Electrical and Computer Engineering — ELEC 301
Electrical and Computer Engineering
Electrical and Computer Engineering — ELEC 301
MINI PROJECT 2
BIASSING AND THE COMMON EMITTER AND COMMON BASE AMPLIFIERS
Objectives
To develop familiarity with the transistor’s hybrid-ð model and issues surrounding the biassing of
transistors as well as to analyse and measure the characteristics of an important transistor amplifier
using 3 commonly available transistors.
Introduction
The bipolar junction transistor can be modelled for small signal operation using the hybrid-ð model,
shown in Figure 2.1. Under the assumption of small signals, the transistor is assumed to work in its
active region. Large signals can cause the transistor to enter the saturation or cut-off modes, where the
hybrid-ð model no longer describes the circuit’s operation.
In this mini project, we examine two basic single transistor amplifier circuits, the common emitter and
common base amplifiers. These amplifiers are shown in figures 2.2 and 2.3, respectively.
This project consists of three parts. The transistors that you will be using/considering in your designs
are the 2N2222A, 2N3904, and 2N4401. The data sheets for these transistors may be obtained from the
manufacturers’ web sites (e.g., the 2N3904 and 2N4401 data sheets can be downloaded from the
Fairchild Semiconductor web site) or otherwise found on the web.
References:
1) ELEC 301 Course Notes.
2) Standard Resistor and Capacitor Values list that can be found in the Other Mini Project Related
Handouts module on the course’s CANVAS web site.
3) A. Sedra and K. Smith, "Microelectronic Circuits," 5th (or higher) Ed., Oxford University Press,
New York.
Background
The hybrid-ð model contains five elements: rð, cð, cì, gm and ro (rx and rì have been ignored). You can
also ignore the small-signal capacitances, cð and cì, at low and mid-band frequencies. The other model
element values are either calculated based on the bias conditions (e.g., gm) or can be obtained from the
“h-parameters”. The h-parameter values may be obtained from the data sheets provided by the
transistor’s manufacturer and may depend on the bias point. For example, gm = IC/VT, where IC is the
collector bias current and VT is the thermal equivalent of voltage given by VT = kT/q, where k is
Boltzmann's constant, T is the temperature in degrees Kelvin, and q is the charge on an electron in
Coulombs. Also, rð is given by rð = â/gm, where â is the common-emitter current gain of the transistor.
Here we should note that, by definition, â at d.c. is the same as the h-parameter hFE. hFE is available in
transistor data sheets and is given for various bias points (often it is also plotted as a function of the bias
point).
There are three main configurations in which single transistors may be connected to amplify signals.
The convention is to name each configuration after the node that is shunted to the common voltage at
mid band, i.e., to VCC or ground. The three configurations are the common-emitter (CE), the
common-base (CB), and the common-collector amplifiers (CC); the collector amplifier is not covered
in this mini project.
In the following sections, you will design bias networks for the common emitter amplifier and you will
characterize the response of each type of amplifier.