I'm currently trying the DIY route for a modular synthesiszer which is going towards my degree in Uni. I'm using the circuit schematic provided by Ray Wilson at MFOS for the VCO (attached), and before physically constructing the circuit I plan to place individual components of each module circuit through electronic simulations using TINA. This is for the sake of the final dissertation which will obviously need to contain the justification of my knowledge of the circuits involved. So i've made and simulated the control voltage summer, have a justification for the component values and the required 18mV (per volt of input) at the output of the summer. The issue i've been having is with the next stage of the VCO: the expo converter (which I have highlighted with a red rectangle in the attachment).
I understand the basic principle of the circuitry, it's just trying to understand how the individual components interact together. My understanding thus far of the functionality of the expo converter comes down to knowing that linear voltage at the input is converted into an exponential scale that we can hear. I know that by having matching pairs of transistors the difference in temperature between each of them during operation will be small, and therefore, easier to compensate for with a tempo resistor (R24) when in direct contact as a heat sink. So the NPN pair has a combined reaction to temperature which the tempo then cancels out, which in turn will decrease the frequency drift / adjust the scale of the incoming CV.
Beyond this I am under the impression that the op amp acts as a current source around one of the transistors which in turn fixes a value of current through the other transistor, and that R17 being fed back to to the emitter junctions determines the boundaries of the maximum current value able to pass through the exponentiator. So i'm sorry to be a pest, but could somebody with the required understanding of this circuitry give me a run through on exactly how each of the components in the highlighted area of the schematic interact?
I understand the basic principle of the circuitry, it's just trying to understand how the individual components interact together. My understanding thus far of the functionality of the expo converter comes down to knowing that linear voltage at the input is converted into an exponential scale that we can hear. I know that by having matching pairs of transistors the difference in temperature between each of them during operation will be small, and therefore, easier to compensate for with a tempo resistor (R24) when in direct contact as a heat sink. So the NPN pair has a combined reaction to temperature which the tempo then cancels out, which in turn will decrease the frequency drift / adjust the scale of the incoming CV.
Beyond this I am under the impression that the op amp acts as a current source around one of the transistors which in turn fixes a value of current through the other transistor, and that R17 being fed back to to the emitter junctions determines the boundaries of the maximum current value able to pass through the exponentiator. So i'm sorry to be a pest, but could somebody with the required understanding of this circuitry give me a run through on exactly how each of the components in the highlighted area of the schematic interact?
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