![]() Thus, the small C-B capacitance appears (1+|Av|) times larger than its actual value. Keep in mind that Av is a negative number for the inverting C-E amplifier. ![]() Moreover, the collector feedback is (1-Av) times larger than the base signal. The collector signal capacitively coupled back opposes the base signal. ![]() However, in the C-E configuration, the collector output signal is out of phase with the input at the base. Thus, one would think that the C-B capacitance would have little effect. This C-B capacitance is smaller than the E-B capacitance. The Miller effect is the multiplication of the bandwidth robbing collector-base capacitance by voltage gain A v. The key to understanding the wide bandwidth of the cascode configuration is the Miller effect. Bandwidth Capacitance and the Miller Effect This is an AC circuit equivalent with batteries and capacitors replaced by short circuits. The cascode amplifier is combined common-emitter and common-base. The cascode amplifier configuration has both wide bandwidth and a moderately high input impedance. “Capacitor coupled three stage common-emitter amplifier” Capacitor coupled for a cascade example. The stages are in a cascode configuration stacked in series, as opposed to cascaded for a standard amplifier chain. The solution is to precede the C-B stage by a low gain C-E stage which has moderately high input impedance (kΩs). While the C-B ( common-base) amplifier is known for wider bandwidth than the C-E ( common-emitter) configuration, the low input impedance (10s of Ω) of C-B is a limitation for many applications.
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