Author Topic: Question on oscillators ?  (Read 1334 times)

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Offline waelTopic starter

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Question on oscillators ?
« on: October 10, 2017, 09:19:29 pm »
Hi,

I'm trying to understand the attached oscillator , when i removed the R2 resistor it stopped oscillating ,(don't know why)
My question is :
1. How to simply understand/analyze such circuits ?
2. Why removing the R2 makes it stop oscillating ?
3. Changing the Q2 PNP have a very strange effects ?
4. Also changing the inductor to a resistor has a strange effect ?

Attached is a ltspice circuit if you wish to test it.

Regards,
Wael
 

Offline ataradov

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Re: Question on oscillators ?
« Reply #1 on: October 10, 2017, 09:46:16 pm »
1. Just follow each transistor on their own.
2. You are removing the feedback loop by completely removing R2.
3. If you are changing it 1:1 (just plug a different transistor into the same circuit), then you are violating basic transistor use principles, so I'm not sure what you have expected?
4. Inductor is a timing element here, so changing it to a resistor will just make this circuit go into some sort of stable state, or some sort of oscillations depending on stray capacitance and inductance. LTSpice may not be able to simulate this accurately.
Alex
 

Offline waelTopic starter

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Re: Question on oscillators ?
« Reply #2 on: October 10, 2017, 09:59:52 pm »
Many thanks, but could you explain how this feedback works, i know that since it's pnp it's on, i don't get why it gets off.
 

Offline ataradov

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Re: Question on oscillators ?
« Reply #3 on: October 10, 2017, 11:00:53 pm »
In a steady state, L2 is a short, so Vb(Q2) is V1 / 2 because of the R1/R2 divider. This is enough to keep Q2 open, but that also opens Q1, which tries to pull Vb(Q2) to ground, but L resists this change, creating delay. Eventually Q2 will close, which will close Q1, and R1+R2 will start charging the inductor, which it will also resist. But eventually Vb(Q2) will rise again, and the cycle repeats.

Attached is a bit easier to read version of the schematic.
Alex
 
The following users thanked this post: nugglix, wael

Offline medical-nerd

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Re: Question on oscillators ?
« Reply #4 on: October 11, 2017, 08:51:27 am »
Hiya

Thank you atarodov for explaining why the circuit would oscillate, can you explain the purpose of the diode across the inductor please?

Cheers
'better to burn out than fade away'
 

Offline waelTopic starter

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Re: Question on oscillators ?
« Reply #5 on: October 11, 2017, 09:09:40 am »
Hi,

The diode in this configuration is to eliminate flyback (voltage spikes when the magnetic field collapses), it's a flyback diode.

Regards,
Wael
 

Offline medical-nerd

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Re: Question on oscillators ?
« Reply #6 on: October 11, 2017, 11:08:06 am »
Hiya

Much obliged, I've only looked at typical rf oscillator circuits so far. I'm familiar with their flyback use in relays and it did look familiar. I didn't realise that they were used in oscillators for this purpose as well.

Cheers
'better to burn out than fade away'
 

Offline ataradov

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Re: Question on oscillators ?
« Reply #7 on: October 11, 2017, 04:19:33 pm »
Much obliged, I've only looked at typical rf oscillator circuits so far. I'm familiar with their flyback use in relays and it did look familiar. I didn't realise that they were used in oscillators for this purpose as well.
Even in this simulation you can see that V(ind) is ~1.5V + 0.7V = 2.2V. Where 0.7V is a diode forward voltage.
Alex
 


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