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Circuit Diagram Help Needed

Discussion in 'General Electronics Discussion' started by webpower, Jun 9, 2017.

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  1. webpower

    webpower

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    Jun 9, 2017
    Hi The following diagram is a circuit from a synthesizer book by R.A Penfold.

    The text that describes the circuit is as follows. My question is can anyone please explain to me what the diode does and why it seems to be shorted out across the 741 opamp.

    Here is the text that describes the circuit (Copied from the book):

    From every log opamp I have seen the diode is part of the feedback of the opamp. But here it is a very strange set up to me. I have no idea what is going on.


    This converter circuit, in common with most other types of logarithmic amplifier, relies for its operation on the fact that the current through a forward biased silicon diode rises exponentially with linear increments [it!] the input voltage. Although this characteristic is only maintained over certain limits, in this application there is no need to exceed these limits as only a modest ,range of output currents is involved (no more than a range of about 100 to 1). What does complicate things is that the voltage across a forward biased silicon diode varies significantly with changes in temperature, and diodes are often used as electronic temperature sensors. The circuit must therefore include temperature compensation to avoid the need for very frequent readjustment to correct tuning drift.
    Having tried a variety of configurations, this one seems to give reasonably accurate and stable results without the need for any “difficult to obtain” components. IC2 is a CA3046 transistor array, which consists of three individual transistors plus two others connected as a long-tailed pair (i.e. having their emitters connected together). In this circuit only two of the individual transistors are used, and no connections are made to the other devices. IC2a operates as the converter, while IC2b provides a degree of temperature compensation. The point of using a transistor array to provide these two devices is that this ensures excellent thermal contact between the two, and consequently gives instant and accurate temperature compensation. The alternative of using two ordinary silicon NPN transistors with their cases glued together seems to work reasonably well, but the additional expense of using a transistor array is probably justified.
    The input voltage range is far too large to directly drive IC2a, and a potential divider to provide a suitable degree of attenuation is therefore included at the input of the circuit. RV1 is adjusted to give the required 1 volt per octave characteristic. RV2 is the frequency control, and this enables the output current of the circuit to be adjusted. In practice this acts as the tuning control, and it is adjusted to give the required pitch range from the VCO. It provides a large control range, and enables the pitch range to be shifted over at least three octaves. FIG07.GIF
     
    Last edited by a moderator: Jun 10, 2017
  2. Alec_t

    Alec_t

    2,998
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    Jul 7, 2015
    Welcome to EP!
    D1 clamps the emitter voltage of IC2a to ground, since it is only the forward current through the transistor which is of interest.
     
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  3. webpower

    webpower

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    Jun 9, 2017
    Many thanks for such a quick reply. Sounds like you understand the circuit.

    Why would this not just short the emitter out to ground (All the charges moving through the diode to ground)?

    Would you be so kind to give me a complete breakdown of this diagram. Explaining what all the components do. I would like to build this and the other associated circuits, but before I do, I want to understand the circuits.

    What I see is two transistor type units (Ic2a,IC2b) feeding an op-amp in some kind of negative feedback with the diode? But I am even more confused now given your reply (Which I am sure is correct).
    Also I am confused as to why we have 0v supply rail (I am assuming that is the mid point potential in the circuit)

    Maybe if this whole diagram was redrawn for me I might follow it better ;-)

    Please can you explain it, in detail for me. Many thanks again. Its fascinating and I want to get it properly understood.
     
  4. Alec_t

    Alec_t

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    813
    Jul 7, 2015
    It would if it were forward-biased, but it is normally reverse-biased. The diode when reverse-biased plays no part in feedback. Feedback to the inverting input (pin 2 of the 741) is via Tr1.
    I haven't fully analysed the circuit.
     
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  5. webpower

    webpower

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    Jun 9, 2017
    Thanks it helps. I still don't understand what this circuit is supposed to achieve.

    So, given that the voltage at 13 is always lower than at the 0volt rail, is this why diode is always reverse biased? Have I got that correct? So given that, why does it need the diode at all? In the text it says something about how the diode conducts exponentially. So that seems to suggest a possible reason why it is there (logarithms aka exponential changes like diode behavior)?

    (somehow it is supposed to convert a linear change in voltage from a keyboard's voltage levels, to logarithmic voltage levels). Which I can kind of see, if it was using the diode as part of some kind of feedback network. But it does not look like it is. Very strange circuit, but book seems good. I don't think their is a mistake in circuit diagram or anything.

    But I am not getting how it works. But your helping me, so thank you very much.
     
  6. Alec_t

    Alec_t

    2,998
    813
    Jul 7, 2015
    Yes.
    I don't think it does, unless you have a really large voltage coming from the keyboard input.
    The non-linear effect is produced in IC2a. The Penfold article explains.
    Here's a simulation of the circuit. The output voltage Vco is plotted for a keyboard input of 5V (yellow trace) and 12V (blue trace) respectively, as a function of the control voltage Ctl provided by RV2. As you can see, the yellow trace shows little variation in the Vco voltage, whereas the blue trace shows a distinct non-linear variation.
    Non-linear-Control.PNG
     
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  7. webpower

    webpower

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    Jun 9, 2017
    I am so impressed and grateful to. How long have you been messing about with electronics? Your very good my friend.

    This has explained it better than article. So, what exactly does he mean when he says this (Do you think?)
    I assumed this (quote below) was in reference to diode D1 in circuit diagram. But we agree it is NOT referring to that diode at all but, in fact to the PN junction within the transistor in IC2a?

    "What does complicate things is that the voltage across a forward biased silicon diode varies significantly with changes in temperature"

    Thanks for doing the plot. Be very interested to see the output without D1 in the circuit. Could you try that for me? I think you got it right in your first reply to me. It must anchor the op-amp to 0volts (else I suppose it has no reference voltage???) But I am only guessing, I have no idea really.
     
    Last edited: Jun 9, 2017
  8. Alec_t

    Alec_t

    2,998
    813
    Jul 7, 2015
    Over 60 years :).
    I tried it without D1. The plot looks identical.
    Penfold's reference to voltage change with temperature applies to any semiconductor junction: the base-emitter junction of IC2a in this case.
     
  9. webpower

    webpower

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    Jun 9, 2017
    You have completely explained this to me plus the spice simulation was very helpful. When Penfold
    talked about this, I just (wrongly) assumed he was referring to D1. So yes the transistor PN junction now makes total sense. I also now understand how the compensating transistor works now given that it is the PN junction and not D1.

    Thanks for doing that spice simulation. How easy is spice to learn? I would love to try that out myself. I have macspice on my mac but it does not have a graphical front end. But it does use a text file where components are numbered which sort of looks logical. So I am assuming I can still replicate what you did, as macspice still does pop up graph plots in frequency and time domains but, just does not show a circuit diagram.

    I don't suppose you could post here a copy of the Spice text file? So I could replicate it. As that is so fascinating.

    Great to be in touch with someone who has so much experience. Brilliant.
     
  10. Alec_t

    Alec_t

    2,998
    813
    Jul 7, 2015
    Here's the text file. LTspice uses the .asc file extension, but you can open the file in any text processor, though you may have to change the extension to .txt. Goodness knows what Macspice will do with the file. Good luck.
    LTspice has a steep learning curve, but it's worth persevering. Its Help is comprehensive, although somewhat terse.
     

    Attached Files:

  11. webpower

    webpower

    6
    0
    Jun 9, 2017
    Thank you so much. Will be back here if I need help again. So, hopefully get to talk to you again sometime soon.
    You are an absolute star.
     
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