US2983877A - Transistor oscillators - Google Patents

Description

May 9, 1961 K. BROERMANN TRANSISTOR OSCILLATORS 2 Sheets-Sheet 1 Filed July 18, 1957 INVENTOR Kenneth Broermann AGENT y 1961 K. BROERMANN 2,983,877

TRANSISTOR OSCILLATORS Filed July 18, 1957 2 Sheets-Sheet 2 7' I Q: H612 T g z V T v 0 TIME g /0.6 Q H61 3 E u Ii 8 Kenna m Broermann INVEN TOR.

United States r TRANSISTOR OSCILLATORS Kenneth Broermann, Cincinnati, Ohio, assignor to The Baldwin Piano Company, Cincinnati, Ohio, a corporation of Ohio Filed July 18, 1957, Ser. No. 672,658

14 Claims. (Cl. 331-52) The present invention relates generally to systems for generating trains of electric wave forms which are harmonically related in frequency, and more particularly to transistorized electronic circuitry for generating octavely related wave forms by frequency division, and to transistorized oscillators and frequency divider circuits for use in such systems.

Electronic musical instruments commonly employ a plurality of master oscillators, each of which generates oscillations corresponding to a note of different nomenclature within one octave of the musical scale. Octavely related notes are generated by cascaded frequency dividers, each divider chain being driven by one of the aforementioned oscillators. Operating characteristics of oscillators suitable for use in electronic musical instruments are stringent. They include generation of:

(a) a wave shape composed of a fundamental frequency and many harmonics;

(b) a stable fundamental frequency;

a rapidly-rising, high-amplitude wave front suitable for triggering a frequency divider accurately and cleanly;

(d) sufiiciently high amplitude of output; and

(2) minimum sub-harmonic content when the oscillator is employed as a trigger source for a frequency divider.

In addition, such oscillators must be susceptible of frequency modulation at low frequency, in order'to permit generation of vibrato effects.

Briefly describing a preferred embodiment of an oscillator according to the present invention, a PN-P junction transistor is employed, having a transformer winding connected between a source of negative bias and a collector electrode. A further transformer winding, inductively associated with the first mentioned winding, is connected via a charging capacitor between the collector and the base electrode of the transistor. The transformer is so poled as to feed back collector current into the base in proper phase to sustain oscillations and to assure rapid charge of the capacitor. A series LC circuit is connected between the base electrode and the emitter electrode of the transistor. In the charge period of each cycle of operations, current flows from the base electrode into the charging capacitor, until the transistor is cut-off in response to voltage across the capacitor. When the conduction period is finished the capacitor commences to discharge through a resistor, and simultaneously a damped sinusoid in the LC circuit commences. The voltage wave shape appearing at the base is then a superposition of waveforms appropriate to decay of current in an RC circuit and of a sinusoid. A firing point for the transistor is reached when the total base voltages passes zero volts, firing time being thus controlled principally according to the period of the sinusoid,;and may occur on any negative peak of the latter.

By reason of the regenerative charging of the capacitor, through the transformer which couples the base to the collector circuit of the transistor, two valuable conse: quences ensue. First, the rise time of the charging curve is extremely short, and second, by employing a greater ice number of turns in the base circuit winding than in the collector circuit Winding the capacitor may be charged to a greater voltage than is available in the bias source of the transistor, and an extremely high amplitude pulse of charging current is made available. The oscillator is, therefore, a saw tooth generator in which a damped sinusoid is superposed on the saw tooth wave form and serves to synchronize initiation of the latter, and frequency is accordingly determined by the constants of the LC circuit, and is independent of transistor parameters.

Vibrato efiects may be produced by introducing a subaudio frequency voltage in series with the resistance of the RC circuit, modulation of frequency thus occurring in response to variation of the times at which base voltage attains a switching value.

It is, accordingly, a broad object of the present invention to provide a novel oscillator employing a semi-conductor amplifying device, and capable of providing a stable wave shape of generally sawtooth form, containing multiple harmonics and a rapidly rising, high amplitude wave front.

