# Function Generators Role and Effect

Discussion in 'General Electronics Discussion' started by Lawrenciumbc, Nov 2, 2015.

1. ### Lawrenciumbc

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Nov 2, 2015
I am new to electronics and just trying to get my head around the use of function generators. I understand that they are used in labs to generate signals for testing, but are they more like a microphone for example which picks up an audio wave and sends it into a circuit (i.e. nothing to do with power) or are they more like a low power supply for AC?

My question is more about how I should think of them within a circuit and whether the peak to peak voltage is just additional voltage into the circuit on top of my power supply.

The particular context is that I am learning about transistor biasing (for a LiFi transmitter) and I have a 5V VCC going into the circuit at the collector of my transistor (via an LED which is also voltage divided to the base too) and then a function generator going to the base which supplies a wave (5Vpk) which will just switch the transistor on and off and I am confused as to whether and the how the peak to peak voltage contributes to the circuit.

2. ### BobK

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Jan 5, 2010
A function generator is for generating a signal for testing. It is not supposed to supply much in the way of power.

You are using it correctly to apply to the base of the transistor, though hopefully this is through a resistor since the BE junction acts just like a diode and does not limit the current. And 5V is a bit high for a signal to an amplifier, your amp cannot put out more than 5V so the signal should be much smaller than that.

Bob

3. ### Lawrenciumbc

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Nov 2, 2015
Ok, so essentially the function generator will increase the voltage in the circuit by whatever its peak voltage setting is? e.g. hypothetically, if I have a voltage divider from my VCC channeling 0.5V to my base and then have 5V coming in on the the FG, then my voltage to the base would be the 0.5 + 5?

If that is the case, then would I be right in thinking that I should use my voltage divider to create most (90% or more) of the voltage I need at the base and then the FG wave just tops it up as it's the fact that its a wave that I need for this use rather than any extra voltage it brings (voltage here being the the byproduct)?

It's going through a resistor though i've not actually built the circuit yet, I'm just simulating to get the numbers right which is where my understanding of the role confused things.

4. ### AnalogKid

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Jun 10, 2015
This discussion will go more smoothly if you post your schematic. In general, you are correct. The signal coming in from the function generator combines (adds or subtracts) with the DC bias at the base, and the combination determines the base current. The circuit you are describing can be either a linear amplifier or a saturating (non-linear) switch, and the relative amplitudes of the signal and the bias can be quite different depending on how your want the transistor to operate.

ak

5. ### Lawrenciumbc

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Nov 2, 2015
Ok no problem, here it is. Sorry its not great quality, I don't have access to the sim at the moment so its a screenshot I took a few days ago.

Bear in mind that I haven't necessarily got the values for everything right yet, i'm just trying to understand as I go!

Linking to my question, you can see the FG on the left coming into the base. Is the 5V peak it is set to literally supplying an extra 5V to the circuit?

If its not clear, the multimeter on the image is set up across the Base and Emitter showing the 551mV. My understanding of the circuit idea was to just get the transistor to turn on and allow the LED to be on but not at full brightness. The FG would then provide a small amount of extra voltage to modulate the LED (not necessarily visible to the human eye so that data can be sent through but it is still a functional light source). I presume the transistor will therefore need to be setup so that it fluctuates between its active and saturated states when the FG voltage is high/low?

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6. ### AnalogKid

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Jun 10, 2015
In your image the scope shows a amplitude of 11 V, but Vcc is 5 V. How do you explain that? Also, connect the other scope channel to the input so you can see exactly what the transistor is doing.

If the intent is to switch the LED on and off efficiently, with little or no intermediate dim operation, then there is a different circuit configuration which is better.

Yes, the signal generator's energy adds to th energies already in the circuit, but don't confuse a 5 V signal with a 5 V power supply. Usually the signal current drawn from the function generator is very small.

ak

7. ### Lawrenciumbc

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Nov 2, 2015
I think this is what I and getting confused with. The VCC supplies the circuit with 5V worth of voltage and the components draw relevant current from that, but when the signal generator is introduced, are you saying that 5V from that (albeit an AC wave), isn't providing a similar effect but from a different point in the circuit?

How do I go about calculating the effect the signal generator is having on the circuit? I understand Ohm's law and voltage dividers and therefore everything up to adding the signal generator makes sense, it's just after that point that I'm not sure how to calculate. For instance without the SG/FG, the voltage delivered to the base is (24/(51+24))*5 = 1.6V, but adding a 5V peak wave from the SG/FG, makes it 551mV, but I don't understand how if it is 'feeding' 5V extra in? - I'm sure it is something at the fundamental level but I can't work out what.

I'm not sure, I don't understand electronics enough yet :s . At a complete guess, is the transistor in the forward active state and amplifying the voltage across the collector and emitter? Even then, I only thought amplification could only be to a maximum of the original voltage coming in?

Well its not to turn it all the way off or necessarily to have a noticeable dim (so as to maintain its use as a solid light to the human eye), the intention is to dim and 'light/bright' it very slightly using the transistor states so in effect, a receiver with a photodiode on could pick up the modulation in it over a short distance (the other half of the project :s). The general layout of the circuit was given to me and I have to work out the correct values to get it working as required. - I hope that is a little clearer

Many thanks for your help btw!

8. ### AnalogKid

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Jun 10, 2015
So this is the front end of a light beam communicator or audio transmitter?

ak

9. ### Martaine2005

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May 12, 2015
Hey Bob,
Looking rather 'shaken and not stirred' these days!
Bob 'licensed to K'... I like that!

Martin

10. ### Lawrenciumbc

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Nov 2, 2015
Its a light beam communicator, nothing to do with audio.

I mentioned audio waves in my original question as I felt it was easier to explain my confusion with function generators with that analogy without going into the actual project I am working on.

11. ### BobK

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Jan 5, 2010
Actually, this was my Halloween costume as Gomez Addams. Here is the full picture, including my lovely wife Morticia.