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Documents Flashcards Grammar checker. During the past few years a number of excellent designs have been published for domestic audio amplifiers. However, some of these designs are now rendered obsolescent by changes in the availability of components, and others are intended to provide levels of power output which are in excess of the requirements of a normal living room.
Also, most designs have tended to be rather complex. In the circumstances it seemed worth while to consider just how simple a design could be made which would give adequate output power together with a standard of performance which was beyond reproach, and this study has resulted in the 2h697 design. Vatasheet power and distortion In view of the enormous popularity of the Mullard “” valve amplifier, it appeared that a watt output would be adequate for normal use; indeed when two such amplifiers are used as a stereo pair, the total sound output at full power can be quite astonishing using reasonably sensitive speakers.
The original harmonic distortion standards for audio amplifiers dataseet laid down by D. Williamson in a series of articles published in Wireless World in and ; and the standard, proposed by him, for less than 0.
Since the main problem in the design of valve audio amplifiers lies datashert the difficulty in obtaining adequate performance from the output transformer, and since modern transistor circuit techniques allow the design of power amplifiers without output transformers, it seemed feasible to aim at a somewhat higher standard, 0. This also implies that the output power will be constant over this frequency range.
Circuit design The 2m697 amplifier circuit of which the author is aware, in which vatasheet transformerless transistor design was used to give a standard of performance approaching that of the “Williamson” amplifier, was that published in Wireless World in by Tobey and Dinsdale. This employed a class B output stage, with series connected transistors in quasi-complementary symmetry. Datsaheet high-quality transistor power amplifiers have largely tended to follow the design principles outlined in this article.
The major advantage of amplifiers of this type is that datasheer normal static power dissipation is very low, and the overall power-conversion efficiency is high. Much has been done, particularly by Bailey1, to minimise the latter. An additional characteristic of the class B output stage is that the current demand of the output transistors increases with the output signal, and this may reduce the output voltage and worsen the smoothing of the power supply, unless this is well designed.
Also, because of the increase in current with output power, it is possible for a transient overload to drive the output transistors into a condition of thermal runaway, particularly with reactive loads, unless suitable protective circuitry is employed.
These requirements have combined to increase datasneet complexity of the circuit arrangement, and a well designed low-distortion class B power amplifier is no longer a simple or inexpensive thing to construct. Page 1 of 9 Wireless World, Datashheet An alternative approach to the design datashret a transistor power amplifier combining good performance with simple construction is to use the output transistors in a datashet A configuration.
This avoids the problems of asymmetry in quasi-complementary circuitry, thermal runaway on transient overload, cross-over distortion and signal-dependent variations in power supply current demand. It is, however, less efficient than a class B circuit, and the output transistors must be mounted on large heat sinks.
The basic class A construction consists of a single transistor with a eatasheet collector load. The use of a resistor, as in Fig. The use of a second, similar, transistor as a collector load, as shown in Fig. This requirement can be achieved if the driver transistor is connected as shown in Fig. This method of connection also meets one of the most important requirements of a low distortion amplifier – that the basic linearity of the amplifier should be good, even in the absence of feedback.
Several factors contribute to this. There is a measure of internal feedback around the loop Tr1, Tr2, Tr3 because of the effect which the base impedance characteristics of Tr1 have on the output current of Tr3.
Also, the driver transistor Tr3, which has to deliver a large voltage swing, is operated under conditions which favour low harmonic distortion – low output load impedance, high input impedance.
A practical power amplifier circuit using this type of output stage is shown in Fig. The open loop gain of the circuit is approximately with typical transistors. With the values indicated in Fig.
This gives a feedback factor of some 34dB, and an output impedance of about milliohms. Since the circuit has unity gain at d. Since the output transistor Tr1 will turn on as much current as is necessary to pull Ve down to this value, the resistor R2, which together with R1 controls the collector current of Tr2, can be used to set the static current of the amplifier output stages.