It is a further object of the present invention to provide a transistor circuit for generating a generally sawtooth wave on which is superposed a sinusoidal wave form, which serves to stabilize the sawtooth wave independently of transistor parameters, bias voltage, temperature, and the like.

There is additionally provided a transistorized cascaded frequency divider chain which is synchronized from the transistor oscillator, the several elements of the chain generating wave forms suitable, after modification in tone color filters, for providing the notes of a given nomenclature in an electrical musical instrument.

The generation of notes of a given nomenclature by means of a master oscillator in cascade with a frequency divider chain is conventional in the art of musical instruments. It is a feature of the present invention that the master oscillator and each of the frequency divider stages associated with a note of a given nomenclature includes a transformer, and that the several transformers are all wound on a common core which acts as a transfer medium for synchronizing signals from one divider stage to the next, as well as for improving the operation of the frequency divider stages.

The first divider stage, and all succeeding stages, employ similar junction transistors, and each constitutes a synchronized relaxation oscillator having one winding of a transformer in its collector circuit. The remaining winding is connected in series with a timing capacitor between emitter and base electrodes of the transistor. A timing resistor is connected across the capacitor. All the similar transformers of a divider chain, as well as the transformer included in the master oscillator, utilize a single common core, made up of one set of laminations, which assures both eificiency of operation and economy of circuit fabrication. V

The application of a synchronizing pulse to any divider stage renders its transistor conductive, which in turn causes regenerative transfer of current from the collector through the emitter and base circuit of the transistor, thus charging the capacitor. When the capacitor is sutficiently charged it drivesthe transistor to cut-off condition, following which the capacitor discharges through the resistor. The transistor is not susceptible to a further synchronizing pulse until the voltage on the capacitor has decreased to a sufiiciently low value, so that successive stages may, by adjustment of the decay times of their timin circuits, possess different division ratios.

Since the base circuit and the collector circuit of the typical junction transistor have widely different resistances, the transformers employed may have turns ratios of other than 1: 1, and ratios of 2:1 have been employed in practice. This makes possible development of a greater voltage on the timing capacitor than is directly available at the transistor bias source, by a factor approximately equal to the turns ratio of the transformer.

It is, accordingly, a further object of the present invention to provide a novel frequency divider chain, the stages of which are electro-magnetically coupled through a common magnetic core for all the transformer windings of the chain.

Frequency modulation of all the stage frequencies is simultaneously accomplished in response to the output of a single oscillator, which develops vibrato signal across a resistor located in series with the discharge-timing resistor of the master oscillator of the system.

It is a further object of the present invention to provide a system of frequency modulating the master oscillator and the frequency divider chain of a transistorized octaval note generator of an electronic musical instrument, by introducing modulating voltage in series with a timing resistor of a capacitor discharge circuit of the master oscillator.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

Figure 1 is a schematic circuit diagram of multiple generating system according to the present invention;

Figure 2 is a plot of base voltage for a master oscillator of the system of Figure 1;

Figure 3 is a plot of collector voltage for the master oscillator; and

Figure 4 is a plot of base voltage, for a frequency divider stage of the system of Figure 1.

Referring more particularly to the accompanying drawings, in Figure l the reference numeral denotes PNP junction transistor including the usual semi-conductive body, and having a base electrode 11, emitter electrode 12 and collector electrode 13. The collector electrode 13 is connected to a source of negative bias potential 14, through a first winding 15 of a transformer 16. The emitter electrode 12 is connected directly to a point of reference potential, such as ground. A further winding, 17, of transformer 16, is connected between collector electrode 13 and base electrode 11 through a suitable charging capacitor 18.

Connected between the base electrode 11 and the point of reference potential is a series resonant circuit comprising two parallel connected capacitors 19 and 20, and an inductor 21. The capacitor 20 may be variable, to enable variation of the operating frequency of the system. A discharge resistor 22 for capacitor 18 is connected in series between the base electrode 11, and a point of reference potential, and may serve for injection in series therewith of signal suitable to frequency modulate the oscillator, and more particularly may serve for injection of vibrato signal, in a preferred application of the present system.

