Measuring transistor gain with Oscilloscope? (And introductions)

cube01

Joined Mar 10, 2011
16
Hey everyone... I've been trolling here for a while and thought I'd pop my head in with a question -

But first off, I thought I'd introduce myself - My name's Dustin... I tinker around with electronics for no other reason than for fun and as a learning experience... Needless to say I'm still learning

I recently acquired a PicoScope digital oscilloscope and waveform generator and am having a blast with it. However I'm not great with it and still don't know how to measure some simple things.

My question is this - I'd like to hook up some transistor circuits on the breadboard and see their gain through my 'scope, but I'm getting inconsistent results - mainly because I don't know where to hook the probes up into the transistor circuit... I'm trying to take the output from the waveform generator, hook it up to the 'scope, and then read the output.

Where should I be hooking the probe and ground clips to in the circuit in order to get "correct" results? That is, if the transistor gain should be X, where do I hook in to see X?

Thanks for your time everyone, and I look forward to contributing here! (or at least benefiting from everyone's collective knowledge and hopefully passing it on elsewhere )

Also, as an afterthought, does anyone know of any good resources on educational experiments to do with an oscilloscopehr I've picked up a few around the web through searches, etc, but my collection is pretty piecemeal.

Thanks again!!

ErnieM

Joined Apr 24, 2011
8,045
<bump>

Well.. there is no short answer as the "gain" of a transistor changes depending on how you are using it. As a minimum you would need a power source and some resistors to turn the transistor on and see what it is doing.

Before you try to do some measurements you would be best served if you first learn how transistors work. There are some lovely tutorials on the top of this page. Introduction: BIPOLAR JUNCTION TRANSISTORS may be a good place to start.

Welcome to the forums!

studiot

Joined Nov 9, 2007
4,998
Well hello cube and welcome.

Firstly it is really important for us to know if you can read circuit diagrams, even if you can't fully understand them.

This ability is vital to progress your aims.

go well

cube01

Joined Mar 10, 2011
16

As far as my ability level, I have a pretty decent understanding of electronics, including transistor usage in common-base, emitter, and collector circuits. I can read circuit diagrams and figure out the transistor gain mathematically of a transistor circuit, no problem... However, this is my first experience with an oscilloscope.

What is throwing me off with my o-scope, specifically, is the ground clips... I'm not sure where to clip them in, mainly because I can see the output of the generator on the 'scope if I touch the waveform generator output probe to the input probe without using the ground clips. This confuses me and causes me to question what I'm supposed to do with the ground clips.
(The waveform generator is a component of the o-scope, if that provides any clues.)

Unfortunately I don't have a manual for my scope or the probes. All I have is the manual for the software that it uses, which is not helpful in terms of setting up the probes, etc.

To put my question in a more straightforward way - Where, in a common-emitter transistor configuration, should I clip in my waveform generators leads? And where, in the same circuit, should I clip in my o-scope probe and ground clip if I want to see the output signal of that circuit?

Thanks again guys!

studiot

Joined Nov 9, 2007
4,998
Your scope ground' connection goes to common, ie the zero voltage rail in the power supply to your transistor circuit under test.

Your scope may have an ac / dc connection for the input - use the ac setting.

If not, you will have to connect the scope input through a series capacitor so as not to disturb the dc levels in your circuit under test.

Similarly if you are injecting a signal from the sig gen you may need to inject through another series capacitor.

The input to the circuit under test goes to the base for a CE or CC amp and the emitter for a CB amp.

The output is taken across the collector resistor ie from the collector, or possibly from the emitter for an emitter follower.

go well

Joined Dec 26, 2010
2,148
Your scope ground' connection goes to common, ie the zero voltage rail in the power supply to your transistor circuit under test.

Your scope may have an ac / dc connection for the input - use the ac setting.

If not, you will have to connect the scope input through a series capacitor so as not to disturb the dc levels in your circuit under test.

Similarly if you are injecting a signal from the sig gen you may need to inject through another series capacitor.

The input to the circuit under test goes to the base for a CE or CC amp and the emitter for a CB amp.

The output is taken across the collector resistor ie from the collector or possibly from the emitter for an emitter follower.

go well
I really cannot agree with the statement (emphasised above) that AC coupling is always necessary to avoid (serious) disturbance to the working point, at least provided that reasonably low resistor values are used in the base bias circuits - of the order of 100kΩ or lower.

Like many instruments, the Picoscope appears to have a 1MΩ input impedance, or at least this model does: http://www.picotech.com/document/datasheets/PicoScope2100.pdf While loading by a 1MΩ impedance to ground will disturb the bias point somewhat, especially when probing the base circuit, this will usually not be critical.

With the impedances normally seen in emitter and collector circuits, connecting such an instrument will often have little effect.

It may be very instructional to the OP to observe the voltages in his circuits complete with DC content, in order to see for himself the effects of biasing.

Joined Dec 26, 2010
2,148
Excellent question! If the OP has some contraption with for instance 50Ω fixed input, my last comments would be rubbish.

studiot

Joined Nov 9, 2007
4,998
I really cannot agree with the statement (emphasised above) that AC coupling is always necessary to avoid (serious) disturbance to the working point, at least provided that reasonably low resistor values are used in the base bias circuits - of the order of 100kΩ or lower.
No it is not always necessary.

But cube is self-confessed new to scopes and has to start somewhere and he (she?) did ask about ac conditions.

Taking a cavalier attitude to poking about the DC side of transistor (or other ) circuits with a scope probe is not to be recommended to experienced users, let alone beginners lest they encounter unexpected high voltage.

