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CLASS B operation of transistors in the power-output stages of battery-operated portable amplifiers offers the important advantages of high collector-circuit efficiency, low standby idling current (and battery power), and a higher ratio of power output to collector dissipation per transistor than can be realized in class A operation. Because the rated power dissipation of many commercially available junction transistors is relatively low, the inherent high efficiency of class B operation is extremely attractive for many transistor applications.

In many respects, the advantages, limitations, and circuit requirements of class B operation are quite similar for both tubes and transistors. For example, low-impedance, well-regulated supply voltages are essential for optimum performance because of the high pulsating peak currents encountered in the output circuit.

If bias is employed, the bias supply should also be well regulated to prevent excessive distortion in the input-circuit voltages. Reasonable matching of tube or transistor characteristics is also required for satisfactory operation.

When either tubes or transistors are used in class B, the driver stage must be capable of supplying the necessary peak input power and the power lost in the interstage transformer without excessive distortion.
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A major disadvantage of class B audio amplifiers is the relatively high value of odd-harmonic distortion that occurs at low power-output levels. This small-signal distortion, however, can be kept within allowable limits provided the transistor is designed for its optimum transfer characteristic and proper circuit-configuration and operation conditions are chosen.
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In some respects, transistors differ from tubes when used in class B operation and display several advantages that make them somewhat easier and more desirable to use. For example, the lower impedance levels involved permit the use of smaller and less expensive interstage and output transformers.

In addition, the variation in input impedance with signal level is not nearly so great with transistors as with tubes when driven into the positive-grid-current region. As a result, design of the driver stage and driver transformer is simplified, and instability due to parasiticus is eliminated.

Because the output characteristics of junction transistors have extremely low-voltage, sharp "knees" in comparison with many vacuum tubes designed for class B operation, circuit designs having higher circuit efficiencies can be used which in some cases closely approach the maximum theoretical value of 78%.

When transistors have controlled large-signal characteristics, i.e., high-current amplification factors at values of high collector peak currents, it is possible to design power amplifiers that utilize very low supply voltages and yet maintain relatively high power sensitivity.

Large-signal "alpha" will be defined as the ratio of collector current to base current at a collector current of 50mA and a collector-to-emitter voltage of 1V. This measurement may be made for d-c or peak values with good correlation. Because the power gain or sensitivity of the transistor in large-signal amplifiers is proportional to this characteristic, high values of large-signal a are extremely desirable.

The peak-collector-current rating, which is determined for a given device by collector-saturation effects and life considerations, is conservatively selected for this transistor at 50mA.

Cross-over distortion can be effectively reduced by the use of a small forward base bias, shown by point A in Fig. 1a, top, which allows a small quiescent collector current to flow during standby.

For any given transistor type, there is a particular value of base bias that results in a good balance between crossover distortion and collector-circuit efficiency.

A convenient method, for determining the operating point is to project the main part of the transfer-characteristic curve in a straight line to the cutoff point, as shown in Fig. 1.

The resulting composite curve is also shown in Fig. 1b, bottom. Use of projected-cutoff bias appreciably reduces cross-over distortion to a point at which any remaining distortion can be reduced by the use of negative feedback.

The change in the slope of the transfer characteristics at high values of base voltage is caused by the fall off in a or collector-current saturation.
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It is assumed that the input voltage is sinusoidal and that the transistors are biased at cutoff, and a one-volt "safe knee" is estimated, the following approximate formulas define the operating conditions - (Anmerkung : We will not show the formulas here, to old und no more interesting)

The frequency response of junction-transistor class B amplifiers is determined primarily by the characteristics of the transformers and transistors used, although the exact relationships are somewhat complicated because of the intermittent collector currents present in the output circuit.

The low-frequency response is dependent upon the primary inductance of the transformers; the frequency at which the power output is 3db down from the output at mid-frequency is reached approximately when the primary reactance of the transformer is equal to the collector load impedance plus the total winding resistance of the transformers referred to the primary.

The high-frequency response is dependent upon the leakage reactance and winding capacitance of the transformers and the cutoff-frequency characteristic of the transistors. It is desirable that well balanced transformers be designed and that the leakage inductance between the two halves of the primary be minimized.

Balanced transformers minimize the d-c-polarizing currents, aid in the cancellation of even-order harmonics in the output, and reduce the possibility of distortion due to unbalance in the input signal.

Because high values of collector peak currents and low supply voltages are involved, the d-c resistance of the primary of the output transformer should be made as low as possible to retain high efficiencies.

The secondary winding of the driver transformer should also have low d-c resistance to minimize the effects of collector back currents upon the operating point. It should be emphasized that the transformer requirements mentioned are not particularly difficult to achieve because of the extremely low impedance levels involved.

Although there are circuit variations that make it possible to eliminate the output transformer, these circuits generally require the use of a power supply or a load impedance that is center-tapped. The driver transformer may also be eliminated in some circuits, although it is difficult in practice to obtain the relatively low d-c impedance required without the use of a transformer.