Output signal may be derived by means of a winding 23, inductively related to winding 21 as by being wound on a common core therewith, a voltage divider consisting of resistors 24 and 25 being connected in series across winding 23. An output lead 26 may be connected to the junction 27 of resistors 24 and 25.

Outlining now the theory of operation of the above described oscillator, and commencing at a time when the base electrode 11 is at reference potential, the transistor is conductive from emitter electrode to collector electrode and collector voltage starts to rise toward reference potential. Current flows from the emitter to the collector 13 and through winding 15 of transformer 16. The transformer action induces voltage in winding 17 of the right polarity to increase the current from the emitter 12 to base 11. This is a regenerative action that further increases the collector current, thus charging the 4 capacitor 18 rapidly. As the capacitor 18 is charged to the voltage across winding 17, the transformer can no longer drive the base circuit. Thus, the base current is reduced, which in turn reduces the collector current. This effect is cumulative in cutting the transistor off. The base electrode rapidly swings positive when the transistor is cut off. The positive potential between the base electrode 11 and the emitter electrode 12 is now impressed across the LC circuit composed of capacitors 19, 20 and inductance 21, and the capacitor 18 is now charged to approximately twice the voltage of source 14, assuming a 2:1 turns patio for transformer 16. The capacitor 18 now commences to discharge in a circuit including resistor 22, at a rate appropriate to the time constant of the RC circuit comprising primarily capacitor 18 and resistor 22. The energy stored in the LC circuit is also dissipated, but in an oscillatory fashion, in the resistance of windings 17 and 15.

The net voltage at the base is, then, that illustrated in Figure 2, where T1 is the commencement of discharge, of capacitor 18 and T2 the wave shape of voltage at base electrode 11, consisting of two superposed wave shapes, one appropriate to an RC decaying transient and one appropriate to an LC decaying sinusoid. Decay continues until the base electrode reaches reference potential, at T3, whereupon the transistor again becomes conductive, and a sharp pulse of current I1, (Figure 3), again flows to recharge the capacitor 18. At this point in the operation of the system the voltage at the collector is zero, substantially.

It will be noted that the sinusoid in Figure 2 is reproduced in Figure 3, but in opposite polarity. The sharp and rapid attainment of reference potential at collector electrode 13 provides the rapidly rising wave front appropriate for application to succeeding frequency dividers as synchronizing pulses, because of the rapidly changing currents, which produce rapid, high-amplitude flux changes in the core of the transformer 16. The preferable current wave shape for application to tone color filters for development of musical notes is that present in the inductor 21. The latter wave shape may be abstracted by associating with winding 21, in inductive relation, the secondary winding 23, from which a signal of desired amplitude may be derived by voltage division across on resistor 27, of a voltage divider consisting of resistors 24, 25 connected in series.

The oscillator comprising transistor 10 synchronizes operation of frequency divider stage FDI, for which the reference numeral 30 denotes a junction transistor having an emitter electrode 31, a base electrode 32 and a collector electrode 33. The source of negative supply voltage 14 is connected to the collector electrode 33 through one winding 35 of a transformer 36. The transformer 36 includes a further winding 37, which may have a greater number of turns than the winding 35, the winding ratio being 2:1, in a preferred form of the invention, although I do not desire to be restricted to any specific ratio.

By having a larger number of turns in the base circuit, the capacitor 18 can be charged to a voltage greater than the bias voltage by a factor of the turns ratio.

The winding 37 is connected to base 32, at one terminal, and at its remaining terminal through a capacitor 38 to a line 39. The latter is supplied with a small negative voltage with respect to ground, from a power supply 40. A preferred value of voltage is 0.6 v. to provide an average of zero volts at 43, thus preventing keying transients. Across the capacitor 38 is connected a voltage divider, consisting of two series resistors 41 and 42, to the junction 43 of which is connected an output lead 44.

The core 45 of transformer 36 is also the core of transformer 16, i.e., both transformers are wound on common larninations. Thereby coupling exists between the windings 15, 17 and 35, 37, via the common core. This coupling, which may be controlled by suitable shunt legs between transformers, is employed to synchronize operation of the frequency divider FDl. The teachings of Jones in U.S. Patent 2,555,038 are applicable here.

