US2360233A - Thermistor as amplifier for low frequency signals - Google Patents

Description

Oct. 10, 1944.

L. w; HussEY 2,360,233

, THERMISTOR AS AMPLIFIER FOR LOWFREQUENCY SIGNALS Filed Dec. 10, 1941 5 NORMAL VARIATION #1111001 COMPENSATION n ranswrrnlsronnou 1 a" cousm-r 04m FIG- 9B a (musmr use. as.

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'EWRHHSTOR AS R FQR LGW FREQUENEY SEGNALS Luther W. Hussey, Madison, N. .l'., assignor to Bell Telephone Laboratories; Incorporated, New York, N. Y; a corporation of New York Application December 14 1941, Serial No. 322316 7 Claims.

This invention relates to amplifying systems systems operative over a band of frequencies. More particularly it relates to the use of semiconductors such as silver sulphide as amplifying elements in such systems and as elements to control the behavior of these amplifying elements as the ambient temper'ature' varies.

It is known that certain compounds which are ordinarily'classed as semiconductors exhibit two very useful phenomena-negative temperature coefficient of resistance and a voltage-current characteristic that rises to a voltage maximum after which the voltage falls off as the current increases thus achieving a negative slope which is practically linear-over a part of its range. It

is in this latter range that these elements f-unction as negative resistances to facilitate amplification in the manner well-known in the art. Also it ,is because of the negative temperature coeflicient theydisplay that they are often .usedas temperature controls especially to regulate electrical devices in accordance with variations in ambient temperature. Both of these desirable properties are made use of in the operation of this invention.

In using semiconductors for' amplification a direct current potential is applied to bias the element into operation over the most linear portion of its negative resistance region so that when it is inserted in a suitable alternating current circuit maximum undistorted amplification is realized. Also, in order to increase the frequency range of the amplification band'and the amount of amplification realizable both of which factors are limited by thethermal inertia of thesemiconductor, it is desirable to keep the ele- .ing various waysof using one or more additional thermistors to modify the action of the amplifier thermistor in accordance withvthis invention; and

Figs. 9a. and 9!) show typeslof impedance used to compensate the variation with frequency of amplifier 'thermistors.

Referring,.now, more particularly to Fig. 1 curves l, 2, 3, l, and 5 represent a family of thermistor-voltage-current characteristics, curve 3 representing the mean, curves l and {representing the effect of high ambient temperature and curves 4 and 5 representing the-effect of low ambient temperature. The curve marked 1/R represents the normal variation of the bias without compensation. Itis the E--I characteristic of a fixed resistance inserted in'series with the thermistor to stabilize its bias. In isthe direct,

current bias, Eb'i's the battery voltage, and V0 a slope equal to the reciprocal of the bias stabilizing resistance is a familiar one and is valid for any kindof resistive device in series with a linear resistance and a battery. In the case of the thermistor it is necessary that the bias ments small in volume in order that they may be sensitively responsive to rapid changes in current and temperature.

In using semiconductors as temperature controls each is chosen to have the proper dimensions and temperature coemcient to regulate the particular variable feature of the amplifier semiconductor that it is designed to control in a Most of these have been discussed in the prior art. Throughout this de- 'scription the word semiconductorand its commercial synonym, thermistor, will be used interchangeably. 4

For a detailed description of this invention and of the component embodiments thereof, reference is made to the drawing, in which:

Fig. 1 shows graphically the variable behavior of a thermistor unit operated in the negative stabilizing resistance and the battery voltage be sumciently large so that the straight line with slope equal to the reciprocal of the bias stabilizing resistance intersects the thermistor direct current voltage characteristic at one point only. Otherwisethe bias will slide to a positive resist ance intersection. Curve a'ob represents the. locus to be followed by the shifting bias if the gain is to remain constant. a."-ob represents the locus of the bias shift if the distortion is to remain constant.

. Fig. 2 shows T1 as a negative resistance thermistor for amplifying the signals generated by source I after which they. are impressed upon the windings of transformer B from which they are-passed along to load 9. The energy for heating and biasing T1 to its negative resistance condition is derived from direct current source 1. Its path is from direct current source I through the primary winding of transformer 8 through starts from source. I, goes through source imcharacteristic in this manner more of retard coil 4. Condenser 3 is merely a blocking condenser to prevent direct current energy from entering the signal source circuit. Its impedance is made sufliciently low so as not to affect transmission.

