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Simple Emitter Follower Circuit Help Plz

Discussion in 'Burnt Fingers DIY Effects' started by sds1, Aug 4, 2018.

  1. sds1

    sds1 Tele-Afflicted

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    I'll try to keep this short because I don't want to muddy it up with a bunch of irrelevant details. I'll add more details as necessary.

    I have been messing with tube circuits for a little while and wanted a primer in transistors. I need a transistor for a switching circuit I'm working on, but as part of the educational initiative I've set out to build a small amplifier circuit.

    All I want to do is build a simple emitter follower amplifier, inject a test signal, and see it working on my oscilloscope. So far I have been unable to do that. I am surely missing something incredibly fundamental.

    Keep in mind the trouble may be with how I'm setting up the test or analyzing my results, not the circuit itself. I just don't know.

    I've looked at a bunch of different basic amplifier circuits but most recently built this one:

    [​IMG]

    For Q I am using BC108, I've also experimented with an MSPA13. For the power supply, I've tried a 9V battery as well as a 9V AC adapter. I get different results.

    Mostly the output sine wave has a bunch of noise on it, it's not clean and I don't know what to make of it. When using the 9V adapter I can generate what appears to be 120Hz wave greatly amplified, with my test signal superimposed (but presumably not amplified). When I use the 9V battery I just get some noise back of no significant increase in amplitude over the test signal.

    I feel like cleaning up all the noise I'm getting on my scope is one objective here. I can generate a clean sine wave on the scope using the test signal, so the signal, probes, and scope are all OK. It's when voltage is applied, or the signal passes thru the transistor, is when noise is being generated or collected.

    Let me leave it at that before I ramble on too long. Anything obvious going on here? What questions do I need to answer in order to move forward?

    Thanks in advance.
     
  2. luckett

    luckett Banned

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    That has a very low input impedance for guitar, but may work with your signal generator depending upon it's output impedance. Remove the 20uf cap to reduce the gain if you want a clean output. You also want to put a cap on the output to filter DC.

    Take a look at the lpb1 input for something more typical for guitar use.
     
  3. Inglese

    Inglese Tele-Meister

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    This circuit has a very high gain and a very low input impedance, too much of the first and too low the second for audio purpose...
    Start removing the cap across emitter resistor as a starting point. This will vive you a small gain of x5 approx but still with a very low input impedance (less than 1k).
    Some theory study won't hurt.
     
  4. Inglese

    Inglese Tele-Meister

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    And this is not an emitter follower...
     
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  5. sds1

    sds1 Tele-Afflicted

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    So I had a few issues at once, none being directly related to any circuit. First, it helps tremendously to connect the output signal probe ground to the circuit ground. This cleaned up a messy scope display.

    But I also had power supply issues. Mainly, the 9V adapter I grabbed out of the closet was noisy. Putting the scope directly on the 9V adapter yielded a huge 120Hz waveform. So the adapter would be considered bad yeah? I reckon when they fail they introduce ripple like the DC power supplies in my tube amps?

    Thanks! I did some reading on input impedance. It seems to mostly affect frequency response? Can there be other more detrimental side effects to low input impedance -- can it just not work at all?

    FYI Adding a 1M resistor to ground at the input, as with lbp1 and basically all tube amps, did not affect my experiments either way.

    I also didn't find any benefit (in the context of this experiment) to the input/output capacitors. I get that they block DC and also participate in RC/CR filters, but in the context of this experiment they seem unnecessary.

    I did! I build the lpb1 circuit but with Q being a BC108. I'm now getting an output waveform about 5X bigger than my test signal.

    The output waveform jumps up and down a little, looks doubled up a bit... besides clipping the ground lead are there any other ways to stabilize this waveform on the display?

    Yeah I've been doing some reading, it doesn't all sink in at once over here. This experiment was a hands-on exercise to supplement my reading.

    Common Emitter... my mistake!
     
  6. Henry Mars

    Henry Mars Tele-Afflicted

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    That is a common emitter ckt the input z at the transistor is is probably around 2K ohms depending on Beta and re'.: the ckt input z is well under 1k .... The unloaded output Z is a little less than 4.7k Ohms. you probably shouldn't be playing around with high voltage electronics ... this is pretty basic stuff.
     
  7. sds1

    sds1 Tele-Afflicted

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    Thanks High Voltage Henry but I've been sticking my tongue on 9V batteries since I was a kid. Nothing bad happens! You're not going to trick me into being too scared to learn and ask questions. Nice try!
     
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  8. Henry Mars

    Henry Mars Tele-Afflicted

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    on the contrary .. I am encouraging you to find things out before you plunge. There are many ways to kill yourself with electricity ...
     
  9. luckett

    luckett Banned

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    You could just have a power supply with poor or no filtering. It sounds like it works fine otherwise.

    If the impedance mismatch is big enough, you could have very low output levels at all frequencies. It depends on the specifics of the circuits.

    You can't just add a 1M resistor to the first circuit. That resistor is in parallel to the other resistors in the circuit that will affect the impedance. You also have to consider changes in bias by using difference resistor values.

    They may not be necessary for what you're doing, but it's good practice and eliminates any potential problems from having DC on the output.

    The lpb1 should be set to around 25x gain and will clip the transistor with a pretty low signal level on the input. Do some reading on setting gain in transistor circuits and you should be able to figure out how to lower the gain to your needs pretty easily.
     
  10. sds1

    sds1 Tele-Afflicted

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    No, you suggested I am over my head and in imminent danger because I don't understand something so "basic" as input impedance. What utter nonsense. I've added you to my ignore list along with the others who like to talk down their noses to DIYers in DIY discussions. Go troll someone else.
     
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  11. sds1

    sds1 Tele-Afflicted

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    I mean it was working in the sense it was generating 9V, but it was impossible to generate a clean waveform with that power supply. My output signal was being added to 120Hz waveform of much greater amplitude. I got another power supply and that problem was correct.

    Lemme ask you this, if I put my oscilloscope directly on some random 9VDC power adapter out of the closet like I did, should I see a massive AC waveform?

    EDIT: here, lemme be specific. If I put my scope directly on the "bad" 9V AC adapter, I get the following waveform on my scope. It's 12Vpp. This waveform dominates the output of any amplifier circuit I tried to build using this adapter.

    [​IMG]

    Sorry, I was talking about the lbp1. Whether the 1M input to ground was present or not in the lpb1 circuit, it made no difference in the context of this experiment. I'm sure it matters greatly when using a guitar and ears though.

    Substituting a BC108 for Q1, I've built the lpb1 circuit per the following schematic and am getting only 5X gain on output. Should I blame the BC108 for the lower gain, or am I looking for some other reason?

    Thanks for your help. :)

    [​IMG]
     
    Last edited: Aug 7, 2018
  12. luckett

    luckett Banned

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    Somethings not right if you're getting 12V AC on the output of a 9V DC adapter.

    A BC108 should be able to handle that amount of gain as long as voltage of the input signal is small. Are you getting a distorted or clean output at 5x gain?
     
  13. sds1

    sds1 Tele-Afflicted

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    Yeah I chucked that adapter.

    The 5X output waveform was clean but bouncing around on the scope.
     
  14. SngleCoil

    SngleCoil Tele-Holic

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    Good grief, yes! Thank you. I'm on almost the same track you are right now. God forbid someone tries to learn by combining theory with a little hands on trial and error experience.
     
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