In operation a synchronizing pulse is supplied to winding 37 by the oscillator by virtue of the interlinkage with the core of transformer 16. The coil is so poled as to induce a voltage in the winding 37, such that current is caused to flow from the emitter 31 to the base 32 through winding 37 and into capacitor 38. Current fiowing out of the collector 33 is a direct function of that flowing out of the base, the collector current being many times greater than the base current. The capacitor 38 appears as a short circuit for rapidly varying voltage, so that the impedance reflected into the collector winding 35 is low. Current amplification occurs in the collector circuit, which regeneratively drives the base circuit, through the transformer 36. The circuit thus rapidly attains a condition in which the capacitor 33 is charged to a voltage which, in the preferred embodiment, is twice the bias voltage, and the transistor is cut off in a manner similar to the master oscillator. The capacitor now discharges through the resistors 41, 42. To a first approximation, by neglecting the non-linear inverse resistance of the base 32, the decay of voltage across the capacitor 38 is exponential.

Discharge continues even when a further synchronizing pulse is applied in the sense in which the voltage of a capacitor 38 is decaying, provided it fails to occur while the condenser 38 is substantially charged. Accordingly, by proper design of circuit parameters which establish the decay current curve of the capacitor 38, every synchronizing pulse, or every alternate pulse, or every third pulse, etc., can be caused to initiate a charge and discharge cycle of capacitor 33. The division ratio of the divider is thus a function of the values of capacitance and resistance employed in the discharge circuit, and of the peak value of voltage to which the capacitor is charged, during the charging part of the cycle.

Divider action cannot occur if successive input pulses eifect or initiate the regenerative action above described. However, if the synchronizing pulse following that which caused a regenerative action is insuificient to cause conduction of base current, divider action will occur.

Consider the two experimentally obtained plots of Figure 4. Let (RC) denote the maximum time constant possible for divider action. Assume that the voltage on the base of the transistor approaches zero volts asymptotically, and that a negatively-going synchronizing pulse is required to initiate synchronizing action. Assume the amplitude of the trigger voltage to be E and that it be substantially independent of the time constant of the circuit. Assume further that the wave shapes illustrated may be represented by E F which in turn equals E, at t=21-, where 7' is the time interval between the trigger pulses. For a given E E, and 7 the value of (RC) is determined. If the time constant were greater than this value, the trigger would not be able to cause conduction on the second pulse, but might on the third pulse.

Let (RC) denote the minimum time constant possible for divider action. Let E, be independent of the time constant in the range considered here. Then in similar action the wave shape for the minimum time-constant may be represented by T r At t=7',

E e )2:Ef

If (RC) were smaller than the value determined by E e- =E then the trigger would cause action where the circuit was sensitive to every pulse. Since E, is the same for both cases -21' -T/ Ro),= 1/(no), and 7 or (RC) =2(RC) Thus the maximum time constant is equal to double the minimum time constant for divider action.

In practice, when the base current is zero, the collector current present can affect the firing time of the base circuit. However, the effect is, small enough at room temperature to be neglected. The above derivation assumes the pulse period is small in comparison to 'r.

Successive divider stages FDZ, FD3, FD4 and FD5 are arranged to have similar circuitry, but to employ RC circuits designed in accordance with the principles hereinabove outlined, such that each successive divider stages effects division by a factor of two, with respect to the preceding stage.

The bias source 40 for the present system includes a source 50 of A.C. power, which is coupled by means of a transformer 51 to input terminals of a full-wave rectifier 52. The output terminals of rectifier 52 are connected to a pi-type smoothing filter and voltage divider. The filter is comprised of a shunt capacitor 53 and a series choke 54, and a further shunt filter 55, connected in conventional pi configuration. Across the capacitor 55 is connected a pair of resistors 56, 57 in series. Lead 14- is connected to the high voltage side of resistor 56, and lead 39, to the junction of resistors 56, 57.

A vibrato signal generator or oscillator V.O. is provided, the output of which serves to frequency modulate the output of oscillator O at a low rate, and thereby to modulate the outputs of frequency dividers FD1 FDS.