Fig. 3 shows a single frequency amplifier without a transformer. Like numbered elements perform similar functions to those of Fig. 2. The tuned circuit composed of condenser l and inductance ll could obviously be replaced by a band-pass network. In this circuit the thermistor amplifier direct current supply is prevented from entering any part of the signal circuit by condenser 3.

With circuits such as those shown in Figs. 2 and 3 the thermistor T1 would .be "initially biased to operate about point 0 on voltage-current characteristic 3 of Fig. 1. This operating point is determined by the negative resistance desired and the required freedom from distortion, This is accomplished by adjusting the direct current source I, which has a battery voltage Eb, to produce a drop Vo across T1 causing current In to flow through thermistor T1 which will then have its optimum bias for maximum undistorted amplification. When signals from the source I are applied to T1, in its biased condition, they will cause the voltage across and current through T1 to oscillate along the tangential direction at point 0 on curve 3 provided the frequency of does notexceed a limiting maximum. By operating over the negative resistance portion of the alternating current power is transferred to the load than is received fromthe signal source, the excess being absorbed from the source of direct current supply in a manner somewhat similar to that of a vacuum tube amplifier. T1 thus provides the negative resistance necessary for very satisfactory ampliflcation over a frequency/range limited only by its thermal lag which can be reduced by reducing the mass of T1. But if the ambient temperature rises, the directv current voltage characteristic shifts toward curve I and if the ambient temperature falls the characteristic shifts toward curve 5 the bias shifting with it along the line aob..

If a circuit such as that shown in Fig. 4 is used,

the shift in locus of the voltage-current characteristic of the negative resistance thermistor with ambient temperature change can be held to within closer limits such as curve 4 for low values and curve 2 for high values. In this circuit all elements similarly numbered perform the same functions as in Figs..2 and 3. A heater winding I 3 shunted by an ambient temperature control thermistor T2 is connected across the battery 7 in series with resistance I5. Heater winding I3 is -woundaround thermistor T1, being electrically but not thermally insulated from it to elevate its temperature to a level somewhat above ambient temperature to efiect closer temperature regulation. Now, if the ambient temperature changes, the resistance of T2 changes with it thereby changing the current through the heater winding which reduces the tendency of the temperature of T1 to change, thus locus of its voltage-current characteristic. An in put transformer l2 couples the input circuit and the signalsdiminishing the-shift in v although the bias operating point shifts along line aob, the sum of the resistances of thermistors T1 and IE; remains nearly constant even though the slope of the voltage-current characteristic changes appreciably.

. Fig. 6 illustrates a circuit' whereby the bias may be controlled in such a way that the operating point will follow a locus that intersects the shifting voltage-current characteristics at points at which the slope is very nearly the same. ferring again to Fig. 1, if the bias is made to vary as the ambient temperature varies in such a way that the operating point shifts along aob' instead of (tab the negative resistance of T1 and hence the gain will remain constant, Tris a slow acting thermistor operating as an ohmic resistance to adjust the bias. It has a negative coeflicient of resistance and decreases the bias When the temperature increases or increases it when the temperature decreases. Resistance I4 is inserted to properly proportion the relationship between the bias and the resistance of the positive thermistor.

Any two or even all three of these methods of controlling the negative resistance of T1 may be combined to realize the close regulation. obtainable by their concurrent action; For example the heater may be used to decrease the ambient temperature variation (say to a range'of curves 2 to 4 instead of I to 5) and then one or both of the other methods may be used for more accurate compensation. Also in case the gain compensated device of Fig. 5 has excessive distortion at some point, the bias variation control of Fig. 6 can be used to. make the operating point vary over that locus of points on the voltage-current characteristics whereat the distortion is the same (such as a"ob of Fig. 1) and either of the other methods used to further minimize the gain variations.

