Exponential current source

The final component of the synthesizer is finished! The exponential current source was the last component which needed to be designed and constructed. Intrinsically it’s no crucial component, in fact the synthesizer would perfectly work without it.

What does it do ?

To understand the use of the exponential current source in the global design, some knowlegde on the human ear is needed. To start, the human does not work in a linear way. Here is an example:

If a certain note is 100 Hz, the note 1 octave up will be 200 Hz. If our ears were linear, then the next few octaves would be 300 Hz, 400 Hz, etc. However, frequency doubles  with each octave. Octaves are 100 Hz, 200 Hz, 400 Hz, 800 Hz, etc. This clearly is not linear.

To create an intuitive interface for the frequency control of the synth, a linear shift at a sensor should create an exponential shift in frequency at the oscillators. The distance sensor we connect to the frequency control already has an exponential response by itself. The reason we still add an exponential source to the design is to encourage creative users to experiment with other interfaces, who very often will give linear responses.

The design

The design is based on the following circuit:

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2 thoughts on “Exponential current source

  1. I’ve noticed the base of the PNP transistor on the left only draws a very low current.
    Is it normal this component is ‘operational’ at 0.06mA? (MOSFET technology?)
    Would this cause any problems in a not so clean environment (a lot of noise)?

  2. Hi Felix,

    A little explanation might be useful here. The 80k and 1k resistors at the left connected to the base of the transistor act as voltage divider and are used here to transform the input voltage in the 0-5V range at the source to a millivolt range at the transistor base. By doing this we try to keep the transistor in the region where it’s collector current has an exponential relation to the base-emitter voltage. This is also the reason why we use bipolar and not FET transistors, the FET doesn’t have this relation.

    This all said, it is true that the exponential source operates with low currents. The reason therefore is that the oscillators need to operate within the range of audible frequencies (20 to 20,000 Hz). Which is very low in comparison to the oscillator’s maximum output. During the testing of our prototype in the electronics lab at Group T (a noisy environment) no unacceptable fluctuations in the frequency were observed.
    In case a better performance on frequency stability is required, a solution is to increase the capacitor value used in the oscillator. This will require larger charging currents which reduces the effect of noise on the frequency fluctuation.

    Thanks for your contribution, hopefully this answered your questions.

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