1. Field of the Invention
This invention relates generally to feedback amplifiers and, more particularly, is directed to an improved feedback amplifier especially suited for use in the video output circuit of a television receiver.
2. Description of the Prior Art
In some existing amplifiers, a negative feedback circuit is provided between the output and input of the amplifying circuit for stabilizing the amplifying operation or for preventing oscillation of the circuit as a result of the positive feedback action occurring, for example, through a stray capacitance between the input and output of the amplifying circuit. A correct and effective negative feedback operation can be achieved only if the phase of the feedback signal is accurately maintained so as to be precisely opposite to the phase of the input signal.
The feedback circuit is usually composed of passive elements and, if the elements of the feedback circuit are all pure resistors, the phase characteristic of the feedback signal and, therefore, the frequency characteristic of the output signal, can be maintained constant without regard to frequency variations of the input signal. However, in actual fact, due to stray capacitance in the feedback circuit, the phase of the feedback signal varies with changes in the frequency of the input signal. The foregoing is particularly a problem if the frequency of the input signal varies often over a wide range, for example, as is the case when the input signal applied to the amplifying circuit is a video signal. In such case, the frequency characteristic of the output signal is not flat and only a particular frequency component of the signal, for example, a relatively low frequency component, is stressed in the output. Thus, with the existing feedback circuit, a very unstable condition or oscillation may occur because of the fluctuation of the phase characteristic of the feedback signal.
When the feedback circuit is formed by a plurality of resistors connected in series between the amplifying circuit output and ground to form a resistance divider circuit and the feedback signal is obtained at a connection point between the resistors, the stray capacitance can be considered to be in parallel with the resistor at the cold side, that is, between the connection point and ground. In such case, the previously mentioned problems can be minimized, that is, the described effects of the stray capacitance can be made negligible, if the resistor in parallel with the stray capacitance is given a small value relative to the impedance of the stray capacitance. However, for any predetermined value of the negative feedback ratio, the relation of the resistance value of the hot side resistor of the resistance divider circuit to the resistance value of the previously mentioned cold side resistor in parallel with the stray capacitance is also predetermined. Thus, providing the cold side resistor with a small resistance value requires that the hot side resistor be similarly provided with a small resistance value, with the result that the composite resistance value of the resistors becomes very small and causes a reduction of the dynamic range of the amplifier. Therefore, in existing feedback amplifiers employing a resistance divider circuit for the feedback, the resistors in such resistance divider circuit have been given intermediate values that are compromises between the small resistance values desired for avoiding fluctuation of the phase characteristic of the feedback signal and the relatively high resistance values required for increasing the dynamic range of the amplifier.