System for automatically controlling the gain-bandwidth product of operational amplifiers

Claim 1

System for automatically controlling the gain-bandwidth product of operational amplifiers, wherein gain-bandwidth product (G*B) of one of the amplifiers (M1) placed on the same chip as the amplifiers (M2, ..., Mn) to be controlled is measured and the resulting signal is used to control through a bias circuit (CP) the gain-bandwidth products of all the ampli­fiers, the value of these products being presettable through the frequency (fc) of a control signal sent to the system input (1), characterized in that it comprises: - a logic circuit (LC), which receives at the input (1) said control sig­nal with predetermined frequency (fc) and duty-cycle equal to 50 per cent and supplies at a first output (2) a signal with a frquency equal to half the input signal frequency and at a second output (7) a signal obtained from the logic AND between the input signal (1) and the signal at the first output (2); - a squaring circuit (SQ), which receives at the input (2) the signal pre­sent at the first output of said logic circuit (LC) and supplies at the output (3) a corresponding square-wave signal with positive or negative amplitudes equal to that of a reference voltage (Ao); - a source (GT) of that reference voltage; - the one (M1) of said operational amplifiers (M1, ..., Mn), highly com­pensated so as to present at a very low frequency the first pole of the transfer function and placed in the voltage-follower configuration, which amplifier receives at the non-inverting input the signal supplied at the output (3) by said squaring circuit (SQ); - a threshold comparator (CA) which receives at the inverting input (4) the signal supplied by said operational amplifier (M1) and at the non-­inverting input both the signal supplied by said squaring circuit (SQ) and aid reference voltage (Ao) through a first and a second resistor (R1, R2), respectively, and supplies at the output (6) a signal with leading edges coincident with those of the non-inverting input signal and trai­ling edges coincident with the instants at which the voltages at the inputs are equal; - a duration comparing circuit (CD), which carries out an EX-OR operation on the signals supplied by said threshold comparator (CA) and by said logic circuit (LC) at the second output (7), supplying at the output (8) a current signal with outgoing direction when the high-level part of the signal supplied by the threshold comparator has a shorter duration than that of the signal supplied by the logic circuit and with incoming direc­tion in the opposite case; - a capacitor (C), with a terminal connected to the negative voltage sour­ce (-Vee) and the other terminal connected to the output (8) of the dura­tion comparing circuit (CD) and to the input of said bias circuit (CP), which controls the amplifier gain-bandwidth product in a way inversely proportional to the voltage across the capacitor terminals.