The oscillator V.O., which is the subject of copending application Serial No. 629,609, in the name of Marion B. Gregory and assigned to the same assignee, employs a transistor 60, having a base electrode 61, an emitter electrode 62 and a collector electrode 63. The collector electrode 63 is connected directly to lead 14, and to a bias setting resistor 64 between collector electrode 63 and base electrode 61. A DC. blocking capacitor 65 couples base electrode 61 to one end of a tank circuit consisting of an inductor 66 and capacitor 67, the other end being connected to ground. The emitter terminal 62 is connected to an intermediate tap 68 of inductor 66.

The tap 68 is connected via a resistor 69 of relatively high value to a slider 70 of a relatively low resistance potentiometer 71, one end of which is connected, as shown, to ground (or, if preferred, to lead 39). The latter may be by-passed to ground (or to lead 39, if preferred) by a capacitor 72 which has negligible reactance for the frequency generated by oscillator O; and the resistor 71 and capacitor 72, taken in parallel, are connected in series with resistor 22 of oscillator O.

In operation, capacitor 72 provides a by-pass for the current through resistor 22. The latter is thus connected to ground for the audio signal, which thereby is not injected into vibrato oscillator V.O., where it might affect the operation of the latter, as by introducing amplitude or frequency modulation, or both.

The output frequency of oscillator V.O. on the other hand, is sufliciently low that the capacitor 72 is not a bypass, but constitutes a relatively high impedance. The vibrato signal appearing at terminal 68 finds a path to ground, then through the resistor 69, to slider 70, and thence via a portion of resistor 71 which lies between slider 76 and ground. Since the resistance of that portion is low relative to that of resistor 69, the setting of slider 70 does not appreciably affect operation of oscillator V.O.; i.e., the latter sees a substantially constant load regardless of the setting of the slider. Hence, the output frequency and amplitude of oscillator V.O. remain constant for all available positions of slider 70.

The voltage at vibrato frequency, which is available at slider 70, is applied to the base electrode 11 of transistor 10, where it is superposed on the voltages in any event there present; and being a slowly varying A.C. voltage, with respect to ground, the times of completion of operative cycles of oscillator O are advanced and retarded, thus producing the affect of frequency modulation.

Since the frequency dividers FDI FDS, are synchronized from the output of oscillator their outputs also are frequency modulated at the frequency of vibrato oscillator V0.

The element 74 represents a conventional multiple terminal plug, the numbered terminals of which are connected to significant points of the system. At the terminals 9, 1, 2, 3, 7, 8, for example, are available the output signals of the system. Although, in the interests of simplification, certain leads, such as 14, in Figure 1, are shown with connections directly to a supply point, it will be understood, as is conventional, that such connections may go via connectors, such as 74. Thus, a series of twelve generators, such as that illustrated in Figure 1, may all be plug-connected to common supply sources such as 40 and the oscillator V0.

The application of a fixed voltage (0.6 v. as mentioned above) to lead 39 balances out the average D.C. component in the output voltages available at terminals 9, 12, 3, 7, and 8. This is a decided advantage in minimizing keying transients in the application of the present system to electronic musical instruments. Further details of similar circuitry are available in US. Patent 2,756,332, to A]. Bissonette, issued July 24, 1956.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the general arrangement and of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims. 7

What I claim is:

1. An oscillator including a semi-conductive device having an emitter electrode, a collector electrode and a base electrode, means connecting said emitter electrode directly to a point of reference potential, a transformer having a first and a second winding, a source of bias voltage connected to said collector electrode through said first winding, a capacitor, means connecting said collector electrode to said base electrode through said second winding and said capacitor in series, a resistance connected between said base electrode and said point of reference potential, and a series resonant circuit connected between said base electrode and said emitter electrode.

2. A transistor oscillator including a transistor having a collector electrode, a base electrode and an emitter electrode, a two winding transformer intercoupling said collector electrode with said base electrode in positive feed back sense, a timing capacitor, means connecting said timing capacitor in series between said collector electrode and said base electrode through one winding of said transformer, a source of bias voltage connected to said collector electrode, means for connecting said collector electrode to said emitter electrode in series with the other winding of said transformer, and a discharge path for said timing capacitor subsisting entirely externally of said transistor.

