Emitter follower unable to drive 16ohm headphone?

Discussion in 'Analog & Mixed-Signal Design' started by yesilovethis, Jul 28, 2016.

  1. yesilovethis

    Thread Starter New Member

    Jul 28, 2016
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    Hi, I am amateur and trying to make a portable VLF whistler receiver working with 9V battery. My circuit consists of 3 active element. The first is a MPF102 FET in common source mode with voltage gain of approx 6db (+2). Then I use a BC547b transistor in common emitter mode with voltage divider bias and emitter register bypassed by 47uf tantalum cap. This transistor gives me voltage gain of Av = 150 (at Ic = 1mA), so the overall gain is 300. With a 2mV sinewave (1KHz), I get about 0.6 volt PP in the collector of BC547b (with a 10uf coupling cap) and 0.3v PP at a load of 5K. So I thought it is enough voltage so I would just add an emitter follower to drive my headphones. So I added another BC547b in emitter follower mode with base register Rb=560K and Re=1.0K (Ic=4mA). The oscilloscope shows nice sinewave with slightly reduced voltage (0.5volt pp) but when I add a headphone with a 100uf cap to emitter of last transistor I only hear put-put-put sound and the scope shows lower voltage spikes. I don't understand what is going wrong? why the last transistor cannot drive the headphone when it supposed to be a low output impedance driver? Thanks for reading the details and any suggestion welcome.
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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  3. yesilovethis

    Thread Starter New Member

    Jul 28, 2016
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    Hi,
    Thanks for your reply with schematic. I have attached my circuit. The 560K is connected from Vcc to base of final transistor. See figure.
    Thanks,
    -R
     
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    I suspect the problem is that the 560k resistor is too big, try 220k.
    Or connect the base of T3 directly to the collector of T2.
     
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  5. #12

    Expert

    Nov 30, 2010
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    I think it is the 1 k emitter resistor. Q3 can dump current into the capacitor and then to the headphone, but the 1k resistor needs to dump that charge on the negative half cycles as fast as the charge is added in by Q3 during the positive half cycles. This amplifier will load up a positive charge on the left of the output capacitor and then most of the current will stop flowing.
     
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  6. yesilovethis

    Thread Starter New Member

    Jul 28, 2016
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    Thanks for the reply but I do not undertand much about the dumping and cycles. Could you please explain in Layman's terms and also what changes I have to make to make this work right way? I learned all the biasing 'theories' from my Bachelor's Electromagnetic text books, so no practical experience (e.g. to make the current in the register divider bias ten times the base current: that I learned from online blog, not from text book).
     
  7. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    As mentioned previously, there are a couple of obvious problems. The 560k and the 10uF capacitor should be removed as they prevent T3 from driving the headphones on the positive half cycle and the 1k resistor cannot sink enough current to drive the headphones on the negative half cycle of the waveform. Think about it, you're trying to drive a 16Ω headphone via a 1kΩ resistor, which one gets all the voltage?

    You should investigate push-pull output stages.
     
  8. yesilovethis

    Thread Starter New Member

    Jul 28, 2016
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    Thanks for explaining the problem. I would modifiy the circuit and check if it is working and post update.
     
  9. BobaMosfet

    Active Member

    Jul 1, 2009
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    Your primary problem is your JFET. Wrong component. Too much impedance at low Vgs. At 2mv, you're not even getting it to transconduct. You need to approach 5V input before it will even start. And even then, output is incredibly low. Because of this, the way the circuit is working, the 9V potential is pulling through T2 and T3 simultaneously and because your output is on the ground side, everything is pulling down.

    Once you choose the proper initial amplifier, you can solve the other issues.
     
