Ece 312 power point

ELECTRONIC CIRCUIT
ANALYSIS AND DESIGN
ECE 312
Alipar, Emmanuel B.
BSECE - 3

C O N T E N T S
I. Large Signal Amplifier
II. Types of Amplifier
●
Class A
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Class AB
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Class B
●
Class C
●
Class D
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Class E
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Class F
●
Class G and H
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Differential Efficiency
III. Field Effect Transistor (FET)

LARGE SIGNAL 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. 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.

Amplifiers are classified by different class ratings (A, AB,
D, etc.) and categorized by the number of channels
they provide (mono, 2-, 4- etc). The class of an
amplifier refers to the amplifier's internal circuitry.
Class A amplifiers have the highest sound quality, but
are the least efficient and do not dissipate heat very
effectively. Class AB amplifiers run more efficiently
and dissipate heat better than Class A amplifiers. This
is why Class AB amps are more reliable and produce
lower distortion in comparison to Class A amps. In
terms of the angle of flow for the input signal, Class A
and Class AB amplifiers have analog designs, while
Class D amplifiers have switching designs.
TYPES OF AMPLIFIER

A. Class A and its Characteristics
●
The output signal varies for full 360° of the cycle.
Figure 15.1 a shows that this requires the Q-point
to be biased at a level so that at least half the
signal swing of the output may vary up and down
without going to a high-enough voltage to be
limited by the supply voltage level or too low to
approach the lower supply level, or 0 V in this
description. It is the highest sound quality, but are
the least efficient and do not dissipate heat very
effectively.
TYPES OF AMPLIFIER

This is the most linear of the classes, meaning the outpu
signal is a truer representation of what was imputed.

●
The output device (transistor) conducts electricity for
the entire cycle of input signal. In other words, they
reproduce the entire waveform in its entirety.
●
These amps run hot, as the transistors in the power
amp are on and running at full power all the time.
●
There is no condition where the transistor(s) is/are
turned off. That doesn't mean that the amplifier is
never or can never be turned off; it means the
transistors doing the work inside the amplifier have a
constant flow of electricity through them. This
constant signal is called "bias".
●
Class A is the most inefficient of all power amplifier
designs, averaging only around 20.
Class A Characteristics

Because of these factors, Class A amplifiers are very
inefficient: for every watt of output power, they
usually waste at least 4-5 watts as heat. They are
usually very large, heavy and because of the 4-5
watts of heat energy released per watt of output,
they run very hot, needing lots of ventilation (not at
all ideal for a car, and rarely acceptable in a home).
All this is due to the amplifier constantly operating at
full power.

The upside is that these amps are the most enjoyed of all
amplifiers. These amps dig out musical detail, since the
transistor reproduces the entire audio waveform without
ever cutting off. As a result the sound is cleaner and
more linear; that is, it contains much lower levels of
distortion. They are the most accurate of all amps
available, but at significant cost to manufacture, because
of tight tolerances, and the additional components for
cooling and heat regulation.

B. Class AB and its Characteristics
An amplifier may be biased at a dc level above the zero
base current level of class B and above one-half the
supply voltage level of class A; this bias condition is
class AB. Class AB operation still requires a push-pull
connection to achieve a full output cycle, but the dc
bias level is usually closer to the zero base current level
for better power efficiency, as described shortly. For
class AB operation, the output signal swing occurs
between 1800 and 3600 and is neither class A nor class
B operation.

There are many implementations of the Class AB
design. A benefit is that the inherent non-
linearity of Class B designs is almost totally
eliminated, while avoiding the heat-generating
and wasteful inefficiencies of the Class A
design. And as stated before, at some output
levels, Class AB amps operate in Class A. It is
this combination of good efficiency (around 50)
with excellent linearity that makes class AB the
most popular audio amplifier design.

Class AB Characteristics
●
In fact, many Class AB amps operate in Class A at lower
output levels, again giving the best of both worlds.
●
The output bias is set so that current flows in a specific
output device for more than a half the signal cycle
but less than the entire cycle.
●
There is enough current flowing through each device
to keep it operating so they respond instantly to
input voltage demands.
In the push-pull output stage, there is some overlap as
each output device assists the other during the short
transition, or crossover period from the positive to the
negative half of the signal.

`
C. Class B and its Characteristics
A class B circuit provides an output signal varying over
one-half input signal cycle, or for180° of signal, as in
Fig. 15.1 b. The dc bias point for class B is therefore at
0 V, with the output then varying from this bias point
for a half-cycle. Obviously, the output is not a faithful
reproduction of the input if only one half-cycle is
present.

