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Transistor Amplifier Circuits

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Transistor Amplifier Circuits

Similar circuit arrangements are used with both vacuum tube and transistor amplifiers. This similarity applies to the circuit arrangement only, however, and it is incorrect to consider the transistor as a direct substitute for the vacuum tube... the differences between the operation of transistors and vacuum tubes are far greater than their similarities. The transistor is basically a current operated device. Thus, the transistor operates as a current amplifier and prefers a constant current power supply. The vacuum tube, on the other hand, operates as a voltage amplifier and works best with a constant voltage power source. In addition, most vacuum tube circuits feature both high input and high output impedances.

Transistor circuits generally have a low to moderate input and a moderate to high output impedance. In vacuum tube circuits, the input and output signals are generally well isolated, whereas this is often not the case with transistor amplifiers. And, as we discussed in the last Chapter, the transit time required for electrons(and holes) in transistors is considerably longer than the transit time required by electrons in a vacuum tube.The similarities in the circuit arrangements of transistor and vacuum tube amplifiers maybe most easily observed by considering the various elements of the transistor as analogous, but not equivalent, to corresponding elements in the vacuum tube.

In the transistor, the collector corresponds to the plate, the base to the grid, and the emitter to the cathode of the vacuum tube.There are three basic vacuum tube amplifier circuits, each of which is identified by the grounded or common element. They are the grounded cathode, the grounded grid, and the grounded plate (or cathode followe"?! amplifiers"! Transistor amplifiers are identified in a similar fashion as grounded emitter, grounded base and grounded collector amplifiers). Let us discuss each of these three basic types.

THE GROUNDED EMITTER AMPLIFIER:
The grounded cathode triode tube circuit is shown at the left in Fig. 42. The input signal is applied between the grid and cathode, while the output signal appears between plate and cathode. Average grid potential is determined by a grid biasing voltage, and average plate potential with reference to the cathode is fixed by a d.c. plate voltage. The cathode is thus common to both the input and output signals, and may be grounded.

The equivalent transistor amplifier is shown at the right. Input is applied between base and emitter. Average d.c. base potential,with reference to the emitter, is determined by the bias voltage. However, it is the bias current that determines the mode of operation of the circuit.The output signal is taken from between the collector and emitter, with average collector potential fixed by the d.c. collector voltage. In many cases, the actual collector voltage has very little effect in determining collector current, as this is primarily a function of the bias current.When a signal is applied to the input, the bias current varies about its average value, and corresponding variations, though of much greater amplitude, take place in the collector circuit. This results in an amplified version of the input signal appearing across the load impedance. Although this circuit is termed a grounded emitter amplifier, the emitter need not necessarily be grounded ... it is simply common to both the input and output circuits.Note that the polarity of the d.c. power supply voltage is not indicated in Fig. 42. The proper polarity will depend on the type of transistor used... this is illustrated in Fig. 45 for the three basic types of amplifier circuits and for different types of transistors.These polarities are indicated as positive or negative when measured between the indicated transistor element (emitter, base, or collector)and the grounded element."Polarities of the output elements, whether collector or emitter, always are as shown, with reference to the grounded or common element. However, polarities of the input elements maybe occasionally reversed in practice.

This may occur when the biasing voltage, at the element itself, is 0.2 volt or less. In a few instances,gain may be increased or distortion lessened by using a 'reversed1 bias polarity."Along the upper row of diagrams are shown the correct polarities for NPN junction type transistors or for point contact types with Ptype base material. The collector emitter polarities are similar to those encountered in vacuum tube circuits. Thus, just as the plate of a vacuum tube is positive with respect to its cathode, so is the collector of an NPN transistor positive with respect to its emitter."The lower row of diagrams shows relative polarities for PNP junction transistors or for point contact units with N type material in the base.

Note that these polarities are exactly the opposite of those shown in the top row."It is extremely important that the proper d.c.polarities always be observed when working with transistor circuits. While the application of a negative voltage on the plate of a vacuum tube,for example, may not injure the tube,the application of improper voltages to a transistor will ruin it almost instantly.Electron current flow, in transistor circuits,is exactly like that in any electrical circuit. The movement of electrons is from the point of highest negative potential through the circuit elements to those points of less negative (or more positive) potential.