The base-input, common-collector circuit, in which the load is in series with the emitter, has "built-in" d-c and a-c degeneration which greatly improves both the temperature stability and the distortion characteristics of the amplifier.

The input resistance in this circuit is relatively high and is more linear with excitation than that of the common-emitter circuit. As a result, high input voltages are required to develop the necessary driving power, the maximum value of which is limited by the supply voltage. This circuit accommodates a much wider variation in transistor characteristics than the common-emitter circuit, and does not require temperature compensation to correct for variations in collector reverse current.

The improved stability at higher temperatures and dissipations permits operation of a given transistor in the common-collector circuit at higher dissipation than in the common-emitter circuit. This circuit is also suitable for use with higher-power junction transistors. Because cross-over distortion is minimized without application of external base bias, better distortion performance is obtained at low power-output levels.

Considerable power sensitivity is sacrificed, however, for these improved characteristics, and it is necessary in many amplifier applications to add an additional driver stage.

The efficiency of either the common-emitter or the common-collector circuit has a maximum theoretical value of 78%. In practice, the actual efficiency depends upon the quiescent value of collector current, the supply voltage, the efficiency of the output transformer and the level of power output.

The efficiency of the two circuits is nearly the same for equal peak power output. In general, the choice of a high supply voltage results in slightly higher efficiencies because the "knee" voltage then becomes a smaller percentage of the total supply voltage. For example, the efficiency can be increased by 5 to 10% at rated output if supply voltage is increased from 4.5V to 9V.

Distortion in junction-transistor class B amplifiers is a function of the power output, the supply voltage, the input and load impedances, and the a characteristic of the transistors.

The effect of these factors is more severe in the common-emitter circuit, which employs no internal degeneration and, consequently, is more sensitive to circuit and characteristic variations.

The common-collector circuit has less distortion under each operating condition, as shown in Fig. 7, and shows the greatest significant improvement at low power-output levels, where cross-over distortion is effectively minimized without the use of external bias. For the common-emitter circuit, the curves show distortion at low power levels for the optimum value of bias.

The over-all distortion of transistor class B amplifiers can be reduced, therefore, by the use of transistors that have well-matched large-signal characteristics, by an increase in the supply voltage and the load impedance, and by the use of negative feedback. The final amplifier should be designed to provide the desired balance between distortion and power sensitivity for the given application.

The transfer-characteristic curves shown in Fig. 1. illustrate the effects of ambient temperature upon transistors in common-emitter class B circuits. The operating point is designated by point A on the 25°C curve. If the common-emitter circuit is operated with a constant bias voltage, an increase in temperature causes an appreciable increase in quiescent output current and a consequent decrease in the maximum power output and output-circuit efficiency.

The curves of Fig. 10 are reasonably linear, even in the low-current regions. As a result, the transistors may be operated at zero bias without introducing the problems of low-level distortion at room temperature or with temperature variations. In this circuit, therefore, no bias supply or temperature-compensating network is required for optimum performance.

The shift of the curve with temperature at the high-current end of the scale causes a slight variation in the maximum power and the power sensitivity of the circuit. This variation may be reduced by the use of an output transformer having increased d-c primary resistance in series with the emitter load, resulting in increased d-c stability.

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A circuit diagram of a part-tube, part-transistor battery-operated portable radio receiver is illustrated.

The tube filaments (die Heizdrähte in den Röhren), which originally were connected in parallel, are connected in series so that a common supply voltage can be used for the filaments and the transistors.

Consideration must be given in this circuit to the normal biasing arrangements required with series-string operation of tubes. The operating conditions of the pentode section of the first audio stage are modified to provide adequate driving power for the transistor output stage and to increase the over-all power sensitivity of the audio system.

The sensitivity of the resulting audio system is equivalent to that of receivers using a subminiature output tube and operating from a supply of 45V or less. This sensitivity is about 10db below the audio sensitivity of receivers using a miniature output tube similar to the 3V4 and operating from a supplv of 67V.

The sensitivity of all-tube receivers, however, generally decreases more rapidly with battery life than the sensitivity of the illustrated receiver. Consequently, any difference in performance between the two systems is reduced with battery life. In some existing receiver designs having an r-f stage, the loss in audio sensitivity is automatically compensated for by the r-f sensitivity, which is normally more than adequate.

In many other receivers, additional sensitivity may be gained by modification of the i-f stage, or by use of more sensitive antennas and higher efficiency speakers.

As compared to an all-tube receiver, therefore, the part-tube, part-transistor receiver has nearly equivalent audio sensitivity, similar distortion characteristics, equal or greater power output with a maximum value that is more nearly constant with battery life, and considerably higher over-all efficiency.

The greater battery efficiency can be used in either of two ways:

• (1) portable equipment can be designed to have extremely small size and light weight by the use of miniature batteries compatible in size with the low power requirements; or
• (2) portable equipment can be designed using conventional battery sizes resulting in substantial improvements in battery life and a sizable reduction in operating cost.

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