`
Two class B operations-one to provide
output on the positive output half-cycle and another to
provide operation on the negative-output half-
cycle are necessary. The combined half-cycles then
provide an output for a full 360° of operation.
This type of connection is referred to as push-pull
operation, which is discussed later in this chapter.
Note that class B operation by itself creates a very
distorted output signal since reproduction of the
input takes place for only 180° of the output signal swing.

Class B Characteristics
●
The input signal has to be a lot larger in order to drive
the transistor appropriately.
●
This is almost the opposite of Class A operation
●
There have to be at least two output devices with this
type of amp. This output stage employs two output
devices so that each side amplifies each half of the
waveform. [li Either both output devices are never
allowed to be on at the same time, or the bias
(remember, that trickle of electricity?) for each device
is set so that current flow in one output device is zero
when not presented with an input signal.
●
Each output device is on for exactly one half of a
complete signal cycle.

D. Class C and its Characteristics
It is an amplifier is biased for operation for
less than 180 of the input signal cycle and
will operate only with a tuned or resonant
circuit which provides a full cycle of
operation for the tuned or resonant
frequency. Such power amplifiers are,
therefore, employed in special areas of
tuned circuits, such as radio or
communications.

`
E. Class D and its characteristics
It is an amplifier that is similar to a switchable power
supply, but with audio signals controlling, or
modulating, the switching action. To do this, you use a
technology called Pulse Width Modulation.
According to experts, audio signals can be used to
modulate a PWM system to create a high power audio
amplifier at fairly low voltages using very small
components.

Class D Characteristics
●
While some Class D amps do run in true digital mode,
using coherent binary data, most do not.
●
They are better termed "switching" amplifiers, because
here the output devices are rapidly switched on and
off at least twice for each cycle.
●
Depending on their switching frequency, they may be
"switched on" or "off" millions of times a second.
●
Class D operation is theoretically 100% efficient, but in
practice, they are closer to 80-90% efficiency.
●
This efficiency gain is at the cost of high-fidelity.

F. Class E and its Characteristics
The class-E/F amplifier is a highly efficient switching
power amplifier, typically used at such high
frequencies that the switching time becomes
comparable to the duty time. As said in the class-D
amplifier, the transistor is connected via a serial LC
circuit to the load, and connected via a large L
(inductor) to the supply voltage. The supply voltage
is connected to ground via a large capacitor to
prevent any RF signals leaking into the supply. The
class-E amplifier adds a C (capacitor) between the
transistor and ground and uses a defined L1 to
connect to the supply voltage.

`
The following description ignores DC, which can be added
easily afterwards. The above mentioned C and L are in
effect a parallel LC circuit to ground. When the transistor
is on, it pushes through the serial LC circuit into the load
and some current begins to flow to the parallel LC circuit
to ground. Then the serial LC circuit swings back and
compensates the current into the parallel LC circuit. At
this point the current through the transistor is zero and it
is switched off. Both LC circuits are now filled with energy
in C and L0.

`
The whole circuit performs a damped oscillation. The
damping by the load has been adjusted so that some
time later the energy from the Ls is gone into the load,
but the energy in both C0 peaks at the original value
to in turn restore the original voltage so that the
voltage across the transistor is zero again and it can be
switched on.

Class E Characteristics
Class E uses a significant amount of second-harmonic voltage.
The second harmonic can be used to reduce the overlap
with edges with finite sharpness. For this to work, energy on
the second harmonic has to flow from the load into the
transistor, and no source for this is visible in the circuit
diagram. In reality, the impedance is mostly reactive and the
only reason for it is that class E is a class F (see below)
amplifier with a much simplified load network and thus has
to deal with imperfections.
In many amateur simulations of class-E amplifiers, sharp
current edges are assumed nullifying the very motivation for
class E and measurements near the transit frequency of the
transistors show very symmetric curves, which look much
similar to class-F simulations.

G. Class F and its Characteristics
In push–pull amplifiers and in CMOS, the even
harmonics of both transistors just cancel. Experiment
shows that a square wave can be generated by those
amplifiers. Theoretically square waves consist of odd
harmonics only. In a class-D amplifier, the output filter
blocks all harmonics; i.e., the harmonics see an open
load. So even small currents in the harmonics suffice
to generate a voltage square wave. The current is in
phase with the voltage applied to the filter, but the
voltage across the transistors is out of phase.
Therefore, there is a minimal overlap between current
through the transistors and voltage across the
transistors. The sharper the edges, the lower the
overlap.

`
While in class D, transistors and the load exist as two
separate modules, class F admits imperfections like the
parasitics of the transistor and tries to optimise the
global system to have a high impedance at the
harmonics. Of course there has to be a finite voltage
across the transistor to push the current across the on-
state resistance. Because the combined current through
both transistors is mostly in the first harmonic, it looks
like a sine. That means that in the middle of the square
the maximum of current has to flow, so it may make
sense to have a dip in the square or in other words to
allow some overswing of the voltage square wave. A
class-F load network by definition has to transmit below
a cutoff frequency and reflect above.

