High precision, high bandwidth variable gain amplifier and method

A controllable gain amplifier comprising:

• a pair of gain elements, each having a pair of electrodes forming a current-conducting channel, and an input electrode for receiving input signal applied thereto to control current conducted between the pair of electrodes, with corresponding ones of the pairs of electrodes connected in common to a current source;
• a pair of elements each having resistance determined in response to control signal applied thereto, the elements being connected in common to receive supply voltage and to corresponding other of the pair of electrodes of the gain elements;
• an auxiliary current source connected to each of said corresponding other of the pair of electrodes of the gain elements for producing at each such electrode an output signal representative of the input signal applied to the corresponding input electrode with a gain determined by control signal applied in common to the pair of elements;
• and
• a control circuit for supplying the control signal, the control circuit comprising:
• a pair of auxiliary gain elements, each having a pair of electrodes forming a current-conducting channel, and an input electrode for receiving input signal applied thereto to control current conducted between the pair of electrodes, with corresponding ones of the pairs of electrodes connected in common to a current source;
• a pair of auxiliary elements each having resistance determined in response to control signal applied thereto, the auxiliary elements being connected in common to receive supply voltage and to corresponding other of the pair of electrodes of the auxiliary gain elements;
• an additional current source connected to each of said corresponding other of the pair of electrodes of the auxiliary gain elements for producing at each such electrode an output signal representative of the input signal applied to the corresponding input electrode with a gain determined by control signal applied in common to the pair of auxiliary elements;
• reference voltage supplies connected to supply input signals to the input electrodes of the auxiliary gain elements;
• a comparator connected to receive the reference voltages and output signals from the auxiliary gain elements for producing a comparator output as a logical combination of the output signals from the auxiliary gain elements and the reference voltages as input signals to the auxiliary gain elements for application as control signal to the auxiliary elements;
• and
• a converter connected to receive the comparator output and an auxiliary reference voltage for supplying a control signal to the pair of elements having amplitude in a range between the levels of comparator output and auxiliary reference voltage in response to a gain-controlling signal supplied to the converter for setting the gain from input signal to output signal of said gain elements.

A method of controlling amplification of applied signal by a plurality of cascaded gain stages that each has gain variable over a range between minimum and maximum levels in response to a gain-controlling signal applied thereto, the method comprising:

• selectively adjusting the gain of one of the plurality of cascaded gain stages toward maximum gain with all other of the plurality of cascaded gain stages substantially at minimum gain;
• and
• selectively adjusting another of the plurality of cascaded gain stages toward maximum gain with said one gain stage substantially at maximum gain and with all remaining other of the plurality of cascaded gain stages substantially at minimum gain;
• and
• selectively adjusting one of the remaining other of the plurality of cascaded gain stages toward maximum gain with said one and said another of the plurality of cascaded gain stages substantially at maximum gains.