Bipolar Junction Transistor Amplifiers:Amplifier Types and Applications

Introduction

The Need for Amplification

The ﬁeld of electronics includes many applications wherein important information is generated in the form of a voltage or current waveform. Often this voltage is too small to perform the function for which it is intended. Examples of this situation are the audio microphone in a sound ampliﬁcation signal, the infrared diode detector of a heat-seeking missile, the output of the light intensity sensor of a CD player, the output of a receiving antenna for an amplitude modulation (AM) or frequency modulation (FM) signal, the cell phone signal received by the base station before retransmission, and the output of a receiving dish antenna in a satellite TV system. Each of these signals have peak voltages in the range of hundreds of microvolts to hundreds of millivolts. To transmit a signal over phone lines or through the atmosphere and then activate a speaker, or to illuminate a cathode-ray tube or other type of TV display may require a signal of several volts rather than millivolts.

An audio ampliﬁcation system may require peak voltages of tens of volts to drive the speaker. The speakers in a CD system or in radio receivers may also require several volts to produce appropriate levels of volume. The video information for a TV signal may need to be ampliﬁed to a value of thousands of volts to drive the picture tube or display. Even in computer systems, the signals recovered from the ﬂoppy or hard disk drives must be ampliﬁed to logic levels to become useful. Ampliﬁcation of each of these signals is imperative to make the signal become useful in its intended application. Consequently, ampliﬁers make up a part of almost every electronic system designed today.

At the present time, the bipolar junction transistor (BJT) and the metal-oxide semiconductor ﬁeld-effect transistor (MOSFET) are the major devices used in the ampliﬁcation of electronic signals.

A Brief History of Amplifying Devices

Ampliﬁcation is very important that the so-called era of electronics was ushered in only after the development of the triode vacuum tube that made ampliﬁcation possible in 1906 [1]. This device enabled public address systems and extended distances over which telephone communications could take place. Later, this element allowed the small signal generated by a receiving antenna located miles away from a transmitter to be ampliﬁed and demodulated by a radio receiver circuit. Without ampliﬁers, communi- cations over long distances would be difﬁcult if not impossible.

Improvement of the vacuum tube continued for several years with the tetrode and pentode tubes emerging in the late 1920s. These devices enabled the rapid development of AM radio and television. Vacuum tubes dominated the electronics ﬁeld well into the 1950s serving as the basic element in radios, television, instrumentation, and communication circuits.

The forerunner of the BJT, the point-contact transistor, was invented in 1947 [2]. The BJT followed shortly and grew to dominance in ampliﬁer applications by the mid-1960s. From then until the 1990s, this device had no peer in the electronics ﬁeld. In the 1990s, the MOSFET became important as an amplifying device, however, the silicon BJT along with the newer heterojunction BJT (HBT), continue to be used in many ampliﬁer applications.

Amplifier Types and Applications

Ampliﬁers can be classiﬁed in various ways. The input and output variables can be used to describe the ampliﬁer. The frequency range of the output variable or the power delivered to a load can also describe a major characteristic of an ampliﬁer. In the case of an operational ampliﬁer (op amp), the mathematical operations that can be performed by the circuit suggest the name of the ampliﬁer.

Input and Output Variables

Electronic transducers generally produce either voltage or current as the quantity to be ampliﬁed. After ampliﬁcation, the signal variable of interest could again be voltage or current, depending on the circuit or device driven. A given ampliﬁer can then have either current or voltage as the input signal and current or voltage as the output signal. As the input variable changes, the ampliﬁer produces a corresponding output variable change. Gain of an ampliﬁer is deﬁned as the ratio of output variable change to the input variable change as shown in Figure 23.1.

There are four possible ratios for the gain. These are summarized in Table 23.1 along with the corresponding ampliﬁer type.

The voltage ampliﬁer is used more often than the others, but each has speciﬁc applications. Some photo detectors generate output current proportional to the light intensity and this current is the input variable to the ampliﬁer. The output variable may be voltage for this application. There are other occasions when an ampliﬁer must generate an output current that is proportional to the input variable, thus, a given application may require any one of the four possible types listed in Table 23.1.

Frequency Range

The frequency range over which an ampliﬁer exhibits useful gain is an important speciﬁcation. An audio ampliﬁer may have a relatively constant gain from tens of Hz up to nearly 20 kHz. The magnitude of this constant gain is often called the midband gain. Those frequencies at which the magnitude of gain falls by 3 dB from the midband value are called 3-dB frequencies. For a high-ﬁdelity audio ampliﬁer, the lower 3-dB frequency may be 20 Hz while the upper 3-dB frequency may be 20 kHz. The bandwidth of this ampliﬁer extends from 20 Hz to 20 kHz. In spite of the relatively small absolute bandwidth, this type of ampliﬁer is referred to as a wideband amplifier. The ratio of the upper 3-dB frequency to the lower 3-dB frequency is 20,000/20, which is 1000 in this case.

Another important type of ampliﬁer is that used in radio receiver circuits. The receiver for an AM signal consists of several stages that amplify over a frequency range from about 450 to about 460 kHz. The gain may be maximum at 455 kHz, but drops very rapidly toward zero when the frequency is below 450 or above 460 kHz. This type of ampliﬁer is called a narrowband amplifier because the ratio of upper 3-dB frequency to lower 3-dB frequency has a near-unity value of 460/450 in this case.

Output Power

Some audio speakers may require hundreds of watts of power to operate at the desired level. An light emitting diode (LED) display may only require milliwatts of power. The ampliﬁers that drive these devices

can be classiﬁed as high power or low power, respectively. While there are no ﬁxed power values to differentiate, ampliﬁers that generate outputs in the milliwatt range are called low-power ampliﬁers or simply ampliﬁers. Ampliﬁers that produce outputs in the watt to several watt range are called power ampliﬁers or high-power ampliﬁers. The bulk of integrated circuit (IC) ampliﬁers are used in low-power applications although pulse-width modulation (PWM) IC ampliﬁer stages can deliver several watts of power to a load.