`
Any frequency lying below the cutoff and having its
second harmonic above the cutoff can be amplified,
that is an octave bandwidth. On the other hand, an
inductive-capacitive series circuit with a large
inductance and a tunable capacitance may be simpler to
implement. By reducing the duty cycle below 0.5, the
output amplitude can be modulated. The voltage square
waveform degrades, but any overheating is
compensated by the lower overall power flowing. Any
load mismatch behind the filter can only act on the first
harmonic current waveform, clearly only a purely
resistive load makes sense, then the lower the
resistance, the higher the current.

H. Class G & H and its Characteristics
There are a variety of amplifier designs that enhance
class-AB output stages with more efficient
techniques to achieve greater efficiencies with low
distortion. These designs are common in large audio
amplifiers since the heatsinks and power
transformers would be prohibitively large (and
costly) without the efficiency increases. The terms
"class G" and "class H" are used interchangeably to
refer to different designs, varying in definition from
one manufacturer or paper to another.

Class-G amplifiers (which use "rail switching" to
decrease power consumption and increase efficiency)
are more efficient than class-AB amplifiers. These
amplifiers provide several power rails at different
voltages and switch between them as the signal output
approaches each level. Thus, the amplifier increases
efficiency by reducing the wasted power at the output
transistors. Class-G amplifiers are more efficient than
class AB but less efficient when compared to class D,
without the negative EMI effects of class D. Class-H
amplifiers take the idea of class G one step further
creating an infinitely variable supply rail. This is done by
modulating the supply rails so that the rails are only a
few volts larger than the output signal at any given
time.

`
The output stage operates at its maximum efficiency all the
time. Switched-mode power supplies can be used to create
the tracking rails. Significant efficiency gains can be achieved
but with the drawback of more complicated supply design
and reduced THD performance. In common designs, a voltage
drop of about 10V is maintained over the output transistors in
Class H circuits. The picture above shows positive supply
voltage of the output stage and the voltage at the speaker
output. The boost of the supply voltage is shown for a real
music signal. The voltage signal shown is thus a larger version
of the input, but has been changed in sign (inverted) by the
amplification. Other arrangements of amplifying device are
possible, but that given (that is, common emitter, common
source or common cathode) is the easiest to understand and
employ in practice. If the amplifying element is linear, the
output is a faithful copy of the input, only larger and inverted.

In summary:
Class G and H amplifiers add complexity to the signal and
degrade it because of the need for switching depending on
the input signal
Class D amplifiers are models of efficiency, but with a loss of
detail and fidelity
Class B amplifiers generally introduce some crossover
distortion, but move away from Class D, G, and H's extreme
non-linearity.
Class AB amplifiers may introduce some crossover distortion,
but they get closer to the ideal of Class A for most of its
operating regime.
They are indeed the best compromise of performance versus
cost.
Class A amplifiers introduce no crossover distortion and are
the most desirable amps to own, but they are expensive, run
hot, and have to be very well-built.

Differential Efficiency
Types or class of amplifier has a different and specific
characteristics, basing on the meaning and the data that
gathered.
The power efficiency of an amplifier, defined as the ratio of
power output to power input, improves
(gets higher) going from class A to class D. In general terms, we
see that a class A amplifier, with dc
bias at one-half the supply voltage level, uses a good amount of
power to maintain bias, even with no
input signal applied. This results in very poor efficiency,
especially with small input signals, when very
little ac power is delivered to the load, In fact, the maximum
efficiency of a class A circuit, occurring
for the largest output voltage and current swing, is only 25%
with a direct or series-fed load
connection and 50% with a transformer connection to the load.

Class B operation, with no dc bias power for no input signal,
can be shown to provide a maximum efficiency that reaches
78.5%. Class D operation can achieve power efficiency over
90% and provides the most efficient operation of all the
operating classes. Since class AB falls between class A and
class B in bias, it also falls between their efficiency ratings-
between 25% (or 50%) and 78.5%. Table 15.1 summarizes
the operation of the various amplifier classes. In class B
operation, a push-pull connection is obtained using either a
transformer coupling or by using complementary (or quasi-
complementary) operation with npn and pnp transistors to
provide operation on opposite polarity cycles. While
transformer operation can provide opposite cycle signals,
the transformer itself is quite large in many application. A
transformer less circuit using complementary transistors
provides the same operation in a much smaller package.

Calculate the input power, output power, and efficiency of the
amplifier circuit in Fig. 15.5 for an input voltage that results in a
base current of 10 mA peak.

