Protection circuit for transistorized audio power amplifier

Claim 2

A power amplifier which is to amplify an input AC signal with a varying amplitude and waveshape with minimum signal distortion, said power amplifier comprising: transistor or transistor-like current amplifying means with load terminal means connected through load circuit means to the terminals of a source of direct current voltage and control terminal means connected to signal drive means which varies the magnitude of drive current fed through the load terminal means, and a protection circuit including means responsive both to various combinations of progressively lower than normal impedances in the load circuit means and progressively increasing degrees of control drive current applied to said transistor or transistor-like current amplifying means for producing a given control signal when such combined conditions approach transistor damaging conditions of operation, and load current termination means responsive to said given control signal by resettably terminating current flow in said transistor or transistor-like current amplifying means for at least a considerable period following the disappearance of said control signal.

Claim 3

The power amplifier of claim 1 wherein said transistor or transistor-like current amplifying means comprise a pair of series connected transistor or transistor-like current amplifying devices having their adjacent load terminals interconnected through a pair of series connected impedance means, their remote load terminals connected to the opposite polarity terminals of a source of direct current energizing voltage whose output terminals present voltages of an opposite polarity relative to a given reference point and their control terminals connected to signal driving means so one of the current amplifying devices is conductive only for that portion of an input AC signal having one polarity and the other current amplifying device is conductive only for that portion of the input AC signal having the opposite polarity and to a degree proportional to the instantaneous magnitude of the input AC signal, and a load device normally of much greater impedance than either of said pair of series connected impedance means and coupled between the juncture of said pair of impedance means and said reference point so the voltage between the adjacent load terminals and said reference point increases, and the voltage drop across the load device decreases with the decrease in impedance value of the load device, and said load current termination means includes means for terminating the load current of the power amplifier when the difference between the voltages derived from the end of one of said impedance means remote from said load device and said reference point, on the one hand, and the voltage drop across said load device, on the other hand, reaChes a predetermined value.

Claim 4

The power amplifier of claim 2 wherein said load current terminating means includes a bridge circuit whose voltage output is a direct function of both the current flow through at least one of said transistor or transistor-like current amplifying devices and the impedance of the load device, and control means responsive to a given magnitude of the voltage output of the bridge circuit.

Claim 5

The power amplifier of claim 3 wherein there is formed with one of said series connected impedance means a bridge-type circuit where first and second arms of the bridge circuit are constituted by the latter impedance means and said load device and third and fourth arms of the bridge circuit are constituted by a pair of series connected impedance elements connected across said first and second bridge arms, said pair of impedance elements constituting the third and fourth arms of the bridge circuit having an impedance value so much greater than that of said impedance means and load device as not to constitute any significant current drain on the power amplifier, and said control means includes means responsive to the difference between the magnitudes of the voltages at the juncture between said first and second bridge arms, on the one hand, and said third and fourth bridge arms on the other hand.

Claim 6

The power amplifier of claim 4 wherein said control means responds to said given magnitude when it is of a reversed polarity from the bridge output at normal load circuit impedance levels.

Claim 7

The power amplifier of claim 5 wherein the value of the bridge-forming impedance element nearest said reference point is greater than that of the impedance element most remote therefrom but the ratio thereof is less than the ratio of the normal impedance value of said load device and said one impedance means so the voltage at the juncture of said bridge-forming impedance elements constituting one of the bridge output terminals is less than the voltage at the juncture of said emitter connected impedance means constituting the other bridge output terminal.

Claim 8

The power amplifier of claim 3 wherein the bridge circuit is formed by a pair of bridge-forming impedance elements coupled across a point of at least one of said pair of impedance means remote from said load device and said reference point, which bridge-forming impedance elements are of such a large value as not to place any significant drain on the current flowing through said pair of impedance means and load device.

Claim 9

The power amplifier of claim 1 wherein there is provided means responsive to the short circuiting of the load terminals of said current amplifying means for operating said load current terminating means.

Claim 10

In a power amplifier which is to amplify an input AC signal with a varying amplitude and waveshape with minimum signal distortion, said power amplifier including a pair of series connected transistor or transistorlike current amplifying devices with the adjacent load terminals thereof interconnected through a pair of series connected impedance means and with their remote load terminals connected to the opposite polarity terminals of a source of direct current energizing voltage whose output terminals present voltages of opposite polarity relative to a given reference point, said transistor or transistor-like current amplifying devices having control terminals connected to a source of AC signals to be amplified where one of the current amplifying devices is conductive only for the portion of the input AC signal having one polarity and the other current amplifying device is conductive only for that portion of the input AC signal having the opposite polarity and to a degree proportional to the instantaneous magnitude of the input AC signal, and a load device of much greater impedance than either of said impedance elements coupled between the juncture of said impedance elements and said reference point so the DC load and AC signal current components flow therethrough and so the voltage between the adjacent load terminals and said reference point increases and the voltage drop across the load device decreases with the decreasing impedance value of said load device, the improvement in a portection circuit for the current amplifying devices comprising control means responsive to voltages derived from the voltage adjacent the end of at least one of said impedance means remote from said load device and the voltage drop across the load device by resettably disconnecting said load device from the power amplifier circuit for at least a considerable period following such disconnection when the difference between said derived voltages reaches a predetermined value.

