Amplifiers.pdf

• In 1904, the vacuum-tube diode was introduced by J. A. Fleming.
• Shortly thereafter, in 1906, Lee De Forest added a third element, called the control
grid, to the vacuum diode, resulting in the first amplifier, the triode.
• In the early 1930s the four-element tetrode and the five-element pentode gained
prominence in the electron-tube industry.
• On December 23, 1947, Dr. S. William Shockley, Walter H. Brattain, and John
Bardeen demonstrated the amplifying action of the first transistor at the Bell
Telephone Laboratories.
• All amplifiers (devices that increase the voltage, current, or power level) have at least
three terminals, with one controlling the flow or potential between the other two.

• This signal processing is usually most conveniently performed by electronic systems.
• For this to be possible, however, the signal must first be converted into an electric
signal, that is, a voltage or a current.
• This process is accomplished by devices known as transducers.
• The simplest signal-processing task is that of signal amplification.
• The need for amplification arises because transducers provide signals that are said to
be "weak," that is, in the microvolt (μV) or millivolt (mV) range and possessing little
energy.
• Such signals are too small for reliable processing, and processing is much easier if
the signal magnitude is made larger.
• The functional block that accomplishes this task is the signal amplifier.

• The signal amplifier is obviously a two-port network. Its function is conveniently represented by the
circuit symbol of Fig. 1(a).
• A more common situation is illustrated in Fig. 1(b), where a common terminal exists between the input
and output ports of the amplifier. This common terminal is used as a reference point and is called the
circuit ground.
• An amplifier contains one or more active devices and transforms power from the dc supply into the
signal pwer at the output, the signal amplitude at the output being proportional to that at the input.

• In the case of a transformer, although the voltage delivered to the load could be greater than the
voltage feeding the input side (the primary), the power delivered to the load (from the secondary side
of the transformer) is less than or at most equal to the power supplied by the signal source.
• On the other hand, an amplifier provides the load with power greater than that obtained from the
signal source. That is, amplifiers have power gain.
• The power gain of the amplifier is defined as:
where ia is the current that the amplifier delivers to the load (RL)' ia = 'oa/RLo and iI is the current the
amplifier draws from the signal source.
• The current gain of the amplifier is defined as:
• Thus,

• The amplifier gains are ratios of similarly dimensioned quantities.
• Alternatively: electronics engineers express amplifier gain with a logarithmic measure.
• Specifically the voltage gain Av can be expressed as:

• Practically speaking, the amplifier transfer characteristic remains linear over only a limited
range of input and output voltages.
• For an amplifier operated from two power supplies the output voltage cannot exceed a
specified positive limit and cannot decrease below a specified negative limit.
• Each of the two saturation levels is usually within a volt or so of the voltage of the
corresponding power supply.
• With the positive saturation levels denoted L+ and L-, respectively, obviously, in order to
avoid distorting the output signal waveform, the input signal swing must be kept within the
linear range of operation,

6. As based on magnitude-response curve (or coupling)
(a) capacitively coupled amplifier (b) direct-coupled amplifier
(c) tuned or bandpass amplifier.

• In small-signal amplifiers, the main factors are usually amplification linearity and
magnitude of gain.
• Since signal voltage and current are small in a small-signal amplifier, the amount of
power-handling capacity and power efficiency are of little concern.
• Large-signal or power amplifiers, on the other hand, primarily provide sufficient
power to an output load to drive a speaker or other power device, typically a few
watts to tens of watts.
• The main features of a large-signal amplifier are the circuit’s power efficiency, the
maximum amount of power that the circuit is capable of handling, and the impedance
matching to the output device.

