November 1960 Electronics WorldTable of ContentsPeople old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Electronics World was published from May 1959 through December 1971. See all Electronics World articles. |
The fundamentals of Class-B push-pull amplifiers have not changed since 1960 when this article appeared in Electronics Word. The transistors for making them have improved in most cases, but the design procedures are basically the same. Class-B amplifiers, in case you are not familiar with the topology, are able to amplify zero-referenced sinusoidal signals throughout the full 360 degrees of rotation signals without an offset voltage bias; they are constructed from two Class-A amplifiers in a cascode configuration. Issues like crossover distortion and thermal runaway are discussed in the amplifier design procedure.
Simple design procedure that will help to understand this very useful and popular transistorized circuitry.
Amplifier Design Procedure
In any power amplifier
design the governing factors will be the desired power output,
available voltage, and component efficiency. Assuming that we
wish to be able to deliver 8 watts to the loudspeaker, we must
first account for the losses in the output transformer, which
may be about 80 percent efficient. This immediately brings the
actual maximum power that the two transistors must deliver up
to about 10 watts. If we want to allow for 25 percent overload
capacity, the power is increased to 12.5 watts or 6.25 watts
per transistor. A typical supply voltage would be 12 volts which
would permit operating the equipment direct from an automobile
battery or two 6-volt lantern batteries. It would also be possible
to build a well regulated supply, using a standard 12.6-volt
filament transformer as power source.
Once the power
and the voltage are determined, we can select a transistor type.
It must be able to handle at least 6.25 watts and must have
a collector-to-emitter voltage rating of at least twice the
supply voltage, which means 24 volts. For our example we have
selected the 2N301 which is made by RCA, Bendix, Sylvania, and
CBS, and is readily available at parts distributors. This transistor
is rated at 40 volts collector potential and can dissipate 11
watts at 80°C which means about 25 watts at room temperature.
These ratings are higher than our minimum requirements but they
afford us a much needed margin of safety.
To get an idea of what the peak-current swing per stage can
be, we can calculate:
6.25 watts x (4 ¸ 12) volts = 2.1 amps
The manufacturers' data shows that the 2N301 can handle up
to 3 amps. peak current. To get the load resistance per stage
we divide the 12-volt supply voltage by the 2.1 amp. peak current
and this is roughly 6 ohms. One of the characteristics of Class
B push-pull circuits is the fact that the total primary impedance
of the output transformer is four times the load resistance
per stage or 24 ohms in our example.
At this point it
is useful to study the characteristic curves of the transistor
in question. Fig. 1 shows the collector characteristics for
the 2N301 for various values of base current. The a.c. load
line for the 6-ohm load resistance is drawn by connecting the
12-volt, zero-current point to the 2.1-amp., zero-voltage point.
To calculate maximum power handling ability we have accounted
for 25 percent overload capacity, but for normal operation the
current and voltage swing will be limited by a factor:
k = sqrt (1/1.25) = 0.896
Therefore, the normal full-load current swing will be only 2.1 amp. x 0.896 = 1.88 amp., as shown by the dotted line in Fig. 1. Another curve, Fig. 2, shows the average transfer characteristic which is simply a plot of collector current vs base voltage. For 1.88 amps. of collector current, the base voltage will have to be 0.78 volt and this, on the base characteristic curve of Fig. 3, will cause a base current of 34 ma. The product of 34 ma. x 0.78 volt is the input power, 26.5 mw.
The input impedance per transistor can be found by Ohm's Law from 0.78 volt divided by 34 ma., or 23 ohms. We can now draw a basic diagram of the Class B amplifier with its input and output requirements, as shown in Fig. 4. This simple circuit will, however, suffer from the disadvantages inherent in Class B transistor amplifiers. If we drive this amplifier with a sine-wave signal of maximum amplitude, 0.78 volt peak at the base of each stage, then the output signal at the loudspeaker will have the waveform of Fig. 5A, which shows the typical symptoms of crossover distortion.