Claim 11

In a power amplifier which is to amplify an input AC signal with a varying amplitude and waveshape with minimum signal distortion, said power amplifier including a pair of series connected transistor or transistorlike current amplifying devices with the adjacent load terminals thereof interconnected through a pair of series connected impedance means and with their remote load terminals connected to the opposite polarity terminals of a source of direct current energizing voltage whose output terminals present voltages of opposite polarity relative to a given reference point, said transistor or transistor-like current amplifying devices having control terminals connected to a source of AC signals to be amplified where one of the current amplifying devices is conductive only for the portion of the input AC signal having one polarity and the other current amplifying device is conductive only for that portion of the input AC signal having the opposite polarity and to a degree proportional to the instantaneous magnitude of the input AC signal, and a load device of much greater impedance than either of said impedance elements coupled between the juncture of said impedance elements and said reference point so the DC load and AC signal current components flow therethrough and so the voltage between the adjacent load terminals and said reference point increases and the voltage drop across the load device decreases with the decreasing impedance value of said load device, the improvement in a portection circuit for the current amplifying devices comprising control means responsive to voltages derived from the voltage adjacent the end of at least one of said impedance means remote from said load device and the voltage drop across the load device by resettably disconnecting said load device from the power amplifier circuit for at least a considerable period following such disconnection when the difference between said derived voltages reaches a predetermined value.

Claim 12

The power amplifier of claim 9 wherein there is formed with one of said series connected impedance means and load device a bridge type circuit where first and second arms of the bridge circuit are constituted by at least one of said pair of impedance means elements and said load device and third and fourth arms of the bridge circuit are constituted by a pair of series connected impedance elements connected across said first and second bridge arms, said pair of impedance elements constituting the third and fourth arms of the bridge circuit being of an impedance value so much greater than that of said impedance means and load device as not to constitute any significant drain from the load circuit, and said control means comprises means responsive to the difference between magnitudes of the voltages at the juncture between said first and second bridge arms, on the one hand, and said third and fourth bridge arms, on the other hand.

Claim 13

The power amplifier of claim 10 wherein the value of the bridge-forming impedance element nearest said reference point is greater than that of the impedance element most remote therefrom and the ratio thereof is less than the ratio of the normal impedance value of said load device and said one impedance means so the voltage at the juncture of said bridge-forming impedance elements is less than the voltage at the juncture of said emitter connected impedance means.

Claim 14

The power amplifier of claim 10 wherein said control means includes a comparison circuit having a pair of input terminals, means for coupling the juncture between said first and second bridge circuit arms and said third and fourth bridge circuit arms respectively to the two input terminals of said comparison circuit, and clipping circuit means for coupling voltages of only one polarity between said juncture points and said input terminals of said comparison circuit, said comparison circuit comparing the difference of the amplitude of the voltage thereto and effecting a load device disconnect operation when the compared voltages reach a given degree of difference.

Claim 15

The power amplifier of claim 12 wherein said comparison circuit responds to said voltage difference when it is of a reverse polarity from the bridge circuit output at normal load device impedance levels.

Claim 16

A power amplifier which is to amplify an input AC signal with a varying amplitude and waveshape with minimum signal distortion, said power amplifier comprising: transistor or transistor-like current amplifying means with load terminal means connected through load circuit means to the terminals of a source of direct current voltage and control terminal means connected to signal drive means which varies the magnitude of drive current fed through the load terminal means, current limiting means connected to the control terminal means of the transistor or transistor-like current amplifying means which control the upper levels of the drive current fed through the control terminal means of said current amplifying means to minimize damage thereto, and an additional protection circuit including means responsive both to various combinations of progressively lower than normal impedances in the load circuit means and progressively increasing degrees of control drive current applied to said transistor or transistor-liKe current amplifying means by resettably terminating power amplifier current for at least a considerable period after such termination is initiated when such combined conditions approach transistor damaging conditions of operation.

Claim 17

A power amplifier which is to amplify an input AC signal with a varying amplitude and waveshape with minimum signal distortion, said power amplifier comprising: transistor or transistor-like current amplifying means with load terminal means connected through load circuit means to the terminals of a source of direct current voltage and control terminal means connected to signal drive means which varies the magnitude of drive current fed through the load terminal means, current limiting means connected to the control terminal means of the transistor or transistor-like current amplifying means which control the upper levels of the drive current fed through the control terminal means of said current amplifying means to minimize damage thereto, and an additional protection circuit including means responsive both to various combinations of progressively lower than normal impedances in the load circuit means and progressively increasing degrees of control drive current applied to said transistor or transistor-liKe current amplifying means by resettably terminating power amplifier current for at least a considerable period after such termination is initiated when such combined conditions approach transistor damaging conditions of operation.