Yes it can be either instructive or annoying to watch the ac trace jump about the screen as you move from dc bias voltage to dc bias voltage when using the dc setting. Further if your input sensitivity is high you can actually drive the trace off the screen. Many a tester has wondered where his trace has gone - that is why trace locator buttons were incorporated in some scopes.

go well

Audioguru

Joined Dec 20, 2007
11,249
A transistor as an amplifier without any negative feedback has a voltage gain of about 180 but when the output signal is anywhere near max output then the top part is compressed and the bottom part is expanded in amplitude. It results in severe distortion.

EDIT: I forgot to say that it is very difficult to measure the gain of a severely distorted circuit.

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ErnieM

Joined Apr 24, 2011
8,045
As the Picoscope is a low cost USB device I would be very surprised if the "ground" clip of the scope probe is not connected some way to the AC main power in your workshop.

Why? The ground of the USB cable for reasons of cost will probably be connected to the measurement circuitry: that's where it's power is coming from. And every PC I have seen (and I have not seen them all) has USB ground and every ground of the PC going back to the AC line, probably to neutral but probably to earth ground.

You can check this out with just an ohmmeter: unplug your PC and see if you get continuity between the probe ground and one or more pins of the PC's plug.

Now that is OK for most work (my \$ scope has the same connection) but something to keep in mind, you can't stick the scope ground everywhere.

Joined Dec 26, 2010
2,148
Note that injecting signals from an oscillator/function generator is typically a very different situation from monitoring them.

Most signal generators do have relatively low output impedances, of the order of 50Ω to 600Ω. In this case a DC blocking capacitor is a wise precaution, for the protection of the generator as well as the circuit you are working on.

studiot

Joined Nov 9, 2007
4,998
As the Picoscope is a low cost USB device I would be very surprised if the "ground" clip of the scope probe is not connected some way to the AC main power in your workshop.
Good point unless the pc used is a laptop.

Joined Dec 26, 2010
2,148
There may be a hidden danger here: such a "scope" driven off an isolated laptop constitutes an "off-ground" scope. If the "ground" clip is then taken to a high potential the whole system may become live.

Doing this with a conventional grounded scope generally results in a loud bang and some damaged equipment. This is unfortunate, but an isolated instrument may silently float up to a hazardous potential.

studiot

Joined Nov 9, 2007
4,998
My guess is that we may be over egging the pudding.

It is right to develop good laboratory/workshop habits but I guess cube is just getting the feel for practical work at low voltage.

With that in mind Adjuster is quite right in suggesting that a scope may be considered as a 'visual voltmeter' and connected to the circuit in much the same way as you would a multimeter. With the exception that the scope is more expensive and more vulnerable to damage so greater safety precautions are in order as well as the usual measurement ones you would take to obtain sensible readings with a meter.

There are also further considerations once to get into higher frequencies.

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cube01

Joined Mar 10, 2011
16
Welcome Dustin,

What model of Picoscope did you get?
http://www.picotech.com/oscilloscope-specifications.html

Sounds like fun,
Ifixit

Hey fixit...
I am having a good time, for sure... I got this guy (I didn't see him in your list):

https://www.alliedelec.com/search/productdetail.aspx?SKU=3890047

To answer some of the other items that came up -I am a 'he' , I am using a laptop, and studiot is correct in that I'm working with low voltages... I'm really just having a great time experimenting and seeing the practical results and mathematical aspect of electronics... I've been at it for about a year or so.

Still unsure what to do with these ground clips though
They don't behave like typical multimeter leads do, like I had expected. If I hook up my scope and generator probes as if they were signal source and multimeter leads, my output flat lines...

Thanks again for everyone's interest. It looks like a good group here...

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cube01

Joined Mar 10, 2011
16
Still unsure what to do with these ground clips though
They don't behave like typical multimeter leads do, like I had expected. If I hook up my scope and generator probes as if they were signal source and multimeter leads, my output flat lines...
To illustrate a bit about what I was saying, please see attached thumbnail...

Obviously it is a simplified version (no base bias, etc) - here is how I am connecting my waveform generator and o-scope - If I connect the probes (without ground clips) to each other, I see a good signal on the 'scope, but if I connect my probes into the transistor circuit, using the ground clips of the probes as shown, I get a 0 V output... These situations both confuse me.
How can I get a signal on my O-scope with only one lead connected? And how come my signal falls flat when connected as shown?
I have verfied that the transistor is good, and not operating in cutoff or saturation during my measurments.
(Again, the waveform generator and o-scope are physically in the same 'box', but are shown separately here for simplicity)

P.S. I also realize that there me be some jealousy concerning my skills with photoshop. Please do not be hard on yourselves - This level of graphic design takes practice.

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Audioguru

Joined Dec 20, 2007
11,249
Without the ground probe connected to the 'scope then it shows mains hum picked up by the "hot" probe.

You don't say what is the output level from the signal generator. It might be too low to turn on the unbiased transistor.

cube01

Joined Mar 10, 2011
16
Without the ground probe connected to the 'scope then it shows mains hum picked up by the "hot" probe.

You don't say what is the output level from the signal generator. It might be too low to turn on the unbiased transistor.

If that were the case, and the transistor was operating in cutoff, then I should see more than 0V, right? I'd see my batter voltage minus the drop across the resistor.

Audioguru

Joined Dec 20, 2007
11,249
If that were the case, and the transistor was operating in cutoff, then I should see more than 0V, right? I'd see my battery voltage minus the drop across the resistor.
Correct.
If the NPN transistor is not biased then it is not turned on and the collector will be at the positive battery voltage, not at 0V which you measured.

But since your extremely simple circuit does not have a series base resistor to limit the base current then maybe your signal generator has blown out the base-emitter of the transistor.