# AC Current sink with op amp and transistor

I want to create an AC load for testing high capacity battery (200Ah) using the circuit below:

Can I put an AC voltage at Vin and get a AC load current through the FET? And why do they need LT1492 with 55mA output current capability while the FET gate current is micro-Amp small?

Another thing that trouble me is that why Iout only depend on that 1 Ohm resistor and not RL?

V+ is connected to the battery.

• What frequency will your 'AC' signal be? The FET's gate current for a DC gate voltage is very small but the gate has a significant capacitance, so the op-amp needs to be able to supply enough current to charge and discharge it through the 100 ohm resistor in order to meet the sub-1µs rise-time specification in the circuit you've posted. If you don't need that speed of response you can get away with a lower current. – nekomatic Jul 21 '16 at 10:13
• Well, my AC signal is around 1kHz and I need about 1.5A to 2A through the FET and load resistance. Can a 16mA TLV2370 handle this current? – kingyo Jul 25 '16 at 2:16
• A good answer to that question is beyond my knowledge level but a quick check of the Si9410DY datasheet shows a maximum gate charge of 50 nC, and a current of 16 mA delivers that charge in just over 3 µs, which is small compared with the interval of 500 µs between switch on and switch off at 1 kHz. The actual current won't be constant but will decay exponentially as the gate charges through the 100 ohm resistor, which you may want to increase in order to limit the current to what the op-amp can handle, but even if we assume a factor of 10 longer as a rough approximation… – nekomatic Jul 25 '16 at 8:24
• …you've still turned the FET on or off within about 30 µs. So my guess is it'll work OK, but the FET is going to spend some time in partial conduction as the gate voltage rises or falls and that means it will need to dissipate some heat - that's the limiting factor, aside from any spec you want to place on how fast the current must rise or fall for your application. – nekomatic Jul 25 '16 at 8:27
• – nekomatic Jul 25 '16 at 8:29