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Shows the functionality of a BJT voltage divider amplifier!
This circuit is designed to demonstrate the operation of a common-emitter BJT amplifier using a resistive voltage divider for biasing.
The input signal is a 1 V, 1 kHz sinusoid, applied directly to the base of the transistor. Note that no source resistance (Rs) and no load resistance (RL) are used in this circuit, allowing the intrinsic amplifier behavior to be observed without external loading effects.
The voltage divider formed by R1 = 15 kΩ and R2 = 50 kΩ establishes a stable DC bias at the base of the transistor. This bias places the transistor in the active region, allowing linear amplification of the AC input signal.
When the input signal is applied, observe that small changes in the base voltage produce larger changes in the collector voltage. The output waveform at the collector is amplified and inverted, demonstrating the characteristic 180° phase shift of a common-emitter amplifier.
Note that the emitter resistor (RE = 1 kΩ) provides negative feedback, stabilizing the operating point and reducing gain, while the collector resistor (RC = 2 kΩ) sets the output voltage swing. In steady operation, the output remains centered around a DC level determined by VCC = 15 V, with the AC signal riding on top of this bias point.
Note the use of the 30 µF capacitors as AC bypass capacitors in this circuit.
These capacitors present a low impedance to the 1 kHz AC signal while blocking DC, allowing the AC components of the signal to pass while preserving the DC biasing of the transistor. At the operating frequency, the capacitive reactance is small compared to the surrounding resistances, effectively shorting AC signals to ground where intended.
In steady state, the capacitors have no effect on the DC operating point but play a critical role in shaping the AC response of the amplifier.
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