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Old 10th May 2016, 07:25 AM   #1
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Default Understanding Transistor amplifier with an online simulation

Transistor amplifier simulator

Not sure if this goes in 'tools and software' but since I ma thinking of building a transistor amplifier

Build low power Transistor Amplifier

I need to undertand the concepts - and this simulator seems to fit the bill.

Circuit Simulator Applet

From the circutis menu, navigate to:

Circuits > Transistors > Common Emitter Amplifier

I changed the Beta to 35 from 100. Next I change the voltage to 9 V what is available.

I get a clipped sine wave and its in negative territory. What next?
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Old 10th May 2016, 09:23 AM   #2
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beta = 35? Seems a bit low.

The problem is twofold:
1. The output RC highpass (5/1meg) has a loooong time constant, and this simulator allows swapping out component values on the fly. Put in 1 + 100k instead, and you should see DC-centered output after a while. (I recommend swapping out the other 5 as well.)

2. This common emitter amplifier has too much gain (~10x) to support 500 mVp / 1 Vpp input on +9 V (1 Vpp x 10 = ?). Even if you optimize the bias resistors by e.g. swapping out the 110k for a 75k, you'll still get rounded tops. Things look better at 300 mVp.

Have you already done a bit of reading yet? Simulators are handy for playing around once you have grasped a few basic concepts.
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Old 10th May 2016, 11:15 AM   #3
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Default Gain

Quote:
beta = 35? Seems a bit low.
Beta for this transistor:

MPS2907. Data sheet gives

DC Current Gain
(IC = −0.1 mAdc, VCE = −10 Vdc)
(IC = −1.0 mAdc, VCE = −10 Vdc)
(IC = −10 mAdc, VCE = −10 Vdc)
(IC = −150 mAdc, VCE = −10 Vdc) (Note 1)
(IC = −500 mAdc, VCE = −10 Vdc) (Note 1)


75
100
100
100
50

Quote:
The problem is twofold:
1. The output RC highpass (5/1meg) has a loooong time constant, and this simulator allows swapping out component values on the fly. Put in 1 + 100k instead, and you should see DC-centered output after a while. (I recommend swapping out the other 5 as well.)
I presume you mean the 5 uF capacitor and the 1Mega Ohm resistor on the right side of the diagram? OK. That did not do any harm, the output sine wave amplitude increased about threefold.

Quote:
2. This common emitter amplifier has too much gain (~10x) to support 500 mVp / 1 Vpp input on +9 V (1 Vpp x 10 = ?). Even if you optimize the bias resistors by e.g. swapping out the 110k for a 75k, you'll still get rounded tops. Things look better at 300 mVp.
I simply do not understand this part - 500 mVp - millivolts ? p? In any case the AC input is set as 500Mv however for an actual input from a mini rca output such as from a phone or MP3 player how can I reduce the output?

I need to do more reading, but these electronics sites are confusing especially the circuit diagram with the infamous "ground". I really wold prefer if they showed two terminals.

Last edited by BasicHIFI1; 10th May 2016 at 11:18 AM.
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Old 10th May 2016, 11:39 AM   #4
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Default Screenshot

Screenshot with the changes
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Old 10th May 2016, 12:30 PM   #5
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For a sine wave:

Vp = peak voltage;
Vpp = peak-to-peak voltage;

Vrms = root mean square voltage;
Vrms = Vp/Sqr(2); that's what you will see if you measure with your DMM (AC).

Why do you call the ground "infamous"? Everything marked with the "ground" symbol is connected together - this is a common ground bus. On the large complicated schematic diagrams it's much easier to use the ground symbols than connecting all these points with a wire - it would be difficult to follow.
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Old 10th May 2016, 12:35 PM   #6
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Quote:
Originally Posted by BasicHIFI1 View Post
Beta for this transistor:

MPS2907. Data sheet gives

DC Current Gain
(IC = −0.1 mAdc, VCE = −10 Vdc)
(IC = −1.0 mAdc, VCE = −10 Vdc)
(IC = −10 mAdc, VCE = −10 Vdc)
(IC = −150 mAdc, VCE = −10 Vdc) (Note 1)
(IC = −500 mAdc, VCE = −10 Vdc) (Note 1)


75
100
100
100
50
So, why only 35 then? This particular circuit (with bias network = 75k/10k) draws about half a mA of quiescent current, so for most of the voltage swing beta should be above 75 and generally closer to 100. Plus you may have noticed that these are given as minimum values, and the graph of typical beta at normal temperatures is well above 100 even at 0.1 mA Ic.