3. The combination according to claim 2, wherein is also provided a source of synchronizing voltage connected to said base for establishing discharge times for said timing capacitor, said source of synchronizing signal being a source of a damped sinusoidal voltage.

4. The combination according to claim 3, wherein is provided a discharge resistance path connected to said timing condenser for discharge of said timing capacitor, and wherein said source of a damped sinusoidal voltage is a series resonant circuit connected across said resistance path.

5. The combination according to claim 4, wherein is further provided means for applying a relatively slowly varying signal to said base electrode to vary said discharge times with respect to a mean value and thereby to frequency modulate said oscillator.

6. The oscillator according to claim 1 wherein is further provided an impedance connected between said point of reference potential and said base electrode via said resistance, said impedance having negligible magnitude at the frequency of said oscillator, said impedance including a resistance and a capacitance in parallel, and means for applying variable voltage to a point of said resistance to vary the frequency of said oscillator.

7. The combination according to claim 6 wherein said series resonant circuit includes a first winding, a second winding electro-magnetically coupled to said first winding, relatively high resistance connected across said second winding, and a signal output connection to said relatively high resistance.

8. The combination according to claim 1 wherein said series resonant circuit includes capacity and an inductance connected in series between said base electrode and a point of reference potential, one terminal of said inductance being directly connected to said point of reference potential.

9. The combination according to claim 20 wherein is further provided an output winding electro-magnetically coupled to said inductance and having one terminal connected directly to said point of reference potential.

10. The combination according to claim 1 wherein said first and second windings are wound on a core, and a plurality of frequency divider stages coupled in cascade to said oscillator, each of said frequency divider stages including a saw-tooth oscillator having a regenerating transformer wound on said core.

11. The combination according to claim 10 wherein is further provided means for applying a variable frequency control voltage to said base, said means comprising an impedance having negligible magnitude at the operating frequency of said oscillator, said impedance being connected between a point of reference potential and said base electrode via said resistance.

12. The combination according to claim 11 wherein said impedance is a capacitance and a relatively high resistance in parallel, said capacitance having negligible resistance at the operating frequency of said oscillator.

13. The combination including, a source of bias voltage, a first bus connected to said source of bias voltage, a second reference voltage bus connected to said source of bias voltage, a transistor oscillator, said transistor oscillator including a transistor having a collector electrode, a base electrode and an emitter electrode, a connection from said emitter electrode to said second bus, a two winding transformer intercoupling said collector electrode with said base electrode in positive feedback sense, a timing capacitor, means connecting said timing capacitor in series between said collector electrode and said base electrode through one winding of said transformer, said collector electrode being connected to said first bus, means connecting said collector electrode to said emitter electrode in series with the other winding of said transformer, and a discharge path for said timing capacitor subsisting entirely externally of said transistor, said transformer being a voltage step up transformer arranged to charge said capacitor from said source of bias voltage to a voltage higher than the voltage of said source of bias voltage, a series-connected inductor and capacitor connected across said discharge path to provide a resonant circuit, said discharge path including a timing resistance.

14. The combination according to claim 13 further including, a chain of frequency divider stages coupled in cascade and connected identically in parallel between said first and second bus, each of said frequency divider stages including a transistorized blocking oscillator having a timing capacitor and a voltage step-up feedback transformer, and circuitry including said transistor and said transformer in each of said stages for substantially instantaneously charging all said last-mentioned timing capacitors in response to a pulse applied to said transformer, and a discharge timing resistor in discharge relation to each of said last mentioned capacitors, the transformers for the several stages and for said oscillator being linked with a magnetic core common to all said transformers.

References Cited in the file of this patent UNITED STATES PATENTS Lindley et a1. Mar. 24, 1953 10 Tourshou Mar. 31, 1953 Felker May 8, 1956 Bissonette July 24, 1956 Sulzer Sept. 25, 1956 Mandelkorn Jan. 8, 1957 OTHER REFERENCES Transistors Theory and Practice, by Rufus P. Turner. Published by Gernback Publications Inc., N. Y., April 2, 10 1954, pages 76-81. v

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