Voltage controlled current source project

335

02:22:59

It’s in development, but this is the early stage so far.  My goal with this is for it to be able to operate at practically any frequency and be of reasonably low current consumption from the control voltage input.  (practically -as in up to maybe a few MHz at absolute maximum, but definitely not a necessity to go quite that high)

I am using a capacitor to make it easier to observe the current sourcing effects at various frequencies.  This is more apparent because when capacitors are charged through a constant current, the waveform becomes very linear and ramp or triangular-like shaped, and with this particular setup it makes it very easy to judge how well it works at pretty much any frequency (within reason of course) (The higher the current, the steeper the rise and visa-versa)

So feel free to test this VCCS circuit by adjusting the frequency source, and then adjusting the control voltage.
 (to make it easier on yourself though, so that you don’t have to be as precise with tuning the control voltage, I would strongly suggest also adjusting the NPN biasing resistor to 10k or higher at low frequencies; and adjusting to 1K or lower at higher frequencies)

-Adjust the frequency using the source highlighted in green (do not alter amplitude or dc voltage spec for this)

-Adjust the Control Voltage by using the voltage source highlighted in orange

-Observe the output highlighted in blue

-The diode is just there to discharge the capacitor on the falling edges of the input signal

published 5 years ago

jason9

5 years ago

One thing you could do for a VCCS is to put the voltage source across a resistor and mirror it with a current mirror to obtain a constant current source that varies with the voltage across the resistor.

Issacsutt

5 years ago

I have considered that, but the downside there is i don’t think it works well enough to be used for a VCO with a capacitor, because a small portion of the current being mirrored has to be shared through the base-emitter junction and collectors have to be tied together in order to maintain the same voltage level and makes it a huge drawback. Basically i have found that the current mirror method is not isolated enough from the control voltage and input signal

Issacsutt

5 years ago

Here check this out, I put the current mirror on the left, kept the original design on the right and they are using the same signal source as well as control voltage for better real time comparison, component values are also the same: http://everycircuit.com/circuit/4704516353818624 What do you think?

jason9

5 years ago

Actually, my comment isn’t right. This current circuit is already a current mirror, just with the NPN that has the collector and base shorted together replaced with a diode since the two behave the same.

Issacsutt

5 years ago

Mmm... very good point. This is the non-feedback version though; I’m sure with a little feedback and a slight circuit variation it will probably perform even better and yet, maybe much less beta dependent

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