We have a mosfet closed-loop programmable current sink that works fine most of the time but behaves badly at startup from zero current or at very low currents. The fix is to drool in a bit of keepalive current so that the loop never goes open and rails low.
The 10 ohm resistor can always be tweaked, but this works in simulation. The opamp that closes the loop has a 3 uV max offset and the fet goes to milli-ohms. My project engineer can make it work; I just donated the idea.
Of course dual diodes are usually separate chips and don't exactly match. But they do have exponential i-v curves and some ohms, not brick walls.
If we specify a minimum input voltage, 2.5 maybe, the keepalive shuts itself off when the voltage is applied so contributes no error.
Ingenious! At very low terminal voltages the user will see it sourcing a low current - don't know if that will be a problem in their application?
piglet
Does the sink device have to be a FET - a BJT or Darlington might be better in startup?
piglet
This will be an 8-channel dummy load module and we'll specify some minimum operating voltage, 2.5 or 3 maybe. The launch customer will almost always use 28 volts DC, the nominal aircraft DC supply voltage.
Once they apply their 3 volts, the keepalive diode back-biases and contributes no error. But this is a dummy load to simulate solenoids and motors, not a precision instrument.
The fet is sure nice. It goes to milliohms and is easy to drive from an opamp.
The thing about startup, with any transistor, is to not let the servo opamp see zero-zero inputs and wind up to some rail. In fact we want the fet to be biased slightly ON and be ready to go. Audio amps have the same issue, wanting to keep the output transistors always on a bit.
Yes, I am hoping with a BJT the base current should guarantee the opamp is always servoing and not railed.
piglet
If both the opamp inputs are zero volts, and it has any offset, it will rail high or low. The keepalive, and a tiny DAC offset, makes the opamp regulate a small constant mosfet current, and that's nice.
This is a textbook opamp+mosfet controlled current sink, which can go bananas at zero current.
Right, a BJT and some minimum demand offset will keep the loop closed even with nothing available on the collector.
A gate-source resistor on the mosfet might have a similar effect. Kill its infinite beta!
The FET avoids the latchup state where the BJT saturates and the loop regulates off the (greatly increased ) base current.
Current sink or resistor. It will be an 8-channel isolated dummy load module. It's easy to have both programmable resistance and constant-current modes, as most electronic load boxes do.
Some even have constant-voltage, essentially zener diode, mode, but I don't think we'll do that. They do want open and short states.
In resistance mode, our customer wants to apply +28 volts to one side and a PWM mosfet to ground. Or connect us to a full-bridge bipolar switcher, like a torque motor or stepper driver. The dynamic issues are horrendous.
The best way to simulate resistors is with resistors, but that had problems too.
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