In BJT (Bipolar Junction Transistor) amplifier modeling, the G-parameters (short for "hybrid" or "short-circuit parameters") are one of the four different sets of parameters commonly used to analyze and characterize the small-signal behavior of a transistor amplifier. The other three sets of parameters are h-parameters, y-parameters, and z-parameters.
The G-parameters are particularly useful when analyzing circuits because they directly relate the input and output current and voltage for the transistor, making them convenient for many circuit design applications. The two sets of G-parameters are:
G<sub>21</sub>: Forward transfer conductance or current gain (Input current to output current ratio).
G<sub>12</sub>: Reverse transfer conductance or voltage gain (Output current to input voltage ratio).
To analyze circuits using G-parameters, you typically follow these steps:
Define the G-parameters: Express the transistor's small-signal behavior in terms of G<sub>21</sub> and G<sub>12</sub>. These parameters can be determined experimentally or calculated from the transistor's h-parameters, which are more commonly provided in datasheets.
Identify the circuit: Determine the circuit configuration you want to analyze using the G-parameters. Common configurations include common-emitter (CE), common-base (CB), and common-collector (CC) amplifiers.
Apply small-signal model: Replace the transistor in the circuit with its small-signal model. This involves replacing the transistor with its equivalent small-signal circuit containing the G-parameters.
Calculate voltage and current gains: Use the G-parameters and the small-signal model of the transistor to calculate the voltage gain (G<sub>12</sub>) and current gain (G<sub>21</sub>) of the amplifier circuit.
Analyze performance: Once you have obtained the voltage and current gains, you can analyze the performance of the amplifier, including input and output impedance, voltage gain, current gain, and power gain.
Design and optimization: Using the G-parameters, you can design and optimize your circuit for specific requirements, such as bandwidth, input/output impedance matching, and gain.
It's essential to note that while G-parameters are useful for certain applications, other parameter sets like h-parameters and y-parameters may be more appropriate in different scenarios. Each parameter set has its advantages and limitations, so selecting the most suitable one depends on the specific requirements of the circuit analysis or design you are working on.