Fast-acting current comparator

A current comparator capable of fast transitions between high and low output states comprising:

• a first comparator stage for combining an input current with a reference current to produce an input signal representing the difference between those currents;
• a second comparator stage providing gain and having an input node to receive said input signal and an output node to produce an output signal which varies between high and low dependent upon the magnitude of said input signal;
• said second stage having a trigger point provided by a high-gain region of its transfer characteristic where the transition between high and low output occurs at a rapid rate;
• first circuit means coupled to said second stage and having first input means responsive to a bias control signal to modify the voltage level of said trigger point of said second stage;
• a bias voltage control circuit including a gain stage matching said second comparator stage and having input and output nodes;
• means for supplying to said input node of said control circuit gain stage a signal matching that on the input node of said second comparator stage when said input current is equal to said reference current;
• second circuit means matching said first circuit means and coupled to said control circuit gain stage;
• said second circuit means including second input means to receive a control signal and responsive thereto for controlling the voltage at one of said control circuit gain stage nodes;
• an amplifier having a pair of input terminals coupled respectively to said input and output nodes of said control circuit gain stage;
• means connecting the output of said amplifier to said input means of said second circuit means as a feedback control signal to force the voltages on said input and output nodes of said control circuit gain stage to be equal;
• and
• means connecting said feedback control signal to said first input means of said second comparator stage to set the trigger point of said second stage so as to provide fast response of said current comparator to a change in said input current.

The method of obtaining fast response from a current comparator having a current-comparison stage wherein an input current is compared to a reference current to produce a comparison signal which is applied to an input node of a first complementary MOSFET inverter gain stage having a transfer characteristic with a high-gain region establishing a trigger point at which the transition between high and low output levels occurs;

• said method comprising the steps of:
• providing a second complementary MOSFET inverter gain stage matching said first inverter gain stage;
• setting the voltage of said input node of said second inverter stage to be identical to that developed by said current-comparison stage when it is at its balance point;
• developing a bias control signal for said second inverter stage to force its input and output nodes to be at the same potential;
• and
• applying said bias control signal to said first inverter stage to modify its trigger point to occur at the voltage of the input node of said first inverter stage when the comparison signal from said current-comparison stage is at its balance point.