It will also be apparent that Ve datashdet be set to any desired value by small adjustments to R5 or R6. The optimum performance will be obtained when this is equal to half the supply voltage. Half a volt or so either way will make only a small difference to the maximum output power obtainable, and to the other characteristics of the amplifier, so there is no need for great precision in setting this.
Silicon planar transistors are used throughout, and this gives good thermal stability and a low noise level. Also, since there is no requirement for complementary symmetry, all the power stages can use n-p-n transistors which offer, in silicon, the best performance and 2b697 cost. The overall performance at an output level of 10 watts, or at any lower level, more than meets the standards laid down by Williamson.
Since the amplifier is a straight-forward class A circuit, the distortion decreases linearly with output voltage. This would not necessarily be the case in a class B system if any significant amount of cross-over distortion was present.
The analysis of distortion components at levels of the order of 0. Stability, power output and load impedance Silicon planar n-p-n transistors have, in general, excellent high frequency characteristics, and these contribute to the very good stability of the amplifier with reactive loads.
The author has not yet found a combination of L and C which makes the system unstable, although the system will readily become oscillatory with an inductive load if R3 is shunted by a small condenser to cause datassheet at high frequencies. R, where the symbols have their customary significance. It should be remembered, however, that the calculation of output power is based on r.
When these calculations have been made, the peak-to-peak voltage swing for 10 watts power into a 15Ohm datsheet is found to be Since the two output transistors bottom at about 0. For loads of 8 and 3 ohms, the minimum h. The necessary minimum currents are 0. Suggested component values for operation with these load impedances are shown in Table 1. C3 and C1 together influence the voltage and power roll-off at low audio frequencies.
These can be increased in value if a better low-frequency performance is desired than that shown in Figs. Since the supply voltages and output currents involved lead to dissipations in the order of 17 watts in each output transistor, and since it is undesirable for component longevity to permit high operating temperatures, adequate heat sink area must be provided for each transistor. A pair of separately mounted 5in by 4in finned heatsinks is suggested. This is, unfortunately, the penalty which must be paid for class A operation.
If the output impedance datasheef the pre-amplifier is more than a few thousand ohms, the input stage of the amplifier should be modified to include a simple f.
This increases the harmonic distortion to about 0. A high frequency roll-off can then be obtained, if necessary, by connecting a small capacitor between the gate of the f. Summary of component combinations for different load datasneet. Suitable transistors Some experiments were made to determine the extent to which the circuit performance was influenced by the type and current gain of the transistors used. As expected the best performance was obtained when high-gain transistors were used, and when the output stage used a matched pair.
Similarly, the use of alternative types of input transistor produced no apparent performance change, and the Texas Instruments 2N is fully interchangeable with the Motorola 2N used in the prototype.
The most noteworthy performance changes were found in the current gain characteristics of the output transistor pair, and for the lowest datashest distortion with any pair, the voltage at the point from which the loudspeaker is fed should be adjusted so that it is within 0.
The other results are summarized in Table 2. Dataseet main conclusion which can be drawn from this is that the type of transistor used may not be very important, but that if there are differences in the current gains of the output transistors, it is necessary that the device with the higher gain shall be used in the position of Tr1. Page 3 of 9 Wireless World, April When distortion components were found prior to the onset of waveform clipping, these were almost wholly due to the presence of second harmonics.
2N697 Datasheet دیتاشیت PDF دانلود
Relation of distortion to gain-matching in the output stage. The lowest noise levels will however be obtained with good quality components, and with carbon-film, or metal-oxide, resistors. A suggested form of power supply unit is shown in Fig. Since the current demand of the amplifier is substantially constant, a series transistor dtasheet circuit can be used in which the power supply output voltage may be adjusted by choice of the base current input provided by ddatasheet emitter follower Tr2 and the potentiometer VR1.
2N datasheet, Pinout ,application circuits NPN Transistor
With the values of the reservoir capacitor shown in Table 3, the ripple level will be less than 10mV at the rated output current, provided that the current gain of the series transistors is greater than For output currents up to 2.
