Current source and current sink
Draw a constant current sink with an op-amp. Is it the same as a current source? Draw a source too.
| Level | ●● onsite technical |
| Track | General board |
| Really testing | Feedback around a transistor; compliance voltage. |
Going deeper
Extra explanation that is not in the book: worked numbers, where people go wrong, and how to say it in the room.
Power in the pass FET — the number most people miss. 12 V supply, 1 A, the load drops 5 V, the sense resistor 0.1 V: the FET drops 6.9 V and burns 6.9 W. That needs a heatsink — and a FET rated for linear operation. Many modern trench MOSFETs have a weak DC safe-operating area (hot-spotting); check the DC SOA line, not just Rds(on).
Accuracy. Op-amp offset sits directly on Vref. 1 mV of offset on a 100 mV sense voltage = 1% current error. Options: a low-offset (zero-drift) op-amp, or a larger Vref — which costs more power in Rs.
Stability recipe that works. 10–100 Ω in series with the gate; 1 kΩ from the sense resistor to the − input; 1–10 nF from the op-amp output to the − input. The local feedback cap dominates at high frequency, so the gate capacitance can't make it ring.
Where people go wrong
- Putting the load in the source leg — then the load changes the feedback.
- Ignoring compliance: Vsupply − Vload must exceed Vref plus the FET's minimum drop at full current.
- An op-amp that can't swing high enough to give the FET its Vgs.
Errata
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