Common series negative feedback amplifiers

Common series negative feedback amplifiers

(2) Series negative feedback amplifier
    The figure shows a common series negative feedback amplifier . The resistor R is a current negative feedback element, because it belongs to both the input loop
and the output loop, linking the input and output loops. First, assume that a certain instantaneous input signal is positive (+). Since the voltage
polarity     of the emitter (e) is the same as the base (b), it is also positive (+), which increases the potential of the emitter of the transistor. Because the emitter voltage of the transistor is equal to the input voltage Ub{-U-Uf, the voltage of the input signal Ube is weakened through the feedback element R , so the feedback element R is negative feedback. The negative feedback resistor R is used to stabilize the amplifier. The larger the resistance value, the smaller the amplification factor of the entire amplifier. The capacitor C connected in parallel with the negative feedback resistor R is a decoupling capacitor, which is equivalent to short-circuiting the AC of the emitter (e), so that the AC signal has no negative feedback effect, thereby obtaining a larger AC amplification factor.