BJT small-signal model (hybrid-π)

Transistors and thyristors · Advanced · about 20 minutes · part of Electronics Engineering

For small signals a biased transistor is a resistor r_π between base and emitter and a current source g_m·v_be into the collector. Both come from the DC collector current: g_m = I_C ÷ V_T and r_π = β ÷ g_m. The voltage gain of a common-emitter stage is then −g_m·R_C.

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How it works

Try it on the bench

1. Change R2 to 12 kΩ: more collector current, so more g_m and more gain, and a smaller r_π. Go to 15 kΩ and the transistor saturates: the small-signal model no longer applies.

Parts on the bench

• Bench power supply
• AC source
• Film capacitor
• 4 × Resistor
• NPN transistor
• Electrolytic capacitor

In the course

Electronics Engineering, module “Transistor amplifiers”.

More on transistors and thyristors

• BJT current gain (β)
• Transistor as a switch
• Emitter follower (buffer)
• MOSFET low-side switch
• Inductive load and the flyback diode
• Darlington pair
• H-bridge motor driver
• Push-pull output stage