldn't be affected by some charging current going into a capacitor.

 

[panic mode]

1. single transition from A to B is not oscillation but if you have system that (upon powerup) keeps on making transition A-B-A-B-A-B... then - you got yourself an oscillator.

2. i'm not sure you understand what is 'instant' and how fast mechanical devices can move.
try to use good function generator, an amplifier and simple coil instead of speaker. set frequency very low (0.1 to 0.5Hz) and observe what compas needle does. see what happens when you slowly increase frequency.

if you don't have access to such gear, try something similar:
stretch one arm straight ahead. now slowly move it few inches left-right-left-right... try to do it faster and faster and faster and faster...
can you still reach those few inches left and right?

 

[lord loh.]

No the problem I had in understanding oscillators was that -
I thought the current (at start up) flowed through the capacitor of the tank circuit and once the capacitor is charged, it would flow through the inductor.

It is the tuned circuit oscillator I am taking about.

The teacher told me that the supplied voltage is a DC and flows through the inductor first. She asked me to ignore the 0->max time. I differ.

I believe that the transient must be considered to start the initial oscillations.

Now the magnetized inductor induces a opposite voltage in a nearby inductor(transformer type setup) and draws current from the base of a transistor (NPN). The tank circuit is at the collector. Now the capacitor starts to discharge through the inductor.

Once the transistor offers higher resistance to the DC bias via the tank circuit, the magnerticv field of the inductor falls in causing a reverse EMF sending current to the base of the transistor and letting the DC bias supply some energy to compensate the loss of energy in the tank circuit.

Am I correct ?

 

[panic mode]

can you provide circuit diagram?