In the grounded emitter circuit, the collector emitter circuit current flow maybe on the order of a few milliamperes, while the base emitter circuit current may be from less than 10 to several hundred microamperes.The grounded emitter circuit features a low to moderate input impedance and a moderate output impedance. With typical junction transistors,the input impedance may be from 300 to 1500ohms, and the output impedance may be on the same order or somewhat higher.Although not quite as stable as the grounded base amplifier, the grounded emitter circuit of fers the highest gain of the three basic transistor amplifier circuit arrangements.Phase reversal occurs in a grounded emitter stage. Thus, if a positive going signal pulse is applied to the input, a negative going signal pulse will be obtained in the output circuit.

THE GROUNDED BASE AMPLIFIER:
A popular triode vacuum tube amplifier circuit is the grounded grid amplifier, shown in simplified schematic form in the diagram to the left in Fig.43. The input signal is applied between the cathode and grid, by way of ground, with bias voltage applied between cathode and ground . . .this is equivalent, of course, to applying the bias between grid and cathode. Output is obtained between the plate and grid, again by way of ground.Average plate potential is determined by the d.c.plate voltage.The analogous transistor circuit, the grounded base amplifier, is shown to the right in Fig. 43.

The input signal is applied between the emitter,corresponding to the tube's cathode, and the base,comparable to the tube's grid. Average emitter potential, with reference to the base, is determined by a bias voltage in the input circuit. The output is taken between the collector and base,with the d.c. collector voltage obtained from a separate power source.As in the case of the grounded emitter circuit,the base of a grounded base amplifier need not be connected to the circuit ground, as long as it is common to both input and output circuits. The input impedance of a grounded base amplifier is very low, generally quite a bit less than100 ohms.* The output impedance, on the other hand, is fairly high. But even with these differences in input and output impedances, the d.c.in the emitter and collector circuits may be on the same order of magnitude. In operation, an a.c. signal applied to the emitter base circuit results in current variations in this circuit. These current changes,in turn, result in corresponding impedance variations in the collector circuit, so that an amplified signal appears across the collector load. Thus, although signal amplification is possible in a grounded base circuit, and a definite power gain maybe obtained, this gain is due almost entirely to the differences between the input and output impedances of the transistor. The actual current amplification, in a grounded base circuit,is always less than 1. The d.c .polarities for the grounded base amplifier are shown in the middle pair of schematics in Fig. 45.

Again, different polarities are employed for different types of transistors.Phase reversal does not take place in a grounded base amplifier.Quite stable, the grounded base circuit provides a moderate power gain and is very popular for some applications. It is more generally used with point contact than with junction transistors,however.

THE GROUNDED COLLECTOR AMPLIFIER:
The grounded plate vacuum tube amplifier is more popularly known as a cathode follower. The basic circuit is shown to the left in Fig. 44,while the equivalent (or, rather, analogous) transistor circuit is shown to the right in the same illustration.In the vacuum tube version of the circuit, the input signal is applied between grid and the grounded plate circuit, with the output signal obtained across a load impedance placed between the cathode and ground. Thus, it is the plate that is common to both input and output circuits.In the grounded collector amplifier, the input signal is applied between base and the grounded side of the collector circuit, with the output obtained across a load impedance placed between emitter and ground. The collector then becomes common to both input and output circuits.As in the other transistor circuits, bias current and collector voltage are supplied by d.c.sources. D.C. voltage polarities in the grounded collector circuit are shown in the last pair of schematics in Fig. 45. The grounded collector circuit differs from the other basic transistor amplifiers in that the impedance maybe relatively high . . sometime seven approaching, in value, the input impedance of a vacuum tube amplifier. The input impedance is very dependent on load impedance, however.Since the output impedance of this circuit is moderate to low, it maybe used as an impedance matching device, if desired, and, in this respect,apes its vacuum tube cousin, the cathode follower.A large current amplification and a definite power gain may be obtained with the grounded collector amplifier. The power gain obtained,unfortunately, is less than that obtained with the grounded emitter and grounded base circuits.The voltage gain of the grounded collector circuit cannot exceed 1... thus, the circuit is of little value as a voltage amplifier.Phase reversal does not take place in the grounded collector stage. The input and output signals therefore have the same phase relationships.The grounded collector amplifier has one feature that is unique... it may serve as a bilateral or "two way" amplifier under some conditions. The connections of the input and output circuits may be interchanged, permitting a signal to be amplified in either the forward or the inverse direction."