P(DC) = Vcc Icq = (20 V)(0.48 A) = 9.6 W
The amplifier's power efficiency can then be calculated using Eq. (15.8)

`
Field Effect Transistor (FET)
The junction field-effect transistor,or JFET, is perhaps
the simplest transistor available. It has some important
characteristics, notably a very high input resistance.
Unfortunately, however (for the JFET), the MOSFET has an even
higher input resistance. This, together with the many other
advantages of MOS transistors, has made the JFET virtually
Obsolete. Currently, its applications are limited to discrete-circuit
design, where it is used both as an amplifier and as a switch.
Its integrated-circuit applications are limited to the design of the
differential input stage of some operational amplifiers, where
advantage is taken of its high input resistance (compared to the
BJT). In this section,we briefly consider JFET operation and
characteristics.

FET's come in five general types, but we will restrict
ourselves to JFET's (for Junction FET) initially and our
examples will only use n-channel JFET's. These have n
channel doping and are similar to npn transistors. The p
channel JFET requires the opposite voltage on the gate.
They usually have poorer performance due to the
lower mobility and shorter lifetimes of holes, as
compared to electrons. As mentioned above, the
source-drain current is the only current that flows
through a FET. The source-drain current is labeled ID.
The voltage applied to the gate terminal enables this
current by creating an electric field inside the channel.
There is no fundamental difference between the
source and drain terminals of a JFET.

This minimum gate voltage, called VP, is a
characteristic that varies from one model of JFET to
the next. It is usually in the range from –3 V to –10
V. Even within the same type of FET this parameter
varies significantly from one device to the next.
For example, the range specified for a 2N5485 the
range is between -0.3 V and -3.0 V. Let’s summarize
these properties: For VGS < Vp: ID = 0 (8.1) For VGS
> 0.6 V: Device Fails! (8.2) When VGS is between
these bounds ID depends on both VGS and VDS. A
complete description of the device would require a
two-dimensional plot showing how ID varies with
both VGS and VDS.

nnnThe common transistor is called a junction transistor, and it was the key device which led to
the solid state electronics revolution. In application, the junction transistor has the
disadvantage of a low input impedance because the base of the transistor is the signal input
and the base-emitter diode is forward biased. Another device achieved transistor action with
the input diode junction reversed biased, and this device is called a "field effect transistor"
or a "junction field effect transistor", JFET. With the reverse biased input junction, it has a
very high input impedance. Having a high input impedance minimizes the interference with
or "loading" of the signal source when a measurement is made.
For an n-channel FET, the device is constructed from a bar of n-type material, with the
shaded areas composed of a p-type material as a Gate. Between the Source and the Drain,
the n-type material acts as a resistor. The current flow consists of the majority carriers
(electrons for n-type material).
Characteristic curves
Common source amplifier
Since the Gate junction is reverse biased and because there is no minority carrier
contribution to the flow through the device, the input impedance is extremely high.
The control element for the JFET comes from depletion of charge carriers from the n-
channel. When the Gate is made more negative, it depletes the majority carriers from a
larger depletion zone around the gate. This reduces the current flow for a given value of
Source-to-Drain voltage. Modulating the Gate voltage modulates the current flow through
the device.

This makes it different from a normal transistor, since
current can flow either from the drain to the source
or from the source to the drain. Note that there is a
maximum value and a minimum value for the gate
voltage in order to keep the device operating. Since
a JFET has a diode junction separating the gate from
the channel, the gate must be held at a voltage of
less than 0.6 V above the channel (usually the
source terminal). If the gate voltage becomes
greater than this, the junction will become
conducting and the gate current will no longer be
zero. Usually, we will not let the voltage between
the gate and the source (VGS) get any greater than
0. If the gate is biased too negative then no current
flows and the channel is said to be “pinched off.”

JFET Characteristic Curves
Characteristic curves for the JFET are shown at left. You
can see that for a given value of Gate voltage, the
current is very nearly constant over a wide range of
Source-to-Drain voltages. The control element for the
JFET comes from depletion of charge carriers from the
n-channel. When the Gate is made more negative, it
depletes the majority carriers from a larger depletion
zone around the gate. This reduces the current flow for
a given value of Source-to-Drain voltage. Modulating
the Gate voltage modulates the current flow through
the device.

The transfer characteristic for the JTET is useful for
visualizing the gain from the device and identifying the
region of linearity. The gain is proportional to the slope
of the transfer curve. The current value IDSS represents
the value when the Gate is shorted to ground, the
maximum current for the device. This value will be part
of the data supplied by the manufacturer. The Gate
voltage at which the current reaches zero is called the
"pinch voltage", VP. Note that the dashed line
representing the gain in the linear region of operation
strikes the zero current line at about half the pinch
voltage.

Ece 312 power point

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