• In capacitively coupled amplifier [Fig. (a)] the gain remains constant over a wide frequency range but
falls off at low and high frequencies. This is a common type of frequency response found in audio
amplifiers.
• Internal capacitances in this type of amplifier (a transistor) cause the falloff of gain at high
frequencies. On the other hand, the falloff of gain at low frequencies is usually caused by coupling
capacitors used to connect one amplifier stage to another. Coupling capacitors will thus cause loss of
gain at low frequencies and cause the gain to be zero at dc.
Frequency response for (a) a capacitively coupled amplifier

• There are many applications in which it is important that the amplifier maintain its gain at low
frequencies down to dc. Furthermore, monolithic integrated-circuit (IC) technology does not
allow the fabrication of large coupling capacitors. Thus IC amplifiers are usually designed as
directly coupled or dc amplifiers (as opposed to capacitively coupled or ac amplifiers). Figure
(b) shows the frequency response of a dc amplifier. Such a frequency response
characterizes what is referred to as a low-pass amplifier.
Frequency response for (b) a direct-coupled amplifier

• In a number of applications, such as in the design of radio and TV receivers, the need
arises for an amplifier whose frequency response peaks around a certain frequency (called
the center frequency) and falls off on both sides of this frequency, as shown in Fig. (c).
Amplifiers with such a response are called tuned amplifiers, bandpass amplifiers, or
bandpass filters. A tuned amplifier forms the heart of the front-end or tuner of a
communication receiver; by adjusting its center frequency to coincide with the frequency of
a desired communications channel (e.g., a radio station), the signal of this particular
channel can be received while those of other channels are attenuated or filtered out.
Frequency response for (c) a tuned or bandpass amplifier.

• A pure sinusoidal signal has a single frequency at which the voltage varies
positive and negative by equal amounts. Any signal varying over less than
the full 360°cycle is considered to have distortion.
• An ideal amplifier is capable of amplifying a pure sinusoidal signal to provide
a larger version, the resulting waveform being a pure single-frequency
sinusoidal signal.
• When distortion occurs, the output will not be an exact duplicate (except for
magnitude) of the input signal.
• Distortion can occur because the device characteristic is not linear, in which
case non-linear or amplitude distortion occurs. This can occur with all classes
of amplifier operation.
• Distortion can also occur because the circuit elements and devices respond
to the input signal differently at various frequencies, this being frequency
distortion.

• The simple fixed-bias circuit connection shown in Fig. 2 can be used to discuss the main features of a
class A series-fed amplifier.

• A number of circuit arrangements for obtaining class B operation are possible.
• The input signals to the amplifier could be a single signal, the circuit then
providing two different output stages, each operating for one-half the cycle.
• If the input is in the form of two opposite-polarity signals, two similar stages could
be used, each operating on the alternate cycle because of the input signal.
• One means of obtaining polarity or phase inversion is using a transformer, and the
transformer-coupled amplifier has been very popular for a long time.
• Opposite-polarity inputs can easily be obtained using an op-amp having two
opposite outputs or using a few op-amp stages to obtain two opposite-polarity
signals.
• An opposite-polarity operation can also be achieved using a single input and
complementary transistors (npn and pnp, or nMOS and pMOS).

• Fig. (a) shows a center-tapped transformer to provide opposite-phase signals.
• If the transformer is exactly center-tapped, the two signals are exactly opposite in
phase and of the same magnitude.

• The circuit of Fig. b uses a BJT stage with in-phase output from the emitter and
opposite-phase output from the collector.
• If the gain is made nearly 1 for each output, the same magnitude results.

• Probably most common would be using op-amp stages, one to provide an inverting
gain of unity and the other a noninverting gain of unity, to provide two outputs of the
same magnitude but of opposite phase.

• Using complementary transistors (npn and pnp) it is possible to obtain a full cycle output across a load
using half-cycles of operation from each transistor, as shown in Fig. a.
• Whereas a single input signal is applied to the base of both transistors, the transistors, being of
opposite type, will conduct on opposite half-cycles of the input.
• The npn transistor will be biased into conduction by the positive half-cycle of signal, with a resulting
half-cycle of signal across the load as shown in Fig. b. During the negative half-cycle of signal, the
pnp transistor is biased into conduction when the input goes negative, as shown in Fig. c.

Amplifiers.pdf

More Related Content

Similar to Amplifiers.pdf

Recently uploaded

Amplifiers.pdf