To calculate the small-signal output resistance of a common-emitter BJT (Bipolar Junction Transistor) amplifier, you need to analyze the small-signal model of the amplifier circuit. The small-signal output resistance, often denoted as "ro," represents the resistance seen at the collector terminal when a small AC signal is applied to the output.
The small-signal model of a common-emitter BJT amplifier typically consists of the following components:
Input impedance (re): Represents the small-signal base-emitter resistance of the transistor.
Output impedance (ro): Represents the small-signal collector-emitter resistance of the transistor.
Voltage gain (Av): Represents the small-signal voltage gain of the amplifier.
Current gain (Ai): Represents the small-signal current gain of the amplifier.
To calculate the small-signal output resistance (ro), you can follow these steps:
Step 1: Short-circuit all the voltage sources in the small-signal model (replace them with a short circuit).
Step 2: Open-circuit all the current sources in the small-signal model (replace them with an open circuit).
Step 3: Apply a small test voltage (Vtest) at the output (collector terminal) and calculate the corresponding small-signal output current (Itest).
Step 4: The small-signal output resistance (ro) is then given by the ratio of Vtest to Itest:
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ro = Vtest / Itest
In practice, you can perform small-signal AC analysis using simulation tools like SPICE (Simulation Program with Integrated Circuit Emphasis) or by solving the small-signal equivalent circuit using appropriate formulas and transistor parameters.
Keep in mind that the small-signal model is an approximation valid only for small variations around the DC operating point, and the actual output resistance of the BJT amplifier might vary with biasing and operating conditions.