US2777904A - Constant output amplifier - Google Patents

Description

Filed April 11, 1952 INI 'ENTOR. 5A M054 6. M/L SOUR/W5 CONSTANT OUTPUT AMPLIFIER Samuel C. Milbourne, Fair Lawn, N.

dix Aviation Corporation, tion of Delaware 1., assignor to Ben- Teterboro, N. J., a corpora- The invention relates to amplifier circuits and more particularly to amplifiers having a regulated output.

In circuits Where a substantially constant A. C. voltage is required, it is usually necessary to use a local oscillator of the Wein bridge type wherein the output is held substantially constant by means of a negative feedback circuit controlled by a current sensitive resistor. To maintain a substantially constant voltage, the current sensitive resistor must be held at a constant ambient temperature and the resistor is installed in an oven having a temperature regulator. This arrangement is satisfactory where the ambient temperature surrounding the oven is also substantially constant, but when the ambient temperature varies over a Wide range, then the oven temperature varies also and the output of the local oscillator cannot be maintained constant.

Since the frequency of the output voltage of the local oscillator is not determined by the frequency of the line voltage energizing the oscillator, the frequency of the oscillator output may vary relative to the frequency of the line voltage. This is objectionable or impractical where the two frequencies are used to operate adevice, such as a two-phase motor, where one phase is energized by the line voltage and the other phase is energized by the output of the oscillator. One remedy is to provide alocal oscillator capable of supplying all the power necessary to operate a single frequency type circuit, but where considerable power is required, the size andweight of the oscillator is impractical.

One object of the present invention is to provide a novel amplifier having its input supplied by a conveniently available line voltage or local power supply and having an output of the same frequency as the line voltage or local power supply.

Another object is to provide an amplifier having a predetermined output which is maintained substantially constant irrespective of variations in line voltage, circuit constants, and output load.

Another object is to provide an amplifier wherein the output is maintained substantially constant even With wide variations in mbient temperature.

The foregoing and other objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawing wherein one embodiment of the invention is illustrated byway of example. It is to be expressly understood, however,.that the drawing is for the purpose of illustration and description only, and is not'intended as a definition of the limits of the invention.

The invention contemplates an amplifier having an input and an output and adapted to have its input supplied from a fluctuating alternating power source. A current sensitive resistor is connected to the input and has a negative temperature coefiicient of resistance. A feedback circuit is connected to the output and includes a current sensitive resistor having a negative temperature 2,777,904 Patented Jan. 15, 1957 2 coefiicient ofresistance. The relative characteristics of the resistors are suchthat a change in'the voltage of: the power source effects a change in the voltage at the input, and a corresponding change in the feedbackvoltage, so that the output of the amplifier remains substantially constant irrespective of fluctuations of the power source.

The single figure of the drawing is a schematic wiring diagram of a novel amplifier constructed according to the invention.

Referring now to the drawing 'for a more detailed description of the novel amplifier constructed according to the invention, the amplifier is shown'as having three stages of amplification 1, 3, 5. The first stage 1 of the amplifier includes a triode 7 having an anode 9,"a cathode 1l,and a grid 13, connected in a conventional bridge-T circuit to eliminate harmonics of the desired frequency.

The second stage 3 of the amplifier includes a pentode 15 having an anode 17, a cathode 19, and a control grid 21, the latter being connected through a resistor 23 and a condenser 25 to the anode'9 of'triode '7. The output m cc stage 5 of the amplifier comprises a pair of triodes 27, 29' connected in push-pull fashion and having cathodes 31, 33,

grids 35, 37, and plates 39; 41. Grid 35 is connectedto the anode 17 of pentode15 through'a condenser 34, and

grids 35, 37 areconnected to ' gro'und'by resistors 36, 33*

and considers 40, 42 in parallel; Cathodes 31, 33 are connected together and to ground through a common cathode resistor 44. Plates 39, 41'are connected 'to'the primary winding-'43 of an output transformer lfi: Grid 37 is connected by a resistor 46 and a condenser 4810' plate39'of tube 27 whereby grid37'is provided witha voltage out of phase with the voltage applied to grid 35 of tube 27 and of substantially the same amplitude.

A center tapped secondarywinding 47 of transformer 45 is connected to a load 'L ofany suitable kind and the center tap 49 is grounded. The amplifier described above is of conventional design and any other suitable amplifier" may be used insteadfi The amplifier has its input-suppliedfrom an alternating power source Es through resistors51, 53,55 connected in series with one another to the-'powersource." Grid '13 of triode-7 in the first amplifier stage is connected through a resistor 57 to the 'junction ofresistors 53, SSso that the voltage across resistor 55 'is coupled to-thegrid.

