Lecture 3: Introduction to electronic analog circuits 361-1-3661
Elementary single-transistor amplifiers
In this lecture, we build
based on a single BJT transistor and a single resistor. (We use
the resistor to translate the output current of the circuit into
voltage; otherwise the circuit will not be able to provide a
voltage gain.) We then analyze and compare the circuits'
small-signals gains to see for what applications they can be
Fig. 1 shows that only three different circuits can be based
on a single transistor. This is so because the collector cannot
serve as an
, and the base cannot serve as an
collector has a negligible effect on the base-emitter junction,
which controls the injection, and, hence, the collector has a
negligible effect on the current gain. The small-signal base
current is much below of that in the collector and emitter, and,
therefore, the voltage, current, and power gains at the base
would be smaller than 1.
Depending on the input-output pair, we will distinguish
among three different configurations (see Fig. 2): common
emitter (CE), common collector (CC), and common base
(CB). In elementary circuits, the common terminal will be
grounded in the small-signal analysis
start the analysis of a circuit from finding
its static state. According to the static state, we will find the
small-signal parameters of the transistor, replace the transistor
by either its small-signal
model, and then
suppress all the static sources. We will then solve the resultant
small-signal equivalent circuit in the most insightful way by
applying superposition, Thévenin, Norton, and Miller
theorems, and recognizing in the small-signal circuit such
elementary sub-circuits as the voltage and current dividers.
This will help us to better understand the circuit architecture
Fig. 1. Connecting practical signals to the transistor.
Fig. 2. Elementary single-transistor amplifiers.
2. Elementary Electronic Circuits with
a BJT Transistor
© Eugene Paperno, 2008