Small signal analysis is a fundamental approach used in electronics to understand how a Bipolar Junction Transistor (BJT) amplifier processes signals. In the active region, the BJT is designed for linear amplification. The transistor's behavior under these conditions is governed by its instantaneous base-emitter voltage VBE, a sum of the DC bias VBE, and a small AC signal VBE, resulting in the collector current iC. Here, the collector current has a DC component and an AC component.
Here, VT is the thermal voltage.
When the AC input signal VBE is significantly smaller than VT, a small signal approximation can be used to get the expression for the AC component of the collector current. The AC component of the collector current is the ratio of the product of the DC component of the collector with time-varying input voltage to the thermal voltage.
The ratio of the AC component of the collector current to the input signal is known as transconductance. The voltage gain of the BJT is a function of the transconductance of the BJT and the load resistance.
Here, the negative sign signifies a 180-degree phase shift between input and output signals.
At the base of the BJT, the small-signal base-emitter resistance rπ is crucial for input impedance considerations and is determined by:
This resistance impacts the transistor's response to input signals and is inversely proportional to the DC bias current.
Through small signal analysis, the detailed performance of BJT amplifiers with small input variations can be precisely characterized and utilized in electronic circuit design.
From Chapter 12:
Now Playing
Transistors
770 Views
Transistors
450 Views
Transistors
315 Views
Transistors
284 Views
Transistors
556 Views
Transistors
793 Views
Transistors
574 Views
Transistors
544 Views
Transistors
336 Views
Transistors
286 Views
Transistors
238 Views
Transistors
288 Views
Transistors
186 Views
Transistors
586 Views
Transistors
356 Views
See More
Copyright © 2025 MyJoVE Corporation. All rights reserved
We use cookies to enhance your experience on our website.
By continuing to use our website or clicking “Continue”, you are agreeing to accept our cookies.