
PushPull Class B Transistor PowerOutput Circuits
November 1960 Electronics World


November 1960 Electronics World
[Table
of Contents] People 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. All copyrights are hereby acknowledged.
See all
Electronics World articles. 
The
fundamentals of
ClassB pushpull 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. ClassB amplifiers, in case you are not familiar with the
topology, are able to amplify zeroreferenced sinusoidal signals throughout
the full 360 degrees of rotation signals without an offset voltage bias;
they are constructed from two ClassA amplifiers in a cascode configuration.
Issues like crossover distortion and thermal runaway are discussed in
the amplifier design procedure.
See all the available
Electronics World articles.PushPull Class B Transistor PowerOutput
CircuitsBy Walter H. Buchsbaum / Industrial
Consultant, Electronics World
Simple
design procedure that will help to understand this very useful and popular
transistorized circuitry.
Transistor audio amplifiers have many
advantages over their vacuumtube counterparts, but one circuit that
really highlights these advantages is the Class B pushpull power amplifier.
The great efficiency and simplicity of transistors really shine in this
application. With no filament power required and a relatively low "B+"
voltage, the power drain during quiet periods is negligible and the
overall circuit efficiency extremely high. These are certainly very
desirable features.
Another attraction is the price of germanium
power transistors which compares favorably with vacuum tubes. In this
article a typical 8watt amplifier design, which uses transistors costing
about $3.00 each, will be illustrated. The same design procedure can
be used for amplifiers at other power levels and for different transistor
types. Those readers who are more concerned with troubleshooting transistor
audio amplifiers will find the material helpful in understanding the
sources of the most frequent defects they are apt to find in transistor
circuits.
Class B operation means that each transistor will
amplify only onehalf of the signal and will "rest" while its mate amplifies
the oppositepolarity portion of the signal. Because power dissipation
is one of the most critical factors in transistors, Class B operation
is particularly suited to the task of insuring low dissipation for each
transistor. There are only two drawbacks to this type of transistor
circuit: crossover distortion and thermal runaway. These will be discussed
below.
Crossover distortion occurs when one transistor stops
conducting before the other has started and is a typical problem with
all Class B circuits. Thermal runaway can occur in any transistor application
but power transistors are especially susceptible. This trouble is due
to the fact that, as a transistor heats up, it will tend to draw more
current which, in turn, will cause the collector to heat up even more
until the unit burns out. There are wellestablished and relatively
simple cures for both crossover distortion and thermal runaway and they
will be described here in some detail. Let us first consider the characteristic
curves.
Most of our readers will be somewhat familiar with the
characteristic curves used in vacuumtube circuit design. Similar curves
are furnished by transistor manufacturers and are used in the same manner.
In last month's article on Class A transistor poweroutput circuits
we described some of the special aspects of transistor characteristic
curves, a discussion which will not be repeated here. Instead, we will
concentrate on the design of a typical amplifier and point out how crossover
distortion, thermal runaway, and some of the minor problems can be handled.
Fig. 1  Collector characteristics of 2N301.
Fig. 2  Transfer characteristics of 2N301.
Fig. 3  Base characteristics of the 2N301. 
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 6volt lantern batteries. It would also be possible to build a well
regulated supply, using a standard 12.6volt 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 collectortoemitter 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 peakcurrent 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 12volt supply voltage by the 2.1
amp. peak current and this is roughly 6 ohms. One of the characteristics
of Class B pushpull 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 6ohm load resistance is drawn by
connecting the 12volt, zerocurrent point to the 2.1amp., zerovoltage
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 fullload 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 sinewave 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 pushpull transistors have the same characteristics,
the distortion will be balanced about the zero line of the sinewave
signal.
To overcome this trouble it will be necessary to prebias
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 prebias voltage is set carefully to minimize
the quiescent collector current.
Fig. 4  Basic design of the 8·watt amplifier.
Fig. 5  The distortion due to the crossover.
Fig. 6  Prebias circuit for the amplifier.
Fig. 7  The final, practical circuit employed. 
Returning to the design of Fig. 4, we can see two places where a prebias
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 12volt 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 prebias 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 prebiasing 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 singlestage amplifiers or Class A pushpull 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 singlestage 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 pushpull
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 pushpull
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 buildup 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 pushpull 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 temperaturesensitive 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 pushpull 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 210ohm 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 prebias
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 10watt 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
TY29X. It has a centertapped primary with an impedance of 24 ohms
and a secondary with either 8 or 4ohm 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 T14 or Chicago TAMS12. 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 100ohm centertapped 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 vacuumtube
pushpull 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



