Introduction
Signal amplification is one of the most important functions of an electronic circuit. In this applet you will play with a single stage amplifier. In practical circuits, amplifiers invariably comprise in more than one stage. We will however consider a single-stage amplifier to focus our attention on the salient characteristics of its performance.
This applet provides an overview of the Single Stage Common Emitter Amplifier. It includes both the amplifier circuit (in the upper part of the applet) and its characteristic curve (in the lower part of the applet) and the Static DC Load Line. The transistor is a bipolar junction transistor (BJT), as BJT plays prominet role in a wide-band amplifier.
The cuicuit shown in the upper part of the applet is termed the Common Emitter Amplifier, since the input signal is fed into the Base of BJT and the output signal is taken out of the Collector, while leaving the Emitter as common reference point for both the Input and the Output.
Rb is a high-valued resistor which determines current in the transistor's Base. The collector current, which is ßIb, all flows through Rc, and Rc is generally chosen so that about half of the supply voltage is dropped across it, leaving the other half to drop across the transistor from the Collector to the Emitter.
When the input signal is in the positive-half of its waveform, then the Base current will increase, causing a large increase in the Collector curent, causing a larger voltage drop across the resistor Rc, and thus resulting in a smaller voltage at the output. Thus a positive-swing in the Input voltage produces a large negative-swing in the Output, and we have 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 results. The maximum possible collector voltage is the voltage supply Vcc, and the minimum is the Collector saturation voltage, which is near zero. The desired bias point is then Vc = 1/2Vcc.
The input resistance to the ac input signal (Zin) for a common Emitter amplifier with its emitter grounded is determined primarily by the dynamic resistance of the Base-Emitter junction diode, which we term rej. The dc current across this junction is IE, which, strictly speaking, equals IC + IB. 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 (IE ~= IC).
In the lower part of the applet, we pesenting the BJT output characteristics along with the dc load line in order to give the user an overview of the working principles of a 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 understanding of the operational principles and the associated limitations of an amplifier.