Introduction

Introduction

Amplifier is one of the most important functions in electronic circuits. In this applet we are presenting an amplifier in a single stage. Though amplifiers invariably comprise more than one stage, we will specifically consider a single-stage amplifier in this applet to focus our attention on the salient characteristics of its performance.

This applet provides the overview of the Single Stage Common Emitter Amplifier. It includes both the amplifier circuit and its characteristic curve and Static Load Line. The transistor we chosen and put into this amplifier is a bipolar junction transistor (BJT), since the prominet role iot plays in wide-band amplifier.

The cuicuit show in the upper part of the applet is termed the Common Emitter Amplifier, since the input is applied at the base and the output is taken from the collector, leaving the emitter as the common element between input and output.

R_b is a high-value resistor which determines the transistor's base current. The collector current, which is ßI_b, all flows through R_c, and R_c is generally chosen so that about half of the supply voltage is dropped across it, leaving the other half across the transistor from collector to emitter.

When the input signal is on the positive-going half the base current will increase, causing a large increase in collector curent. Thus a positive-going input produces a large negative-going output in this type of amplifier, and we term it an inverting amplifier.

The ideal bias condition for this amplifier exists when the collector voltage is balanced halfway between its maximum and minimum values. In this way, the ac signal will be able to swing equally far in the positive and negative directions, and the maximum undistorted output signal will be abtained. In the circuit of the applet the maximum possible collector voltage is the suply Vcc, and the minimum is the collector saturation voltage, which is near zero. The desred bias point is then Vc = 1/2Vcc.

The input resistance to the ac signal (Z_in) for a common emitter amplifier with its emitter grounded directly is determined primarily by the dynamic resistance of the base base-emitter diode junction, which we term r_ej. The dc current across this junction is I_E, which strictly speaking equals I_C + I_B. In virtually all modern transistors, beta is much greater than 10, so it is an adequate approximation to say that the collector and emitter currents are indentical (I_E ~= I_C).

In the lower part of the applet, we pesenting the BJT characteristics along with the dc load line in order to give the user and overview of the work process of the common emitter amplifier. An analysis of a transistor amplifier can be made using the transistor's characteristic curves. The graphical analysis will provide a clearer visulization of the operation and limitations of the amplifier.