- Fig. 7 shows a circuit which uses both heater and bias control to facilitate the controlling effect of thermistor Also one thermistor may be designed to perform the function of both heater control and bias to change, the shift in locus of the volt- Re-v The c ange in the replaced with a series resonant circuit such as condenser I! in series with inductance l6, tuned to the midband frequency, in order to pass frequencies in the amplifier band but keep the impedance facing the thermistor high at low frequencies. In this particular circuit the amplifier thermistor direct current is allowed to pass through the high impedance winding of. both transformers.

To facilitate band-pass operation an electrical network such as those shown in Figs. 9a and 9b designed to take account of the effective reactive component of the amplifier thermistor (due to its thermal inertia) may be put in series with it. A resistance shunted by a condenser in series with the thermistor gives a fair compensation for a fiat gain over a band of frequencies. If an impedance consisting of an inductance and resistance in series and a capacitance in parallel is connected in series with the thermistor a considerably better compensation is obtained.

Self-oscillation will not occur in a negative resistance thermistor circuit used for amplification purposes if the sum of the, thermistor impedance and the total impedance facing it has a positive resistance component at all frequencies. sumcient positive coefficient resistance should, there- .fore,.be provided in the loop containing the thermistor for all embodiments of this invention The fact that all the amplifier circuits embodying the invention-have been described with reference to transformer coupled load and source applied to said first thermistor as the ambient temperature increases.

4. In an amplifying circuit for amplifying signals, a negative temperature cocfiicient thermiscircuits in no way limits its application to such circuits. Resistance, capacitance and network coupling may be used, the complexity of such coupling depending in each instance on the particular circuit application.

These specific and preferred embodiments illustrate the utility of my invention which is defined by the following claims.

What is claimed is:

1. A semiconductor operated in its negative resistance region, electrically biased for optimum amplification and a thermistor connected in said biasing circuit to tend to cause said bias to be always optimum as the ambient temperature changes.

2. A thermistor having a negative resistance that varies with ambient temperature and means I for operating it in the negative resistance part of its range'including electrical biasing means and means for controlling said negative resistance by changing the bias, said means so operating that the change in negative resistance it introduces is equal and opposite to the change introduced by the change in ambient temperature.

3. In an amplifying system, a source of waves to be amplified, a load circuit, and a negative temperature coeflicient thermistor included in circuit with said source and said load circuit, means to supply a bias current to said thermistor ofsuch value as to bias the thermistor into the 7 negative resistance region of its volt-ampere characteristic, said thermistor having a sumciently small temperature lag to enable its temperature to vary at the frequency of said waves to be amplified. and means toreduce the effect tor that is temperature responsive at the frequencies of said signals, a source of steady bias current for biasing said thermistor into the nega... tive resistance region of its characteristic, and means to compensate for variable amplification with varying ambient temperature comprising a relatively slow-speed thermistor connected in circuit with said first thermistor and said source to change the bias current in response to change in ambient temperature.

5. In combination, an amplifier comprising a thermistor having a speed of temperature response sufficiently high to follow variations to be amplified, means to bias said thermistor into the negative resistance region of its voltage-current characteristic, said characteristic having difierent slopes at different operating temperatures and the same value of bias and means to compensate for the 'efiect on the, amplifying properties of the thermistor of changes in ambient temperature comprising means responsive to ambient temperature variations for changing the amount of bias on said thermistor in the direction to tend to maintain at constant value the slope of the portion of the characterisic over which the thermistor is operated by said variations to be amplified.

6. In an amplifier for electrical waves a nega-- tive temperature coeflicient thermistor capable of having its temperature changed at the frequency of said waves, means 'to bias said thermlstorinto the negative resistance region of its voltage-current characteristic, and means responsive to changes in ambient temperature for decreasing the value of bias as the ambient temperature inperature response to heating current to have its temperature and therefore its resistance changed at the frequency of said waves, a sourceof bias current connected to pass a current through said thermistor of such magnitude as to enable the of ambient temperature variations upon the amplifying action of said system comprisin a secthermistor isexhibit a negative resistance effect to said waves, and a slow speed thermistor connected in circuit between said source of bias current and said first thermistor and responsive to relatively slow changes in ambient temperatures aifecting said first thermistor and, in response to such changes inambient temperature, changing the amount of bias current and thereby maintaining the negative resistance eifect exhibited by said first thermistor'substantlally constant.

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