Amplifier circuit and focus voltage supply circuit incorporating such an amplifier circuit

Claim 1

A dynamic focus voltage supply circuit for display apparatus having a cathode ray tube, said supply circuit having an amplifier circuit comprising first and second transistors of like conductivity type with the emitter of the first transistor being coupled to the collector of the second transistor, the transistors being arranged with their main current paths in series in a voltage sharing configuration between first and second supply terminals for a voltage supply, means for biasing the base of said first transistor for setting the quiescent voltage at the connection between the first and second transistors, means for applying an input signal having relatively high and low frequency components to the second transistor, said components comprising a parabolic component at line frequency and a parabolic component at field frequency, respectively, means for deriving an output signal having a parabolic component at line frequency and a parabolic component at field frequency, from a load connected between the collector of the first transistor and the first supply terminal, and means for applying the output signal to a focus electrode of said cathode ray tube, characterized in that said means for biasing the base of said first transistor includes a capacitor having a low capacitance, said biasing means thereby presenting to said first transistor base a bias potential unaffected by said high frequency component which bias varies at said low frequency component rate, whereby the first transistor amplifies the high frequency component but substantially not the low frequency component while the second transistor amplifies the low frequency component but substantially not the high frequency component.