Breadboard and meter a circuit.

Nigel Goodwin

Super Moderator

Most Helpful Member

• Dec 18, 2024

• #2

Well it looks completely non-functional, what's with Q2 base/emitter?.

 

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Lightium

Active Member

• Dec 18, 2024

• #3

Oh no, it oscillates at a variety of frequencies.

 

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Lightium

Active Member

• Dec 18, 2024

• #4

Q2? The power supply is so small there is no need for biasing.

 

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Nigel Goodwin

Super Moderator

Most Helpful Member

• Dec 18, 2024

• #5

Lightium said:

Q2? The power supply is so small there is no need for biasing.

Click to expand...

Q2 base emitter junction connects the supply directly to the top of R2, so the emitter of Q1 is only 0.7V below it's collector, with 2.3V just wasted across R2. I'm doubtful if it will work on only 0.7V, but in any case why use a 3V supply and throw 2.3V of it away?

 

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Lightium

Active Member

• Dec 18, 2024

• #6

mmm, I'll keep going and look at what you say and fix it.

 

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AnalogKid

Well-Known Member

Most Helpful Member

• Dec 18, 2024

• #7

Your question is lacking some important information, such as What is the circuit supposed to do? What are the inputs and where are they connected? What is the output and where is it connected?

ak

 

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Lightium

Active Member

• Dec 18, 2024

• #8

It's just an oscillator I am playing with. L1 determines frequency. R1 is the load.

 

Last edited: Dec 18, 2024

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N

NightMoth

New Member

• Dec 21, 2024

• #9

Hello

I breadboarded this circuit. My Vcc is 3.3V, L1 = 1uH||10uH||10uH||22uH = 803nH.
Here is what I got:


here is breadboard:

 

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Lightium

Active Member

• Dec 23, 2024

• #10

NightMoth, thank you so much. 16.7MHz, wow!

 

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rjenkinsgb

Well-Known Member

Most Helpful Member

• Dec 23, 2024

• #11

That could be the voltage regulator IC oscillating as it does not appear to have any decoupling??

The LT1086 must have a capacitor on its output, with fairly critical characteristics.

See "Stability" on page 8 of the datasheet:

https://www.digikey.co.uk/en/htmldatasheets/production/79267/0/0/1/lt1086cm-3-6-tr

 

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Lightium

Active Member

• Dec 23, 2024

• #12

The application info mentions a 150uF cap, but in my sim I get similar values.

 

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N

NightMoth

New Member

• Dec 24, 2024

• #13

Hello

I have added 10uF input and 100uF output cap to LT1086 voltage regulator.
Oscillations still here, but its frequency drops to 15.86MHz.








Also I have checked values of inductors and capacitors from LC tank with RLC meter, and it confirms why oscillations is about 16MHz but not 26MHz:
Ideal: L1 = 1uH||10uH||10uH||22uH = 803nH, C1 = 47pf ==> Fres = 25.907 MHz
Real: L1 = 3uH||10.2uH||10.2uH||21uH = 1733nH, C1 = 56pf ==> Fres = 16.156 MHz
(My 1uH inductor acually is 3uH...and 47pf cap is actually 56pF...)

Also without C1 capacitor, only breadboard parasitic capacitance left, frequency is 20.77MHz.
According to tank frequency formula it means that parasitic capacitance between three rows on my breadboard is 34pF, but I think it is too high capacitance, maybe 20.77MHz is a limit of frequency 2N2222 can support?

 

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Lightium

Active Member

• Dec 24, 2024

• #14

Wow, you got right into it. Thank you NightMoth. Yes, I've notices I can remove C1 and the circuit oscillates differently.

 

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Lightium

Active Member

• Dec 24, 2024

• #15

With C1=34pF and L1=800nH I simulate 23.011MHz.

 

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