How can I get the desired output signal BJT

Discussion in 'The Projects Forum' started by awesome2999, Sep 7, 2010.

  1. awesome2999

    Thread Starter New Member

    Apr 10, 2010
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    Hi All,
    I have got a few questions about how to get the desired output signal from the transistor which this signal will be going to another stage which is an envelope detector (using a germanium diode)and some amplification to generate a square wave at the final output (AM receiver circuit).
    The aim at this stage is to get an output signal about 2 v p-p AC with 2 v DC from the transistor.

    From the datasheet for the transistor BF199
    Bmin=40 & IC min =7mA
    IC max is 25mA so I tried to get about 9 to 12mA so to be on the safe side (avoid Sat or Cutoff)
    From spice model the BF199 Transistor has B =122

    My approach to find the right parameters, Please correct me if I made mistakes,
    Writing kVL
    Vcc-(I*R)-(IB*R2)-VBE=0 where I= (B+1) IB=B*IB
    IB=VCC-VBE/R2+B*R
    I selected R as 150 ohm
    R2 =29K ohm
    R1=200 ohm
    I used B (hFE) as 90.
    I found the I =IE=9mA
    Vout=1.8 volts which it’s close to get the signal to swing over it

    Problems:
    My problem is when I do insert coupling capacitors I do get a non linear signal like shown in the attachments and some distortion also there is positive cycle is wider than negative cycle. I selected the coupling capacitor as 0.01 and 0.1micro farad as my frequency is 12MHZ to get a reactance XC nearly as zero which it is acting as short circuit for the Ac and open circuit for the DC.
    I’m not sure how to start correct these problems (i tried to change the cap values slighty but it doesn't work). Any help will be much appreciate
    Thanks
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    You can do a DC operating point check in LTSpice to verify that your BF199 is biased correctly and the collector is at roughly half the supply voltage.
     
  3. awesome2999

    Thread Starter New Member

    Apr 10, 2010
    7
    0
    Thanks for the reply, i have just changed the value for R1 from 200 to 175 ohm. i checked the DC operating point ,it is about 2 volts and i do get a signal swing between about 1 to 3 volts but i'm not quite sure how to get a clean and even signal-i changed the vlaues for the coupling and decoupling caps a bit but doesn't change alot.and the other issue is when i measure the output after decoupling cap it should drop the swing signal by 2 volts (DC Operating point) but it does drop more than 2.3 volts?. as shown in the new attachment.

    any idea or a hint so i can work around to fix this problem.

    thank you for the help!

    Regards;
     
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    You would need to attach the LTSpice .asc file too.

    Nobody here knows which node in the schematic is N005 because it depends on how you have drawn the circuit up.
     
  5. awesome2999

    Thread Starter New Member

    Apr 10, 2010
    7
    0
    sorry, it was my fault, this node is the output from the bjt and before the decoupling cap. i have attached the .asc
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Can't help you much with the remaining circuit but the following component values produces a fairly good output waveform at 12MHz.

    The Q1 collector output is meant to be connected to circuit with high impedance so I added a second transistorQ2 (emitter follower) to obtain a low impedance output too.

    Here is the simulation result with the actual circuit file for you to play along.

    [​IMG]
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    if this is suppose to be an AM (Amplitude Modulation) detection, I don't see any modulation. I would be expecting the generator to produce something like this
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A transistor has severe distortion at high levels when it has no negative feedback. Your signal generator in the simulation has a zero output impedance which cancels the negative feedback through the collector-base biasing resistor.
     
  9. awesome2999

    Thread Starter New Member

    Apr 10, 2010
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    0
    Thanks guys for helping me. It is an AM receiver circuit, I have to stick with this kind of design (as a part of my course).which it has antena 50 ohm ,tuning circuit, transistor, envelop detector, amplifier and comparator circuit .carrier frequency is fc= 12 MHZ and the fm=1 KHz and at the end of the comparator I should get as much as possible a clean square wave which it will go to another stage (Digital part).the other fact i just want to understand if it says in the datasheet min IC =7 mA at hFE=40 and ICmax=25 mA, the circuit you designed (eblc1388)it works nice under ic=3mA how come does it work under this condition i was thinking will be going into cutoff region

    thank you

     
    Last edited: Sep 8, 2010
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896

    The datasheet from Fairchild says its mimimum hFE (DC current gain) is only 38 at 7mA. You don't know how much (or how little) your transitors have since it is a range and you didn't measure it. Besides, the hFE is at DC. The hfe is AC at at hgher frequencies and is much less.
     