Crossover Distortion
Fig. 5 illustrates one of the
most frequent defects in Class B transistor amplifiers and the
technician working with these circuits will soon become quite
familiar with the appearance of these distorted output signals,
To understand the cause for crossover distortion we need only
look at the average transfer characteristic curve of Fig. 2
and study the lower portion of the curve below 0.2 volt of base
bias. Here the curve flattens out and stops completely at 0.13
volt. This means that from about zero to 0.13 volt of base signal,
no current flows in the collector circuit. When the base signal
is very small the collector signal will take the form of the
curve of Fig. 5B, with current flowing only during that portion
of the sine wave which corresponds to more than 0.13 volt of
base voltage. Since the two push-pull transistors have the same
characteristics, the distortion will be balanced about the zero
line of the sine-wave signal.
To overcome this trouble it will be necessary to pre-bias each transistor slightly. In effect, for small signals, the transistors then operate as Class A amplifiers. This method can completely eliminate crossover distortion but it means that each transistor draws some current at all times and this reduces the efficiency of the Class B stages. In a good design, the pre-bias voltage is set carefully to minimize the quiescent collector current.
Returning to the design of Fig. 4, we can see two places where a pre-bias might be inserted. We could insert an additional battery between ground and the two emitters or, and this is the simpler approach, we can utilize a portion of the 12-volt collector supply to bleed a small negative voltage to the two bases. This will cause a loss in input signal due to the impedance or the biasing network. In a circuit of this type the bias resistors cannot be bypassed because the capacitor would charge up to the signal voltage level and this would increase the fixed bias too much.
The 2N301 characteristic curves of Figs. 2 and 3 indicate
that a pre-bias of about - 0.13 volt is required. Previously
we calculated the peak input impedance to be about 23 ohms.
The bias resistor should usually be about three times as large
so we select a value of 68 ohms, a readily available and standard
unit. By using Ohm's Law we find the current through the resistor
to be 1.91 ma. At 0.13 volt the base current itself is practically
zero so that only about 2 ma. will flow in the bias resistor.
Because of the variations among individual transistors and other
circuit constants, it is advisable to include a potentiometer
in the bias voltage divider and set the actual voltage level
for minimum collector current, commensurate with minimum crossover
distortion.
Looking at the revised circuit of Fig. 6, we can now
see that the input power of 26.5 mw. which we calculated previously
will not be sufficient because the actual input impedance, per
stage, is not now 23 ohms but a total of 91 ohms. The ratio
between the I^{2}R input power for equal current is
simply the ratio of 23 and 91 ohms, which is 3.95. This corresponds
to approximately 6 db, the increase in input needed to overcome
the effect of the pre-biasing network. The input power required
to drive the circuit of Fig. 6 for peak output is, therefore,
105 mw. Accounting for transformer losses of 20 percent, we
find that the actual peak driving power delivered by a driver
amplifier to the primary of the input transformer will have
to be about 135 mw.
Driving circuits for Class B power amplifiers deserve a lengthy
article in themselves, but in this limited space we can only
point out that Class A single-stage amplifiers or Class A push-pull
circuits are usually used in this application. There are a number
of circuits which avoid the use of a driving transformer and
provide phase splitting and impedance matching directly. For
the example cited here it would be possible to use a single-stage
Class A 2N301 as driver or else a Type 2N32, 2N44, or 2N226,
all suitable and readily available from jobber stocks. The prime
requirement of a driver stage is power handling ability. If
the reader refers to last month's article on Class A power amplifiers,
all of the circuit designs can be followed except that the Class
B push-pull stage input impedance must be substituted for the
load on the output transformer secondary.
Thermal Runaway
One of the basic facts about semiconductor physics is
the interdependence of current flow and temperature. As the
temperature goes up, more current flows. As current flows, heat
is generated in the transistor. These two facts impose a severe
restriction on the circuit designer, especially when germanium
transistors are used where the critical temperatures are relatively
close to room temperature. Silicon devices have a somewhat higher
critical temperature. The transistor characteristic curves of
Figs. 1, 2, and 3 all bear the notation "mounting flange temperature
25°C" and this means that at a higher or lower temperature the
characteristics shown may be changed somewhat.
