Linearised transistor models using gm and ro for AC analysis.
Linearising around the Q-point yields g_m, r_π, r_o (BJT) or g_m, r_o (MOSFET). Hybrid-π and T models enable AC gain, input/output impedance and bandwidth calculations. DC and AC analyses are separated by superposition.
g_m=40·I_C for a BJT at room temperature (A/V per A).
Small-signal models let you predict how an amplifier responds to small AC signals, which is the basis of gain, bandwidth and impedance design.
Linearised transistor models using gm and ro for AC analysis.
Small-signal models let you predict how an amplifier responds to small AC signals, which is the basis of gain, bandwidth and impedance design.
The change in output current per change in input voltage, gm, which sets the gain of a transistor stage.
It linearises the device around its operating point, which is valid as long as the AC signal stays small.
Typical applications include calculating the gain of a transistor amplifier at mid-band, finding the input and output impedance of a common-source stage and estimating the frequency response of a feedback amplifier.
Study "CMOS inverter" first (topic 15 of 104 in Electronics & Embedded). It covers the groundwork this topic builds on.
Short-circuit DC sources for AC. High-frequency models add capacitances. Valid for small signals only.
g_m=40·I_C for a BJT at room temperature (A/V per A).
The next topic is "Operational amplifier", which builds directly on this one.
It is classed as Beginner in Electronics & Embedded (topic 16 of 104). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.
Want to practise? Open the interactive course for quick checks, quizzes, simulations and progress tracking.