  11. clashinfolk

    New Member

    Nov 24, 2012
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    my teacher asked me to use the same circuit as you mentioned awesome2999, but for FM frequencies, around 80-85MHz. I doubt that its possible. If you completed this design, could you help me out?
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    You did not post the schematic so we don't see it.
    FM in Japan might be 80-85MHz but in most of most of the world it is 88MHz to 108MHz.
     
  13. clashinfolk

    New Member

    Nov 24, 2012
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    the circuit is the same as awesome2999, but my teacher asked me to design it for 80-85MHz. I have spent my entire weekend trying to design this thing, and then i came across this thread, and this is designed for 12MHz. Could you help me.
    And sorry for the confusion, what I meant by FM was that for Higher Frequencies :)
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    Then please copy the circuit and post is as an attachment to your reply so we can see what you are talking about.

    You are not designing a circuit, instead you are simply copying it and changing its frequency.
    But we don't know if your's and Awesome's circuit is a transmitter or is an AM receiver as in this thread.
     
  15. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    We often are shown block diagrams where the blocks perform only one circuit function,with no effect on the previous circuitry.
    In this case,D1 would just rectify the signal

    Even with ideal components this doesn't happen.

    For the half cycle when D1 is not conducting,Q1's collector load is effectively R2 & R3 in series,as an ideal D1 is ∞ resistance.

    For the other half cycle,you have C1, D1 in series with the parallel combination of R4 & C6 between Q1 collector & earth.

    This circuit appears in parallel with the existing R2 ,R3 series combination as part of the collector load for Q1 during one half cycle only,hence the gain of the transistor will vary between positive & negative half cycles,causing the distortion you noted.

    ieblc1388 has given you one way to minimise this effect.
    With the low output impedance of an Emitter follower,the change in load between D1's conducting & non conducting states will be so small that it may be neglected.


     
  16. clashinfolk

    New Member

    Nov 24, 2012
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    Please see attached. The problem is that the circuit works fine upto 10Mhz, but when I do calculations, the response is not so good for 80Mhz.

    The problem is that I have to pick up a signal of 2mV and amplify it with a transistor so that it can pass through a germanium diode easily. That's why i need the circuit to provide a gain of about 250 or 300. So that 300mV drops at diode and the rest can go to the rectifier and a then to an op-amp which can amplify the signal to about 5V.

    The rest of the part is easy, but designing bjt for such a high gain is a pain in the sitter. I have done numerous iterations and the result is not very good.

    PS: If you want the model of BF-199, I can post that too.
     
  17. vk6zgo

    Active Member

    Jul 21, 2012
    677
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    In Radio receiver terms,2mv is not a tiny signal.
    "Crystal Radios" used a germanium diode & no RF amplification.

    Are you perhaps neglecting the "signal magnification" which takes place when a signal is at the resonant frequency of a tuned circuit.

    I would suggest you spend a bit of time reading up on Radio receiver circuits.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    I never made a crystal radio but I remember they used a long antenna and an earth ground. They picked up STRONG local stations that produced hundreds or thousands of mV at the input.

    My FM receiver has a sensitivity of 1.4uV. Yes, slightly more than one microvolt.
     
  19. vk6zgo

    Active Member

    Jul 21, 2012
    677
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    Before the advent of vacuum tubes,Crystal Radios were the only game in town,& signals were received over very much longer distances than"local".
    To get thousands of mV,it would have to be "local",indeed!

    That said,diodes don't just turn on "like a switch" when the signal level reaches 0.2V,so the tuned circuit would not have to contribute much.
    Also on reflection,voltage magnification needs a series tuned circuit at the Rx input,so my original comment wouldn't work.
    Germanium diodes have a non linear transfer function active at considerably lower voltages,& can demodulate a signal.
     
  20. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    At 80MHz,Q1's gain will be extremely low,as at that frequency,the capacitive reactance of the circuit stray capacitances will be very much lower in value than,& in parallel with,its collector load resistance.

    The gain will increase considerably,if you replace the collector resistor/s with a tuned resonant circuit. ----These things weren't called TRFs for nothing!

    You should also change the bias circuitry to a voltage divider type.
    As the BJT has a low input Z,it would be a good idea to tap its input down on L1.
     
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