GAIN IN TRANSISTOR CIRCUITS
Gains of transistor circuits usually are specified in decibels of power, instead of as multiplications of voltage, as is more common with vacuum tube circuits (except power output stages). Fig. 46 shows, in graphical form, the relationship between power gain in decibels (db) and gain as an actual ratio between output and input powers. Power in milliwatts is indicated on the chart, although any other common unit of power may be used. Milliwatts are indicated simply because power outputs and permissible power dissipation in transistor circuits are usually measured in these units.Graph showing the relations between gains measured in decibels of power and in ratios of output to input milliwatts. This chart may also be used to compare dt> gain for other power units. Transistor Amplifier Circuits pedance,and that the input signal current remains constant, the input power of a transistor stage is directly proportional to its input impedance.Comparatively little power will be needed at the input when the input impedance is small.Similarly, if the output signal current is constant, output power will be directly proportional to output in pedance,with a high output impedance giving a comparatively high output power for any given signal current. Thus, as in the case of the grounded base amplifier, where input and output currents maybe on the same order of magnitude,the ratio of gain is equal to the ratio of output to input impedances.On the other hand,with input and output signal currents which are not alike,the input and output powers are proportional to the squares of the currents, as well as being directly proportional to the impedances.Actual power gains depend not only on the characteristic impedances of the transistor, but,to a great extent, on how well these impedances are matched by the source and the load, on the type of circuit employed, and on the relative potentials at the high side input and output elements with respect to the common or grounded element. Gain which may be realized in practice is a matter of correct circuit design.Generally speaking, junction transistors are capable of giving more power gain than point contact types,when both types are used in similar circuits. However, gain does depend on the basic circuit employed and, as we have seen, the grounded collector circuit provides considerably less gain then either the grounded base or grounded emitter circuits. There are so many variables that it is difficult to give specific values as representative of typical power gains. However, a figure of 40db represents an average gain for a number of well designed circuits using junction transistors in the grounded emitter arrangement, while about 25db might be obtained using typical point contact units. Grounded collector circuits give a power gain on the order of 15db, while grounded base circuits give a gain somewhat less than,but approaching, the gain of the grounded emitter circuits.

COUPLING TRANSISTOR AMPLIFIERS
For maximum transfer of signal power from a source to a load the impedances of source and load must be equal. There is a rapid decrease of power transfer as the load impedance is made less then the source impedance, and a less rapid decrease as the impedance of the load is made greater than that of the source.In a single amplifier stage there are two sources and two loads requiring matching of impedances. There is first the signal source connected to the transistor input, and for which the input impedance of the transistor stage forms the load. The transistor itself is the second source, at its output terminals. For maximum output from the transistor, the impedance of the load connected across its output terminals must approach or equal its own internal impedance.As we have seen, the input and output impedances of transistors depend on the type of transistor employed on the operating conditions,and on the circuit arrangement used. However, the input impedances of transistor stages are likely to be moderate to small, while the output impedances are likely to be large... this brings up special problems when coupling transistor stages.Probably the simplest means of coupling two transistor stages is to employ an impedance matching transformer between the two stages.Such a circuit is illustrated in Fig. 47. In this circuit, two junction transistor amplifier stages are transformer coupled and are used to drive abeam power output tube. The grounded emitter circuit arrangement is employed.In order to match the high output impedance of the first transistor stage to the rlow input impedance of the second transistor, a step downturns ratio is used in the interstage coupling transformer. Resistance capacity and impedance coupling may be used between transistor stages on occasion.

Typical circuit arrangements are given in Fig. 48. With such circuits, it is assumed that the capacitance of blocking capacitor C is large (in audio circuits, on the order of several microfarads) so that its reactance will be small at the signal frequencies used. Thus, as far as the signal is concerned, there is a direct connection between the output of one stage and the input of the following stage, as shown by the Equivalent Signal Circuit. This means that the output impedance of one stage and the input impedance of the next stage are essentially in parallel, giving a total impedance which is less than the smaller value.Because of the differences in the internal impedances of the transistors, a match is impossible, and maximum power transfer between stages cannot occur.This means that the gain of a resistance capacity coupled multi stage transistor amplifier will be considerably less than that obtained with transformer coupling. In some instances, a three or four stage RC coupled amplifier will be required to give the same gain as obtained with a two stage transformer coupled circuit.Because there is less difference between the input and output impedances, the grounded emitter arrangement is most often used where RC or impedance coupling is necessary.

Source: http://dev.emcelettronica.com/transistors-practical-application-televisi...

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