A portion ofthe output' of the third amplifienstage is fed back to the input of the second"stage througha circuit connecting secondary winding 47hr transformer 45 to control grid 21 'ofpentode 15 and compris'inga resistor 59, a potentiometer 61', and resistors 63,- 65, 6'7, 69. The feedback voltage across resistors 65;67, 69-is 180 outof phase with'the-voltage at'theoutputof" the first amplifier stage at resistor 23, and the output-of the" first amplifier stage andthe feedback voltage are combined algebraically andapplied to'the control grid 21 of pentode 15. The magnitude-of the output voltageacross secondary Winding-47 of transformer ES-may be adjusted to anydesired value by adjusting'the feedback voltage by means of potentiometer 61.

Resistors 53, 63 and'67 "preferably have negative tem-' changes in temperature.

are enclosed within a single envelope'a'nd are heated by heaters .71, 73 closely associatedwith the resistors within the envelope. Heaters 71', 73 are connected in series across power source Es. Various types of negative temperature coefiicient resistors are available commercially, certain of these types being current sensitive and other of these types being current insensitive. Representative constructions of current sensitive negative temperature coefliclent resistors and of current insensitive negative temperature coefficient resistors are described, for example, in U. S. Patent No. 2,332,073 to C. A. W. Grierson, granted October 19, 1943.

An increase in voltage Es produces increased current through heaters 71, 73 so that the ambient temperature of the associated resistors 53, 63 increases and their resistances decrease. The increase in voltage Es also increases current flow through the circuit including resistors 51, 53, 55 and this increased current fiow through resistor 53 further decreases its resistance. The combined effect of the change in ambient temperature of resistor 53 and the increased current flow therethrough results in increased voltage across resistor 55 which is applied to grid 13 of tube 7 through resistor 57. The voltage at the output of the first amplifier stage increases accordingly. Thus, an increase in voltage Es produces an increased voltage at the output of the first amplifier stage, and this is then conveniently balanced out by feedback means about to be described.

The increase in ambient temperature of resistor 63, caused by the increased heater current, decreases its resistance and current fiow increases in the circuit includmg resistors 63, 65, 67 and a portion of potentiometer 61. The voltage across resistors 65, 67 increases accordingly. Since the output of the first amplifier stage and the feedback voltage across resistors 65, 67, 69 are subtracted from one another, the increased output of the first amplifier stage is substantially offset by the increased feedback voltage across resistors 65, 67, 69 so that the input to the second amplifier stage is substantially constant.

Likewise, a decrease in voltage Es produces a decreased voltage at the output of the first amplifier stage and a corresponding decreased voltage across resistors 65, 67, 69. The decreased output of the first amplifier stage is substantially offset by the decreased voltage across resistors 65, 67, 69 so that the input to the second amplifier stage is substantially constant with a decrease in Es.

If the output E across transformer 45 should increase instantaneously, then current flow increases through the circuit including resistor 59, potentiometer 61, and resistors 63, 65, 67. This increased current flow through resistor 63 decreases its resistance, and the feedback voltage across resistors 65, 67, 69 increases whereby the input to the second amplifier stage 3 decreases and the output voltage E0 returns to the desired value.

If the output E0 across transformer 45 should decrease instantaneously, the current flow decreases through the circuit including resistor 59, potentiometer 61, and resistors 63, 65, 67. This decreased current flow through resistor 63 increases its resistance and the feedback volt age across resistors 65, 67 69 decreases whereby the input to the second amplifier stage 3 increases and the output voltage E0 returns to the desired value.

If the gain of one or the other stages of amplification changes due to a change in the B power supply or the lb of one or more of the tubes, then as the voltage at the output of the first amplifier stage changes, the feedback voltage changes in substantially the same proportion so that the output E0 across transformer 45 is substantially constant.

Wide variations in ambient temperature surrounding the tube containing resistors 53, 63 results in a change in resistance of resistors 53, 63, but the change in voltage at the output of the first amplifier stage resulting from the change in resistance of resistor 53 is ofiset by the change in feedback voltage as a result of change in resistance of resistor 63 so that E0 across secondary transformer 45 remains substantially constant even with wide variations in ambient temperature.

For example, if the ambient temperature rises and the resistance of resistor 53 decreases, the voltage at the input of the first amplifier stage increases accordingly. Likewise, the resistance of resistor 63 decreases and the feedback voltage increases. The increased voltage at the output of the first amplifier stage corresponds to the increased feedback voltage, and since the two voltages are added algebraically, the input to the second amplifier stage 3 remains substantially the same.

Also, when the ambient temperature falls, then the decrease in the output of the first amplifier stage is offset by the decrease in the feedback voltage and the output voltage E0 remains substantially constant. Any residual change in output voltage E0 due to change in ambient temperature is compensated by resistor 67 which is selected to compensate for any differences in the negative temperature coefficient characteristics of resistors 53, 63.

The amplifier described herein provides an output voltage E0 which may be adjusted within limits to any desired value and which is maintained substantially constant irrespective of variations in line voltage, circuit constants, output load, and wide variations in ambient temperature. Also, the output voltage E0 has the same frequency as the local power supply so that the amplifier may be used to energize a device such as a two phase motor where one phase is energized by the line voltage and the other phase is energized by the output of the amplifier.

