Java Physics



NPN transistor common-emitter amplifier



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Vin-Vout circuit
  • Characteristics of the (ideal) NPN transistor:
    1. When VBE < 0.7V, IB= 0; when IB ≠ 0, VBE is always equal to 0.7V.
    2. Before saturation, IC = βIB, where β is the current gain of the transistor.
    3. IC is saturated when VCE is dropped to zero.
  • Voltage Relationships:
    1. Input : Vin = IBRB + VBE
    2. Output: VCE + ICRC = VCC
  • Quiescent Point (Q-point)
    Before an input signal is fed into the amplifier, a steady base current flows into the transistor to cause a steady Vout. When a time-varying input signal is added to this steady input, the useful output will then fluctuate about the steady output (the Q-point). An optimal Q-point is chosen at the mid-way of the linear sloping part of the Vout - Vin graph in order to maximize the input amplitude that can be amplified with no distortion. Vin-Vout graph
  • Capacitors
    The two capacitors in the circuit are called “d.c. blocking capacitors”. They are used to prevent currents from flowing into the input device (e.g. microphone) and the output device (e.g. earphone)



Author and Copyright:

Chiu-king Ng

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