Distortion in Power Stages

There are two major causes of nonlinear distortion in BJT stages [3]. The ﬁrst is the change of current gain b with changes in IC or VCE. If an undistorted base current enters the device, the output current is distorted as a result of the change in current gain as the output quantities vary. This effect can also be explained for a common-emitter stage in terms of the output characteristics which show unequal spacing as IC or VCE is changed. Obviously, if the output signal amplitude is limited, less distortion occurs. In power stages, very large output-current changes may be prevalent, and higher distortion levels are to be expected.

The second important cause of distortion arises from the nonlinearity of the base–emitter character- istics of the transistor. When a voltage source drives a common-emitter stage with little series resistance, the base current can be quite distorted. This current varies with voltage in the same way that diode current varies with diode voltage. It is possible to use this input distortion to partially offset the output distortion; however, feedback techniques are most often used in distortion reduction. A perfect, distortion- free ampliﬁer would have transfer characteristics that form a straight line as shown in Figure 23.2.

The BJT with no emitter degeneration has a collector current that relates to base–emitter voltage given by

where k is the Boltzmann’s constant. This function is highly nonlinear, and only small variations of VBE can be applied to approximate linear behavior. An emitter resistance or emitter degeneration can be added to make the circuit more linear at the expense of reduced gain. This amounts to feedback to improve the nonlinear distortion. Another method of reducing distortion is to use several cascaded stages to achieve a very high gain; then feedback is applied around the ampliﬁer to decrease the distortion.

Total Harmonic Distortion

In an ampliﬁer circuit, the output signal should ideally contain only those frequency components that appear in the input signal. The nonlinear nature of amplifying devices introduces extraneous frequen- cies in the output signal that are not contained in the input signal. These unwanted signals are referred

to as harmonic distortion. One measure of the ampliﬁer's performance is called the total harmonic distortion (THD).

If a sinusoidal signal is applied to the ampliﬁer input, the output will also contain a major component of this signal at the fundamental frequency, with amplitude designated vf . In addition, there will be smaller harmonic components in the output signal. These amplitude values will be designated v2, v3, v4, . . . ,vn. Fortunately, in engineering applications, the harmonics decrease in amplitude as frequency increases. Thus, perhaps only the second or third harmonic is large enough to affect the THD. The THD can be deﬁned as a percentage by

Intercept Points

The unwanted harmonic distortion of an ampliﬁer is a function of output signal size. As the signal becomes larger, the linear output will increase, and so will the distortion. In fact, unwanted components due to harmonic distortion may increase more rapidly than the linear component.

Typically, an ampliﬁer with a small output signal, compared with the size of the active region, will have a large linear output component or fundamental component relative to the harmonic components. As signal size increases, the size of the second or third harmonic may become as large as the fundamental component. Generally, this will not occur before the output signal reaches the extremes of the active region, consequently it would be impossible to measure the size of the output signal that results in a harmonic component that is equal in amplitude to the fundamental component.

The concept of the intercept point allows this output to be approximated. A plot, often in dB, is made of the input power versus the output power of the fundamental component. This plot is a straight line with a slope of unity for smaller output signals. As the output approaches the edges of the active region, the output power increases much more slowly than does the input power. Finally, the output power is limited to some ﬁxed value as the output signal size is limited by the active region boundaries. Figure 23.3 shows this variation.

For small signals, each harmonic component is small. The third harmonic is plotted in Figure 23.3 in addition to the fundamental component. As input power is increased, the third harmonic component may increase at three times the rate of increase in the fundamental component. Before the input power is high enough to signiﬁcantly limit the fundamental power, a straight-line extension is added to the fundamental power plot and the third harmonic power plot. The point where these two extensions intersect is called the third-order intercept point (TOI). Note that this point often falls beyond the actual output power that can be achieved by the ampliﬁer. The TOI is speciﬁed in terms of the input power required at this point. For example, in Figure 23.3, the TOI is -10 dB. The higher the intercept point, the more linear is the ampliﬁer.

A second-order intercept can also be deﬁned in much the same way as the TOI is deﬁned. For communication ampliﬁers, the TOI is generally more important than the second-order intercept because the third-order term can result in a frequency that falls within the desired passband of the ampliﬁer. The TOI is an oft-used speciﬁcation for power stages as well as high-frequency ampliﬁers.

Operational Amplifiers

The op amp was developed long before the IC was invented. It was used in analog computers that were designed to solve differential equations by simulation. This ampliﬁer formed the basis of circuits that performed mathematical operations such as weighting, summing, subtracting, and integrating electrical signals. The op amp has two features that allow these operations to be accomplished. The ﬁrst feature is a differential input to allow subtraction. The second feature is that a virtual ground can be created when a feedback resistor is connected between the ampliﬁer output and the inverting or negative input terminal. This virtual ground allows perfect summation of currents into the inverting terminal and also allows perfect integration of an input signal.

Before the IC op amp was developed, the discrete circuit op amp was quite expensive and large, perhaps costing $200 and occupying a volume of 250 cm3. With its low cost and small size, the IC op amp has now made a rarely used component, one of the most popular IC chips in the electronics ﬁeld. The major limitation of the op amp is its frequency response that limits its use in very high-frequency applications. The near-ideal op amp has a very high gain, a high input impedance, and a low output impedance. The symbol for this device is shown in Figure 23.4(a) along with the ideal equivalent circuit of Figure 23.4(b).

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