Be careful to avoid using transistors in areas of heavy beta droop, which is above about 200-400 mA in this case. This could lead to some nasty effects, not to mention that GBW (datasheet Fig. 10) is dropping like a rock and may be good for some nasty intermodulation distortion.
Quote:
Originally Posted by BasicHIFI1 View Post
I presume you mean the 5 uF capacitor and the 1Mega Ohm resistor on the right side of the diagram? OK. That did not do any harm, the output sine wave amplitude increased about threefold.
Are you sure it's not just the scaling that changed?
Quote:
Originally Posted by BasicHIFI1 View Post
I simply do not understand this part - 500 mVp - millivolts ? p?
There are a few different ways of specifying AC amplitudes in periodic signals:
peak-to-peak: The distance between the very minimum and maximum of the waveform. If the latter is described as f(t) = A sin(ωt), the peak-peak amplitude is 2 A. Peak-peak voltages are given in Vpp or Vp-p (using any SI prefixes you can think of).
peak: The difference between zero volts and maximum amplitude, assuming a symmetrical waveform. For our sine f(t) = A sin(ωt), the peak amplitude is A, exactly one half of peak-peak voltage. Peak voltages are given in Vp. I used this because it is what the voltage source in simulation is set up with.
RMS (root mean square): The time integral of the waveform. This is useful for power comparisons to DC. For our sine f(t) = A sin(ωt), the RMS amplitude is sqrt(2) A. RMS voltages are given in Vrms.
In systems of known (load) impedance, it is common to specify load power levels instead, so you might see mW or W into N ohms. dBs referred to known power levels are also commonly used, e.g. dBm (re: 1 mW) or dBf (re: 1 fW) in RF. The dBu you find in pro audio also derives from there originally (nowadays re: 775 mV, which happens to be 1 mW @ 600 ohms - but nobody uses 600 ohm inputs in PA any more).
Quote:
Originally Posted by BasicHIFI1 View Post
In any case the AC input is set as 500Mv however for an actual input from a mini rca output such as from a phone or MP3 player how can I reduce the output?
Using the volume control on said device sounds too simple...?
Otherwise, a voltage divider?

BTW, there is no such thing as a "mini RCA", you surely mean a 1/4" / 3.5 mm minijack.
Quote:
Originally Posted by BasicHIFI1 View Post
I need to do more reading, but these electronics sites are confusing especially the circuit diagram with the infamous "ground". I really wold prefer if they showed two terminals.
You're actually in good company there (Bruno Putzeys claims to never have seen a one-terminal voltmeter either). "Ground" is a shortcut that makes for neat and tidy schematics, nothing more. It's a node that you declare Your Reference Potential. Now a physically extended node that keeps the same potential everywhere under any and all conditions does not exist, of course - finite conductivity and inductance have a say in that. Hence why in layout, considerable effort may be invested into getting the grounding right. Currents do, after all, flow in loops, and things may get ugly if large and distorted currents share the same paths with very small but critical ones.

I do think you should look for a good electronics textbook.
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Old 11th May 2016, 05:54 AM   #7
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Default OK

I must say you guys are an extremely helpful and patient bunch. Has it do do with working with lethal voltages?

Yes, the amplitude has changed.

Recommend a good electronics textbook - something on Kindle and less than $10? OR a website?

The second one in this list is nicely illustrated.

Electronic Circuits: The Definitive Guide to Circuit Boards, Testing Circuits and Electricity PrinciplesMar 19, 2016
by Wayne Charles

Make: Electronics: Learning Through Discovery 2nd Edition
by Charles Platt (Author)


https://www.amazon.com/Electronics-G...s=books&sr=1-4

I learned my electricity from this book, 1982 or so edition: so will download and review.
https://archive.org/details/AdvancedLevelPhysics

Last edited by BasicHIFI1; 11th May 2016 at 06:03 AM. Reason: added info
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Old 11th May 2016, 08:07 AM   #8
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Quote:
Originally Posted by BasicHIFI1 View Post
Recommend a good electronics textbook - something on Kindle and less than $10? OR a website?
How about both? And zero cost?

Six free, open-source, electronics textbooks:
https://www.ibiblio.org/kuphaldt/electricCircuits/

A knowledgeable enthusiast's website, full of non-technical explanations of electronics and electricity concepts: FUNWITHTRANSISTORS.NET

-Gnobuddy
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Old 11th May 2016, 10:32 AM   #9
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Zero cost is good, thanks.

Of course I have to pay the few fractions of a cent for the download.

Not strictly zero then.
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Old 11th May 2016, 10:48 AM   #10
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About the ground symbol:

As a beginner I find the earth symbol confusing. I believe any diagram should be clear and unambigous. To complete a circuit you need all the components connected, a simple earth symbol without knowing where it connects to, is confusing. Also, there is nothing about wire lengths or how they are to be routed so as not to cause interference.

The "AC Source" has a single connection to the base, there is no indication of where the other end goes, presumably to the earth.

This online app displays a proper two-terminal source:

Online Circuit Simulator with SPICE
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