Why is the current in this Colpitt oscillator distorted ?

Thread Starter

silv3r.m00n

Joined Apr 15, 2010
70
I am fiddling with a colpitt oscillator in falstad simulator.
Here is the simulation link: https://bit.ly/2kgtRID

The current in the inductor of the tank circuit is nice sine wave, but the current in the capacitor (right side) is distorted in the negative half.
Here is a screenshot,


Why is this distortion happening ?
 

danadak

Joined Mar 10, 2018
4,057
Is it Vbe of transistor in reverse bias, limited by its breakdown,
that is causing this ? Seems like step size is ~ brkdwn of a NPN
emitter base junction.

Not sure.

Regards, Dana.
 

Danko

Joined Nov 22, 2017
1,201
Distortions of tank capacitor current are from too big and hard positive feedback.
In my circuit feedback current is lowered and smoothed by resistor R4.
 

Thread Starter

silv3r.m00n

Joined Apr 15, 2010
70
During negative current cycle, the phase shift between voltage and current seems to be higher than the phase
shift in the positive part.

it seems like the current flows quicker (sudden loss, due to amplification) than it should, around the negative peak, which causes a sudden break path to 0.

May be this phenomenon, is reduced by the 3K resistor, as far i can guess.
 

SLK001

Joined Nov 29, 2011
1,548
I am fiddling with a colpitt oscillator in falstad simulator.
The current in the inductor of the tank circuit is nice sine wave, but the current in the capacitor (right side) is distorted in the negative half.
Why is this distortion happening ?
The distortion is happening in ALL the sine waves that are shown. It is greater at the output, because of AMPLIFICATION!
 

Danko

Joined Nov 22, 2017
1,201
@silv3r.m00n:
Your circuit 1kHz, enhanced.
Added R1 and R6.
C tank = C4 and C5 in series = 1uF
upload_2019-9-14_23-17-9.png
It is how your circuit works now in web simulator:
circuit-20190915-0402.png
 
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Thread Starter

silv3r.m00n

Joined Apr 15, 2010
70
That probably has something to do with the biasing of the transistor - it looks like it momentarily cuts off.
The distortion happens during the negative current cycle in the tank capacitor
During that time, the transistor is in high conduction (low output).

So transistor cutoff may not be playing any role in the distortions.
 

Thread Starter

silv3r.m00n

Joined Apr 15, 2010
70
@silv3r.m00n:
Your circuit 1kHz, enhanced.
Added R1 and R6.
C tank = C4 and C5 in series = 1uF
View attachment 186171
It is how your circuit works now in web simulator:
View attachment 186190
Thanks, the circuit is indeed much better now. Falstad link: https://bit.ly/2kOFWF9

What does the 200 ohm resistor in series with the bypass capacitor do ?
My first guess is that it reduces AC gain by increase the emitter resistance.
But couldn't that be done by increasing the capacitive impedance ?

I tried removing the 200 ohm capacitor and increase impedance by altering the value of capacitor,
but that brings back the distortions.

Whats the difference between increasing impedance by adding a resistor VS by increasing capacitive impedance alone ?
 
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Danko

Joined Nov 22, 2017
1,201
What effect does the 200 ohm capacitor in series with Re do ?
My first guess is that it reduces AC gain by increase the emitter resistance.
But couldn't that be done by increasing the capacitive impedance ?
Whats the difference between increasing impedance by adding a resistor VS by increasing capacitive impedance alone ?
Resistor 200Ω reduces AC gain up to 10 and linearizes transfer function of amplifier.
It is not good idea to remove resistor 200Ω and reduce emitter capacitor 10uF to 0.77uF (200Ω impedance),
because circuit simple will not oscillate at all (due phase shift at 1 kHz).
Diagrams below show how shapes of collector current and collector voltage are affected by resistor 200Ω.
upload_2019-9-17_4-35-48.png
 
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Thread Starter

silv3r.m00n

Joined Apr 15, 2010
70
Resistor 200Ω reduces AC gain up to 10 and linearizes transfer function of amplifier.
It is not good idea to remove resistor 200Ω and reduce emitter capacitor 10uF to 0.77uF (200Ω impedance),
because circuit simple will not oscillate at all (due phase shift at 1 kHz).
Diagrams below show how shapes of collector current and collector voltage are affected by resistor 200Ω.
View attachment 186345
guess i was thinking along similar lines. would it be correct to say -

1. adding a resistor reduces ac gain without inducing phase shift, which keeps closed loop phase shift to minimum.
2. if no resistor is added, and capacitor reactance is reduced alone, then a significant phase shift is induced, inhibiting oscillations.
 

MisterBill2

Joined Jan 23, 2018
10,493
With oscillators there are also concerns about fast starting and frequency stability. Of course increasing the feedback to the point of running the active device into it's non-linear areas is always going to produce distortion. That is inescapable. An optimum design includes negative feedback that tends to keep the oscillation in the linear area of the active device, presuming that the intention is to produce a low distortion sine wave. Oscillators intended to produce strong harmonics are a different case.
 

Danko

Joined Nov 22, 2017
1,201
guess i was thinking along similar lines. would it be correct to say -
1. adding a resistor reduces ac gain without inducing phase shift, which keeps closed loop phase shift to minimum.
2. if no resistor is added, and capacitor reactance is reduced alone, then a significant phase shift is induced, inhibiting oscillations.
Yes, it is correct.
I slightly modified your circuit.
No capacitors, except for tank caps.
No phase shift.
Quite clear, 11Vp-p sine:
upload_2019-9-18_0-0-57.png

EDIT:
@MisterBill2
Probably changing R1 to 100K, R 2 to 27K, and R4 to 1K will reduce the harmonics quite a bit.
upload_2019-9-18_2-49-13.png
 

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MisterBill2

Joined Jan 23, 2018
10,493
Based on the harmonic content shown in post 19, just because I don't see the distortion does not mean it is not there. BUT certainly there is no obvious tendency to break into a higher order oscillation either.So the circuit is an improvement. Probably changing R1 to 100K, R 2 to 27K, and R4 to 1K will reduce the harmonics quite a bit.
 
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