Chapter 10 Other BJT Amplifiers Common Collector CC
Chapter 10 Other BJT Amplifiers
Common Collector (CC) Amplifier n n n A CC amplifier provides current and power gain. CC input and output signal voltages are in phase. A CC amplifier is also referred to as an emitter follower.
Emitter Follower DC Relationships
AC Equivalent Circuit n The ac equivalent of the emitter follower shows that the output signal voltage is developed across r. E. n r. E = RE || RL
Emitter Follower Gain n n Circuit current gain is significantly lower than the transistor hfe rating. Voltage gain is less than unity (1). n n Av is assumed to be approximately equal to 1 when r. E >> r’e. Ap < Ai because Av < 1.
Emitter Follower Impedances n Input impedance is relatively high; higher than that of a comparable CE amplifier. n n The value of hfc is equal to (hfe + 1). In most cases, we assume that hfc hfe. Output impedance is relatively low; lower than that of a comparable CE amplifier.
Using Emitter-Feedback Bias n Emitter followers are often emitter-feedback biased to provide higher amplifier input impedance.
Decoupling Capacitor n Decoupling capacitor – A capacitor connected between VCC and ground, in parallel with the dc power supply. n n Used to prevent emitter-follower output distortion (shown in the lower of the two waveforms). A regulated dc power supply can also be used to prevent this distortion.
Buffers n n Buffer – A circuit used to compensate for an impedance mismatch between a source and its load. An emitter follower can be used to couple a highimpedance voltage source to a low-impedance load.
Darlington Pair n Darlington pair – A pair of transistors connected to provide higher current gain and input impedance than a single transistor.
Darlington Emitter Follower
Common Base (CB) Amplifiers n n A CB amplifier provides voltage and power gain. CB input and output signal voltages are in phase.
CB Amplifier AC Analysis
CB Amplifier Buffer n The CB amplifier can be used to couple a lowimpedance voltage source to a higher-impedance load.
High-Frequency Characteristics n The ground at the base terminal results in relatively high stability. n n Changes in the output circuit have little effect on the input circuit. CB amplifiers generate little noise. n n Noise – Any undesired voltage or current generated within, or externally to, an electronic system. Noise has natural and man-made sources. High frequency, low-amplitude signals are highly susceptible to noise.
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