1. ELECTRONIC CIRCUIT ANALYSIS AND DESIGN ECE 312 Alipar,
Emmanuel B. BSECE - 3
2. C O N T E N T S I. Large Signal Amplifier II. Types of
Amplifier Class A Class AB Class B Class C Class D Class E Class F
Class G and H Differential Efficiency III. Field Effect Transistor
(FET)
3. 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.
4. 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
5. 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
6. This is the most linear of the classes, meaning the outpu
signal is a truer representation of what was imputed.
7. 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
8. 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.
9. Class A Circuit
10. 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.
11. 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.
12. 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.
13. 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.
14. ` 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.
15. ` 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.
16. 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.
17. 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.
18. ` 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.
19. 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.
20. 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.
21. ` 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.
22. ` 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.
23. ` Class E Amplifier
24. 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.
25. G. Class F and its Characteristics In pushpull 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.
26. ` 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.
27. ` 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.
28. 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.
29. 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.
30. ` 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.
31. ` Class H Amplifier
32. 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.
33. 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.
34. 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.
35. 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.
36. 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)
37. ` 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.
38. 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.
39. 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. Lets 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.
40. 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.
41. Sample Problem
42. 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.
43. 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.
44. 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.