Although but one embodiment of the invention has been illustrated and described in detail, it is to be expressly understood that the invention is not limited thereto. Various changes can be made in the design and arrangement of the parts without departing from the spirit and scope of the invention as the same will now be understood by those skilled in the art.

I claim:

1. In an amplifier having an output and having an input adapted to receive an A. C. input signal of varying amplitude, means for deriving a first A. C. signal from said input signal including a current-sensitive first resistance element having a negative temperature coeificicnt of resistance and electrically coupled in circuit between the source .of input signals'and said amplifier input to vary the amplitude of said first signal as a direct function of said input signal, means including a negative feedback circuit coupled to said amplifier output for developing a second A. C. signal of the same frequency as said first signal but substantially out of phase therewith, said feedback circuit including a second resistance element having a negative temperature coetficient of resistance, means for additionally modifying the amplitude of said first signal .as a function of said input signal and for modifying the amplitude of said second signal as a function of said input signal so that the amplitudes of said modified first and second signals vary directly by substantially equal amounts in response to variation in the amplitude of said input signal, said modifying means including heater means for indirectly heating the resistance of said first and second elements as a function of said input signal, means for algebraically adding said modified first and second signals to produce a resultant A. C. signal having a substantially constant magnitude corresponding to the difference in the amplitudes of said modified first and second signals, and means for amplifying said resultant signal to develop an amplified resultant signal at said amplifier output.

2. In an amplifier having an output and having an input adapted to receive an A. C. input signal of varying amplitude, means for deriving a first A. C. signal from said input signal including a current-sensitive first resistance element having a negative temperature ooeflicient of resistance and electrically coupled in circuit between the source of input signals and said amplifier input to vary the amplitude of said first signal as a direct function of said input signal, means including a negative feedback circuit coupled to said amplifier output for developing a second A. C. signal of the same frequency as said first signal but substantially 180 out of phase therewith, said feedback circuit including a second resistance element having a negative temperature coefiicient of resistance, means for additionally modifying the amplitude of said first signal as a function of said input signal and for modifying the amplitude of said second signal as a function of said input signal so that the amplitudes of said modified first and second signals vary directly by substantially equal amounts in response to variation in the amplitude of said input signal, said modifying means in cluding heater means for indirectly heating the resistance of said first and second elements as a function of said input signal, means for algebraically adding said modified first and second signals to produce a resultant A. C. signal having a substantially constant amplitude corresponding to the difference in the amplitudes of said modified first and second signals, said first and second resistance elements being substantially equally responsive to changes in ambient temperature to change the amplitudes of said modified first and second signals by substantially equal amounts, whereby the amplitude of said resultant signal is substantially unaffected by changes in ambient temperature, and means for amplifying said resultant signal to develop an amplified resultant signal at said amplifier output.

3. In a constant-output amplifier having an output circuit and having an input circuit including a pair of terminals adapted to be coupled to a source of fluctuating A. C. voltage of predetermined frequency to receive an input signal therefrom, a plurality of amplifying stages for developing an amplified signal in said output circuit, means for producing a first A. C. signal of said predetermined frequency including the first amplifying stage and a current-sensitive first resistance element having a negative temperature coefiicient of resistance and coupled in circuit between said input terminals and the input to said first amplifying stage to vary the amplitude of said first signal as a direct function of said input signal, means including a negative feedback circuit coupled between said output circuit and the input to the second stage for developing a second A. C. signal of said predetermined frequency but substantially 180 out of phase therewith,

said feedback circuit including a second current-sensitive resistance element having a. negative temperature coefficient of resistance to vary the amplitude of said second signal as a direct function of said amplified signal, a common envelope enclosing said first and second resistance elements so that said elements are subjected to substantially the same ambient temperature, means for additionally modifying the amplitude of said first signal as, a function of said input signal and for additionally modifying the amplitude of said second signal as a function of said input signal so that the amplitudes of said modified first and second signals vary directly by substantially equal amounts in response to variation in the amplitude of said input signal, said modifying means including heater means inside said envelope for indirectly heating said resistance elements as a function of said input signal, a circuit for energizing said heater means coupled to said pair of terminals, means for algebraically stantially equal amounts when changes in ambient temperature occur, whereby said amplified signal is substantially unaffected by changes in ambient temperature.

References Cited in the file of this patent UNITED STATES PATENTS 2,183,731 Wright Dec. 19, 1939 2,290,084 Albright July 14, 1942 2,307,308 Sorensen Jan. 5, 1943 2,378,620 Chatterjea et al. June 19, 1945 2,426,589 Bollman Sept. 2, 1947 2,428,363 Elmendorf Oct. 7, 1947 2,559,587 Black July 10, 1951 2,572,108 Chalhoub Oct. 23, 1951 2,581,953 Hecht et a1 Jan. 8, 1952

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