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Lesson 1 - Application of Transistors: AmplifiersTransistors can be used in many applications, but the primary usage is to build amplifiers.
The basic purpose of an amplifier is to raise the level of a signal, to create gain.
In many cases, amplifiers amplify all aspects of a signal, voltage, current and power, but some circuits are optimized for one application while others are better suited to another.
Some amplifiers are optimized to add minimum distortion to the signal, while other designs are optimized for high output power or efficiency (ratio of output power versus power drawn from the supply). Others may be optimized for wide bandwidth, while others may be optimized for lowest noise. Some amplifiers are optimized for pulse operation, while others operate continuously. Believe it or not, there are many more variations and other modes of operation that need to be optimized, depending on the application.
It is generally difficult to achieve at the same time low distortion, high efficiency, wide bandwidth and low noise and designers are often faced with tough choices when trying to optimize more than one parameter without sacrificing the others too much.
For the sake of simplicity, we will use bipolar transistors in the following examples. We will later study other types of transistors, such as the JFET and the MOSFET (see definition below), and compare their capabilities to the bipolar transistors when making amplifiers and other circuits.
How a transistor amplifies a signal?We have seen before that a bipolar transistor will let current flow through the collector-emitter junction when current is injected in the base-emitter junction.
The ratio of collector current available when a certain amount of base current is injected into the transistor is called the current gain. It is denoted with the Greek letter Beta:
ß = Ic/IbThe current gain is the main characteristic that allows a transistor to amplify a signal. Yet, in the following lessons, I will show you how to design a circuit that does not depend too strongly on gain, in order to achieve repeatable performance.
Please note that the current gain is often referred to as Hfe in data sheets.
Classes of amplification
Amplifiers are often categorized by their CLASS of OPERATION. The class of operation of an amplifier describes how the transistors of the amplifier are biased, because this will affect the tradeoff between distortion and efficiency. The class of operation is a quick way to establish if a design will be suited to high efficiency (Class C) or low distortion (Class A). Let's take a look at the 3 most common classes of operation, and determine what makes them work the way they do.
Class A Amplifiers:
Class A amplifiers are designs where the transistors are biased near the middle of their linear range, midway between being completely turned off and completely turned on (saturated). This way, the transistor is as far away as possible from the non-linear portions of its operating range.
When the input signal changes, the operating point of the transistor moves up and down, following the changes in input voltage, creating a higher version of the input signal on its output.
Class A amplifiers optimize linearity (the absence of distortion) at the expense of efficiency. They are usually simple, the main requirement being a stable bias circuit so the transistor remains in the middle of its linear region. Because of low efficiency, they are usually used for low level amplification.
Class A amplifiers have high quiescent current (current in the absence of input signal), so they dissipates power all the time. Actually, a class A amplifier dissipates even more heat when there is no input signal. Typical efficiency is about 33% at full power, and much less at lower power levels.
Class B Amplifiers:Class B amplifiers use transistors that are biased just at their cutoff point. The transistors are turned off unless there is a signal to amplify. If we were to make a class B amplifier with a single transistor, it would only amplify one side of the input signal, leading to considerable distortion. By combining two transistors, we can make class B amplifiers that have relatively low distortion. Each transistor amplifies one side of the signal. This is called a push-pull configuration.
Class B amplifiers have good efficiency (much better than class A) of about 50% over a wide power range, have moderate distortion and can be used in "linear" applications such as audio amplifiers.
Class B amplifiers have virtually no quiescent current, so they stays cool in the absence of input signal.
A variation of class B is called class AB, where the operating point is moved up a little bit, so that the amplifier behaves like a class A amplifier for small signals (where the distortion of a class B amplifier is most noticeable) and like a class B amplifier (where efficiency is much better) for large signals.
Class AB amplifiers have a small quiescent current.
Class C AmplifiersClass C amplifiers are biased very far in the cutoff region, so they will only amplify a signal large enough to get the transistors out of cutoff. They are very non-linear and are only used in certain applications such as RF amplification. They generate a lot of distortion, so they must be followed by good filters to remove the unwanted harmonics and other distortion products. The reason why someone would want to use class C amplifiers is that they are very efficient, up to 70%.
Amplifiers as interface to digital logic
With the pervasive use of microcontrollers and other digital logic in modern electronics, There is often the need to amplify a small signal to levels sufficient to drive logic gates, with typically require a signal between 0 and 5 V (in some cases between 0 and 3 V).
I will use a sound detector as an example, such as the clapper key-ring that makes noise when you clap your hands so you can find your keys.
The clapper has a small microphone that only generates millivolts when receiving a sound. We must amplifiy this signal to the 3 V or 5 V needed by the logic chip which generates the sound.
It is not important to amplify the sound with low distortion, we just want gain, and we want to be able to drive the logic with an on/off signal.
This function is performed using a special type of amplifier called a comparator. You can imagine a comparator as a high gain amplifier with the output voltage limited to known values that are appropriate to drive logic gates, 0 to 3 V or 0 to 5 V.
With such specialized amplifiers, the gain is so great that a small variation of the input signal (less than a millivolt) is sufficient to cause the output to quickly jump from the minimum to the maximum voltage the device is capable of providing.
We will study comparators and their applications in more details in a future lesson.
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