Author Topic: Transistor amplifier parasitic oscilation  (Read 1096 times)

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Offline ovidiu231Topic starter

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Transistor amplifier parasitic oscilation
« on: September 13, 2019, 10:36:45 pm »
Hello,I'm new here. I got a big problem, I build a class A amp with 2n2222a, biass point consist of 2x 10k resistors, on colector I have 4.7k and on emitor 2.2k. When I inject the signal from my function generator i get this...
Anyone have an ideea what is going on?
 

Offline bob91343

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Re: Transistor amplifier parasitic oscilation
« Reply #1 on: September 13, 2019, 11:12:16 pm »
I can't read the voltage scale but I suspect you are overdriving it.
 

Online Circlotron

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Re: Transistor amplifier parasitic oscilation
« Reply #2 on: September 13, 2019, 11:45:05 pm »
Is there a coupling capacitor between your signal generator and the base of the transistor? A schematic diagram would help.
 

Offline ovidiu231Topic starter

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Re: Transistor amplifier parasitic oscilation
« Reply #3 on: September 13, 2019, 11:47:23 pm »
It's 1V/div
 

Offline ovidiu231Topic starter

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Re: Transistor amplifier parasitic oscilation
« Reply #4 on: September 13, 2019, 11:52:52 pm »
I tried with all kinds of caps. When I increase the amplitude, the bump increases too.
 

Offline kg4arn

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Re: Transistor amplifier parasitic oscilation
« Reply #5 on: September 14, 2019, 12:00:35 am »
Schematic please. Show the test point also.
Sorry I missed it :palm:
« Last Edit: September 14, 2019, 03:04:03 am by kg4arn »
 

Online ledtester

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Re: Transistor amplifier parasitic oscilation
« Reply #6 on: September 14, 2019, 01:36:18 am »
Check the DC biasing of the collector. With 10K / 16K biasing of the base LTspice says it is around 3.3V which is much too low.

Change the 16K resistor to 2K and see what happens. The collector should be around 7.4V and you should see an amplification of about 1.75.

834819-0

Update: Attached the ltspice files.
« Last Edit: September 14, 2019, 01:56:56 am by ledtester »
 

Offline T3sl4co1l

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Re: Transistor amplifier parasitic oscilation
« Reply #7 on: September 14, 2019, 03:34:25 am »
That's what happens when the transistor saturates and the function generator is driving the emitter resistor.  Draw out the voltages, you will see why this is so. :)

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline bob91343

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Re: Transistor amplifier parasitic oscilation
« Reply #8 on: September 14, 2019, 04:38:54 am »
As I said, you are overdriving it.  Back off on the input signal and it should clean up.
 

Offline ovidiu231Topic starter

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Re: Transistor amplifier parasitic oscilation
« Reply #9 on: September 14, 2019, 10:27:17 am »
It worked, thank you so much❤
 


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