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Common Emitter Amplifier - and Diagnosing Problems with an Oscilloscope

richardeberhardt

Sep 30, 2022
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My goal is to learn enough electronics that I can diagnose my tape recorder (circa 1981) problem - there appears to be a problem with the amplification part of the circuit board.

I read a couple of overview books and decided that, before jumping into the tape recorder problem, I would start off by building a breadboard to replicate the design and implementation of a simple common emitter amplifier.

I chose these two YouTube videos for this purpose:

Theory

Implementation

This is my version of the breadboard
Common emitter amplifier Test Circuit -1.jpeg

Testing Set Up

Oscilloscope is DSO Shell by JyeTech
Breadboard
Using the iPhone app Signal Gen
Power supply via Arduino

Results

Input Signal
Oscilloscope Sample on Input.jpg

Output Signal
Oscilliscope Sample on Output.jpg
The input measurement showed what I expected it to show.

The output measurement is not what I was expecting
The general shape of the wave is there, but the image is not stable
The measured frequency of the wave goes up - that looks wrong

The Vpp is higher on the output side than on the input side (23mV to 70mV)

Any help would be appreciated

Thanks,

Rich
 

Bluejets

Oct 5, 2014
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You have no negative supply........it arrives at the breadboard from the Uno but goes no further.
 

danadak

Feb 19, 2021
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Your ground lead to breadboard from UNO R3 is on upper ground rail, but
your circuit is all tied to lower ground rail, you should move that ground to the
lower ground rail. The two red and two blue rails are not tied together in the
protoboard unless you put in a jumper red to red, blue to blue.

Look with your scope at power to ground coming into board. If that is full of
trash signal use a .1 uF ceramic disk and a 10 uF or greater tantalum from the
power to ground at the protoboard. Then look at power rail noise again. If you
do not have a tantalum then use a standard electrolytic for the 10 uF or greater
cap.

IU am not sure but I do not think your base bias R's are wired correctly, check that.

What is your Vce on the transistor and Vbase of the transistor to ground ?


Regards, Dana.
 
Last edited:

richardeberhardt

Sep 30, 2022
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Thank your for your help.

I have corrected the breadboard as directed. It now looks like this:

Breadboard with Notes.jpg


Using the corrected breadboard, here are the testing set ups for input and output measurement:

Testing Input.jpg

Testing Output.jpg

And hear are the corresponding closeups of the scope wave and readings:

Scope - Probe at Input Position.jpg

Scope - Probe at Output Position.jpg

This is closer to what I was expecting.

The capacitors I am currently using are "104"s which I believe have a capacitance of 0.1uF.

I don't think that I have tantalum capacitors, but might have an electrolytic.

Dana, can you give me some clarification on where I should mount the 10uF capacitor (to try to reduce the noise)?
 

Bluejets

Oct 5, 2014
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You cannot use a breadboard and it's layout and expect to get noise free output.

Nil input shielding adds to the dilemma.
 

danadak

Feb 19, 2021
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The 10 uF goes from the + supply rail on the protoboard to the negative (ground) rail
on the protoboard.

The sine out has a high freq noise on it, using your time/div on scope increase it so you
can measure the time between two adjacent peaks of the high freq and calculate that
freq, what is it ? Try to trigger on the highest peak to get a stable waveform at the
faster sweep speed. Could be layout on board is causing transistor to oscillate at a higher
frequency, or something in lab coupling into protoboard.

Scope your power rail, what does that look like ? Suggest AC couple so you can lower
the V/div on scope to see details. What does the peak to peak noise look like ? Use a
battery in lieu of UNO R3 to power, to see if UNO is generating a lot of crap. "Normally"
on a circuit like this on your power rail < 100 mV of noise should be present.

Good practice, short leads, layout.....

1670898634930.png

Regards, Dana.
 
Last edited:

richardeberhardt

Sep 30, 2022
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Joined
Sep 30, 2022
Messages
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The 10 uF goes from the + supply rail on the protoboard to the negative (ground) rail
on the protoboard.

The sine out has a high freq noise on it, using your time/div on scope increase it so you
can measure the time between two adjacent peaks of the high freq and calculate that
freq, what is it ? Try to trigger on the highest peak to get a stable waveform at the
faster sweep speed. Could be layout on board is causing transistor to oscillate at a higher
frequency, or something in lab coupling into protoboard.

Scope your power rail, what does that look like ? Suggest AC couple so you can lower
the V/div on scope to see details. What does the peak to peak noise look like ? Use a
battery in lieu of UNO R3 to power, to see if UNO is generating a lot of crap. "Normally"
on a circuit like this on your power rail < 100 mV of noise should be present.

Good practice, short leads, layout.....

View attachment 57349

Regards, Dana.
Dana,

I tightened up my breadboard using some of the techniques you recommended, it looks like this:
NewBreadboard.jpeg

The display is a lot more stable.

Thank you.

I have a question about the voltage divider resistors. This is in column 10 - across the transistor base.

The R1 is 20k ohms

Since I did not have a 3.6K ohms resistor for the R2, I used three resistors in series to supply the 3.6K : 3.3K ohms + 200 ohms + 100 ohms

Is this OK when using a Voltage divider?

Is my attempt at biasing the transistor messed up?

My next step on your suggestions is to replace the capacitors .

Thanks again.
 

danadak

Feb 19, 2021
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Dana,

I tightened up my breadboard using some of the techniques you recommended, it looks like this:
View attachment 58453

The display is a lot more stable.

Thank you.

I have a question about the voltage divider resistors. This is in column 10 - across the transistor base.

The R1 is 20k ohms

Since I did not have a 3.6K ohms resistor for the R2, I used three resistors in series to supply the 3.6K : 3.3K ohms + 200 ohms + 100 ohms

Is this OK when using a Voltage divider?

Is my attempt at biasing the transistor messed up?

My next step on your suggestions is to replace the capacitors .

Thanks again.

What do you measure with a voltmeter, no signal input to Transistor, on the collector and on the
emitter ?

A tool for transistor biasing : http://www.guitarscience.net/calcs/ce.htm


Regards, Dana.
 
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