If a
transistor were 100 percent efficient it would have no power
loss in the transistor itself but such a device is, of course,
unavailable. The2N301, according to the published data, will
dissipate 3 watts for an output of 12 watts in a Class B push-pull
circuit. The 3 watts of heat must be conducted away from the
transistor or else it will build up to a progressively higher
temperature. If the flange temperature increases, the current
through the collector increases and the amount of power which
is dissipated in the transistor also increases. In other words,
if the mounting arrangement of the transistor cannot dissipate
3 watts of power, the resulting temperature build-up will burn
out the transistor. It is possible, by careful design, to mount
each transistor so that it can easily radiate 3 watts into the
surrounding air, but on a hot day the efficiency of this heat
transfer will suffer because the surrounding air itself is warmer.
In last month's article on Class A power amplifiers
we illustrated several techniques for mounting power transistors
for maximum heat radiation. The same approach should be used
for Class B push-pull circuits except that we must be careful
not to allow the heat from one transistor to contribute to the
other, unless the dissipation from both can be radiated readily.
In addition to the mechanical mounting methods, there
are simple electronic means of preventing thermal runaway and
maintaining stability with varying temperatures. The most widely
used method involves base bias control by means of a temperature-sensitive
resistor or "thermistor." A thermistor can either increase or
decrease in resistance with temperature, but the latter type
is more common. In using a thermistor to control the bias of
a transistor the thermistor is mounted close to the transistor
heat sink so that any heat rise in the transistor will affect
the resistance of the thermistor.
Returning once again
to our example of a typical Class B push-pull circuit we can
see how a thermistor is used in the circuit of Fig. 7. Here
the thermistor, R_{T}, part of the bias network and
in parallel with the 210-ohm carbon resistor, provides the desired
68 ohms at 25°C. When the temperature increases to 50°C the
thermistor will be only 40 ohms and this will reduce the pre-bias
enough to cut the transistor collector current down to a safe
value.
The principle is simply to reduce bias as temperature goes
up and to do this in conformity with the transistor characteristics.
If the right thermistor were used, the fixed resistance R_{1}
would not be required, but most commercially available thermistors
are more temperature sensitive than needed and therefore a fixed
shunting resistor is often used.
Transformer Considerations
Before we can build the final circuit of Fig. 7, we
must give some thought to the two transformers. These items
must be selected from available stock types and, if a perfect
match is not possible, the effects of mismatch will have to
be evaluated. Starting at the output stage, we immediately know
that this transformer will have to be the bigger one since it
must handle at least 8 watts. Actually a 10-watt unit will be
the standard size. In checking through the catalogue we find
that a number of manufacturers make suitable units of this type.
To cite just one, consider the Triad TY-29X. It has a center-tapped
primary with an impedance of 24 ohms and a secondary with either
8- or 4-ohm impedances. It can handle 10 watts of audio power.
We can see that it meets our requirements exactly.
There
are a number of other transformers equally suitable, such as
the Stancor T-14 or Chicago TAMS-12. It may happen that in another
design the calculated output impedance turns out to be a value
which cannot be matched by a commercial output transformer.
We can then use the nearest available transformer type or change
the design of our circuit. The first method is recommended when
the nearest available transformer impedance is not more than
25 percent higher than the transistor output impedance. Otherwise
it is possible to change the output impedance by redrawing the
load line. We can change either the supply voltage or the peak
power and, if necessary, select a different transistor type.
When we consider the input transformer we see that,
as far as the Class B amplifier is concerned, we can select
only the secondary and the power ratings. In our example the
input transformer must be capable of handling at least 135 mw.
and must match the input impedance of 182 ohms. Actually, to
minimize distortion, the source impedance should be lower than
the input impedance so we should select a transformer which
has less than 182 ohms secondary impedance. A good choice would
be the Thordarson TR64 which has a 100-ohm center-tapped secondary
and a primary impedance of 100 ohms. It can handle 0.5 watt
which is ample for this circuit. Because the secondary impedance
is lower, some loss in power transfer will occur here which
means that the driver stage will have to deliver more than the
135 mw. previously calculated. In a conservative design the
driver stage will be capable of delivering at least twice the
driving power calculated for the Class B output stages.
Conclusion
Class B transistor amplifiers have the
great advantage of requiring almost no quiescent power and are,
therefore, very efficient. The design principles are the same
as for vacuum-tube push-pull Class B circuits, except that the
transistor's temperature characteristics must be taken into
account. The detailed design procedure presented here requires
only a knowledge of Ohm's Law, arithmetic, and an understanding
of characteristics curves.
Posted January 3, 2013