  10. yesilovethis

    Thread Starter New Member

    Jul 28, 2016
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    I have to use a JFET input stage. Because I am trying to catch the VLF radio transmission only using half a meter whip antenna, so I cannot load the antenna by using a regular transistor, I have to have a very high input impedance pre-amp to get the signal out to a lower impedance. 2mV in the circuit is only a test signal from a signal generator to see if my circuit is working properly without any distortion. I am getting a undistorted signal of 500 mV Peak-to-peak at the emitter of Q3 (confirmed using oscilloscope) which I can easily amplify using a separate commercial stereo amplifier. But I want to make it portable and so I am trying to make the circuit just enough to drive 4 or 5mW of power to the usual 16ohm headphones. I think if I tweak the last transistor biasing and/or add another emitter follower then it might be sufficient.
     
  11. BobaMosfet

    Active Member

    Jul 1, 2009
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    Understood. But the specific JFET you're using is a _VHF_ FET. Not sure it will even work @ 1kHz (I don't see the datasheet really going below 11Mhz). Your first capacitor on the gate of T1 can present its own impedance (at some set of frequencies) on top of the 2K-Ohm impedance that T1 presents at Vgs = 0. Your 1M-Ohm resistor prior to gate T1 acts as a pull-down resistor, pulling Vgs = 0. The rest of the circuit beyond T1 is irrelevant until you correct your transistor choice for T1.
     
  12. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Vgs will not be zero because the FET is a depletion mode device and there is a source resistor to bias it properly. With the correct negative Vgs bias the JFET will be very high impedance input as required.
    JFET, MOSFET, BJT will all work from DC up to some maximum frequency.
    MPF102 will function correctly in this circuit.
     
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  13. BobaMosfet

    Active Member

    Jul 1, 2009
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    I appreciate what you're saying, but having built the circuit, it's behaving differently. The T1 source resistor is tied to ground, as is the resistor on the T1 gate. The capacitor on the gate stops DC. Irregardless of how you think the FET is supposed to behave, the circuit is causing it to be bypassed.

    But seriously, I've stated my opinion. You all will figure it out, eventually, I'm sure. Happng hunting.
     
  14. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Have a look at the self bias section here: http://www.circuitstoday.com/fet-biasing
     
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  15. yesilovethis

    Thread Starter New Member

    Jul 28, 2016
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    I did some study and also found the actual calculation of input output impedance of different FET biasing circuits. The common source FET biasing has input impedance = Rg (1Mohm in this case). I also saw a high input impedance guitar amplifier on a webpage using MPF102 with Rg = 3.3 Mohm. So I think MPF102 works fine in the audio frequency range which I also see using my oscilloscope. On that page the guy explains everything why the circuit build in that way it is and also that MPF102 is the best suitable FET for this type of project. So, I build it in similar way. Here is the page:
    http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp2.htm

    Thanks,
    -Rihan
     
  16. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    I don't see how any of those confused/ confusing statements are true for this circuit.
     
    Last edited: Jul 29, 2016
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  17. Bordodynov

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    May 20, 2015
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    See

    Draft320.png
     
  18. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    In the schematic in post #3, T3 is not biased correctly. What is shown is called "dangle-biased". This requires exact knowledge of the transistor's gain and that gain must be extremely stable. The problem is that a transistor's gain wanders all over the place during operation as it warms up. Dangle-biasing is the worst of all possible methods. I would eliminate the 10 uF coupling capacitor between T2 and T3 and the 560 K resistor, and connect the T3 base directly to the T2 collector as an emitter follower.

    Also, the 1K resistor in the T3 emitter is too large. The smaller it is, the better the circuit will reproduce the negative half-cycles of the audio. BUT, the smaller it is, the hotter T3 will get. Since you have a 9 V battery, a better solution is to replace everything after T1 with an LM386 audio power amplifier.

    ak
     
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  19. BobTPH

    Active Member

    Jun 5, 2013
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    Here is a good push-pull stage to replace everything after your JFET stage. R9 reperesents the 32 Ohm headphones. I get 500mV+ out of a 4mV input.

    Oops. You don't need both C1 and C2. I clipped out another stage from my original design.

    upload_2016-8-6_15-50-26.png
     
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  20. Bordodynov

    Active Member

    May 20, 2015
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    See
    Vin_amp=2mV Vout_amp=850mV

    Draft320_.png
     
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