I have a problem with the presented current source amplifier. Two attached scope figures show voltage measured across the load for two different input sinusoidal signals of 100Hz and 1000Hz.
The input signal is feed in from a signal generator without any offset, and voltage across the load has a "step" rise from zero instead raising smoothly zero no matter how small amlitudes are (see scope figures). The step is some kind of delay as it increases with frequency meaning that there is a constant phase delay that is more visible at higer frequencies and still present even at 1Hz. It totaly not visible when input signal is a DC rised up in steps of 0.1mV, that is the current can be very low.
I cannot figure out how to get rid of the step and what is causing it. My firt guess is that is related to turn on time of a MOSFET and a sleeve rate of an op-amp, but AD848 has a sleeve rate of 300V/us which is more than enough. I tried also another op-am with a sleeve rate of 20V/us, different MOSFETs with different gate capacitance, but the step always stay the same so there has to be something else going on?
The input signal is feed in from a signal generator without any offset, and voltage across the load has a "step" rise from zero instead raising smoothly zero no matter how small amlitudes are (see scope figures). The step is some kind of delay as it increases with frequency meaning that there is a constant phase delay that is more visible at higer frequencies and still present even at 1Hz. It totaly not visible when input signal is a DC rised up in steps of 0.1mV, that is the current can be very low.
I cannot figure out how to get rid of the step and what is causing it. My firt guess is that is related to turn on time of a MOSFET and a sleeve rate of an op-amp, but AD848 has a sleeve rate of 300V/us which is more than enough. I tried also another op-am with a sleeve rate of 20V/us, different MOSFETs with different gate capacitance, but the step always stay the same so there has to be something else going on?
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