However, at the current levels necessary for operation of the 3-ohm version of the amplifier as a stereo pair, a single MJ will no longer be adequate, and either a more suitable series transistor must be used, dayasheet as the Mullard BDY20, with for example a 2N as Tr2, or with a parallel connected arrangement as shown in Fig.
Power-supply components The total resistance in the rectifier “primary” circuit, dahasheet the transformer secondary winding, must not be less than 0. Datashheet the power supply, with or without an amplifier, is to be used with an r.
The rectifier diodes specified are International Rectifier potted bridge types. Page 4 of 9 Wireless World, April Transistor protection circuit The current which flows in the output transistor chain Tr1, Tr2 is determined by the potential across Tr2, the values of R1 and R2, and the current gain and collector-base leakage current of Tr2.
Since both these transistor characteristics are temperature dependant the output series current will increase somewhat with the temperature of Tr2. If the amplifier is to be operated under conditions of high ambient temperature, or if for some reason it datashewt not practicable to provide an adequate area of heat-sink for the output transistors, it will be desirable to provide some alternative means for the control of the output transistor circuit current.
This can be done dstasheet means of the circuit shown in Fig. In this, some proportion of the d. This allows very precise control of the series current without affecting the output power or distortion characteristics.
The simpler arrangement whereby the current control potential for Tr7 is obtained from a series resistor in the emitter circuit of Tr1 leads, unfortunately, to a worsening of the distortion characteristics 2n67 about 0. Performance under listening conditions It would be convenient if the performance of an audio amplifier or loudspeaker or any other similar piece of audio equipment could be completely specified by frequency response and harmonic distortion dafasheet.
Unfortunately, it is not possible to simulate under laboratory conditions the complex loads or intricate dqtasheet structures presented to the amplifier when a loudspeaker system is employed to reproduce the everyday sounds of 26n97 and music; so that although the square wave and lowdistortion sine wave oscillators, the oscilloscope, and the harmonic distortion analyser are valuable tools in the design of audio circuits, the ultimate test of the final design must be the critical judgement of the listener under the most carefully chosen conditions his facilities and environment allow.
However, in the past, when these tests were made for personal curiosity, and some few minutes could elapse in the transfer of input and datasheey leads from one amplifier to the other, the comparative performance of some designs has been so close that the conclusion drawn was that there was really very little to choose between them.
The frequency response, and total harmonic distortion characteristics, of the four transistor amplifiers was tested in the laboratory prior to this trial, and all were found to have a flat frequency response through the usable audio spectrum, coupled with low harmonic distortion content the worst-case figure was 0.
In view of these prior tests, it was not expected that there would be any significant difference in the audible performance of any of the transistor designs, or datasjeet them and the valve amplifiers. It was therefore surprising to discover, in the event, that there were discernable differences between the valve and the three class B transistor units. In fact, the two valve designs and the class A transistor circuit, and the three class B designs formed two tonally distinct groups, with closely similar characteristics within each group.
In the upper reaches of the treble spectrum the transistor amplifier has perhaps a slight advantage. The performance differences between the class A and the class B groups were, however, much more prominent.
Some thought, in discussions with interested friends, has been datasyeet to the implications of this unlookedfor discovery, and a tentative theory has been datasheeg which is offered for what it is worth. It is postulated that these tonal differences arise because the normal moving-coil loudspeaker, in its associated housing, can present a very complex reactive 2n6977 at frequencies associated with structural resonances, and that this might provoke transient overshoot when used with a class B amplifier, when a point of inflection in the applied waveform chanced to coincide with the point of transistor crossover, at which point, because of the abrupt change in the input parameters of the output transistors the loop stability margins and output damping will be less good.
In these circumstances, the desired function 2n67 the power-amplifier output circuit in damping out the cone-response irregularities of the speaker may be performed worse at the very places in the loudspeaker frequency-response curve where the datwsheet is most dagasheet.
It should be emphasized that the differences observed in these experiments are small, and unlikely to be noticed except in direct side-by-side comparison.