US3882407A

US3882407A - Amplifier blanking circuit

Info

Publication number: US3882407A
Application number: US154855A
Authority: US
Inventors: A Donald Arsem
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1950-04-08
Filing date: 1950-04-08
Publication date: 1975-05-06
Anticipated expiration: 1992-05-06

Abstract

1. In combination, an amplifier for amplifying applied signals comprising an amplifier tube having an anode, a cathode, and at least one control grid, an anode resistor through which an operating voltage is applied to said anode, means for biasing said tube so that it has good transconductance, whereby there is subbstantial anode current flow in the absence of applied signals, said tube having an input circuit through which signal may be applied to said tube with its polarity in the direction to increase the flow of anode current in said tube, said tube having a blanking circuit through which a blanking pulse may be applied to drive said tube to anode current cut-off, means for applying said blanking pulse to said blanking circuit, said anode being at a certain potential in the absence of applied input signal and applied blanking pulses, and an un-bypassed unidirectional conducting device that in direct-current connected between said anode and a point of substantially the same potential as said certain potential, said device having an anode that is connected to the anode of said amplifier tube to permit current flow from said anode to said point.

Description

United States Patent 119] Arsem [451 May 6,1975

[ AMPLIFIER BLANKING CIRCUIT [75] Inventor: A. Donald Arsem, Liverpool, NY.

[73] Assignee: RCA Corporation, New York, NY.

[22] Filed: Apr. 8, 1950 [21] Appl. N0.: 154,855

[52] US. Cl. 328/221; 328/99; 328/172; 330/11 [51] Int. Cl. H0lj 19/82 [58] Field 0fSearch...250/20.52, 13, 27 GT, 27 PSC, 250/27 TR; 178/7.l DC, 7.2 C, 7.2 DC, 7.2

[56] References Cited UNITED STATES PATENTS 2,166,995 7/1939 Koch 250/20.53

2,402,091 6/1946 Schade 178/7.2 C 2,406,760 9/1946 Goldmark 250/27 CCU X 2,412,542 12/1946 Smith 250/27 PSC 2,448,070 8/1946 Sunstein 1. 250/27 PSC 2,495,780 l/1950 Shapherd et a1. 250/27 PSC 2,508,672 5/1950 Guanella 179/15 A UX 2,535,912 12/1950 Frank et al. I. 250/27 GT 2,548,436 4/1951 Loughren l78/7.3 DC 2,572.080 10/1951 Wallace 250/27 PSC 2,619,548 11/1952 Lesti .1 179/15 A X FOREIGN PATENTS OR APPLICATIONS 580,527 9/1946 United Kingdom 250/27 PSC 624,413 6/1949 United Kingdom 179/15 A OTHER PUBLICATIONS Electronics, Clipping and Clamping Circuits, July, 1947, pp. 111-113. Photostat in 250-27 PSC.

Primary Examiner-Maynard R. Wilbur Assistant Examiner-Gregory E. Montone Attorney, Agent, or Firm-Edward J. Norton EXEMPLARY CLAIM 1. In combination, an amplifier for amplifying applied signals comprising an amplifier tube having an anode, a cathode, and at least one control grid, an anode resistor throughwhich an operating voltage is applied to said anode, means for biasing said tube so that it has good transconductance, whereby there is subbstantial anode current flow in the absence of applied signals, said tube having an input circuit through which signal may be applied to said tube with its polarity in the direction to increase the flow of anode current in said tube, said tube having a blanking circuit through which a blanking pulse may be applied to drive said tube to anode current cut-off, means for applying said blanking pulse to said blanking circuit, said anode being at a certain potential in the absence of applied input signal and applied blanking pulses, and an unbypassed unidirectional conducting device that in direct-current connected between said anode and a point of substantially the same potential as said certain potential, said device having an anode that is connected to the anode of said amplifier tube to permit current flow from said anode to said point.

5 Claims, 4 Drawing Figures PUT 376N124 w ur ATTORNEY AMPLIFIER BLANKING CIRCUIT My invention relates to blanking or blocking circuits for amplifiers and particularly to such circuits wherein the amplifier is to be periodically blocked as in pulse echo radar equipment, for example.

It is usually desirable in radar receivers to block the receiver in some way during the instant a pulse is transmitted to prevent passage of this pulse through the re ceiver. This blocking is often accomplished sufficiently by means of cyclic gain control. Howver, in some sys' terns it is desirable to apply a blanking or blocking pulse to one or more amplifier stages so that the stage cannot pass signal during the application of the blanking pulse.

An object of the present invention is to provide an improved blanking circuit wherein the blanked ampli fier has good gain and wherein no undesired pulse is produced by the blanking action.

A further object of the invention is to provide improved means for periodically blocking an amplifier.

According to one preferred embodiment of the invention a pentode is suitably biased to function as a good amplifier and the signal is applied to its control grid. A negative blanking pulse is applied to its suppressor grid to drive the tube to cutoff. This normally would result in a large positive pulse at the anode. To avoid this, a diode is connected to the anode in such a way that as soon as the anode of the amplifier tube starts to go more positive the diode conducts and prevents said anode from going increasingly positive any substantial amount.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a block diagram of a radar system illustrating one application of the present invention;

FIG. 2 is a circuit diagram illustrating one preferred embodiment of the invention;

FIG. 3 is a group of graphs illustrating the circuit operation; and

FIG. 4 is a circuit diagram showing a modification of the circuit shown in FIG. 2.

Referring to FIG. I, there is shown a pulse echo radar system comprising a pulse transmitter which is pulse modulated by periodic pulses ll supplied from a pulse generator 12.

The receiver is of the superheterodyne type comprising a converter 13, a local oscillator 14, an LP. amplifier 16, a detector 17 and a video frequency amplifier 18. The amplifier 18 comprises a first stage 18a.

The output pulses of amplifier 18 are supplied to a suitable indicator such as a cathode ray tube 19. The cathode ray of the tube 19 is deflected along a time axis in synchronism with the pulse transmission by means of a sawtooth generator 21 that is synchronized by pulses from the pulse generator 12.

In the example shown, the video amplifier stage 18a is blanked or blocked by the negative pulses 22 supplied from a blanking pulse generator 23. The generator 23 is synchronized by pulses from pulse generator 12 so that each of the pulses 22 occurs during each of the transmitted pulses 1 1. Thus the stage 18a is blocked during each pulse transmission and the initially transmitted signal cannot pass through the amplifier 18.

The circuit of the video stage 18a is shown in detail in FIG. 2. In this example the amplifier tube is a pen tode 26 having a suitable negative bias applied through a grid resistor 20 to its control grid to make it function as a good amplifier. In other words, the tube 26 is biased to insure a high transconductance. Under this condition there is a substantial flow of anode current. It should be noted in particular that the tube 26 is not operated close to a cut-off condition as is so commonly the practice for the purpose of minimizing undesired pulse output resulting from application of a blanking pulse.

Input signal is applied to the control grid of tube 26 through a lead 25 and a coupling capacitor 30.

The screen grid of tube 26 has normal operating voltage applied to it, plus volts in this example.

The negative blanking pulses 22 are applied to the suppressor grid of the tube 26 by way of a coupling capacitor 27, a resistor 28 being connected between the suppressor grid and ground.

The plus B voltage is applied through an anode resistor 29 to the anode of tube 26. As previously pointed out, the application of a negative blanking pulse 22 would normally cause the anode (point A) of tube 26 to go to the maximum plus B voltage, 300 volts in this example, to cause an undesired positive pulse to appear in the output.

This undesired pulse is avoided by means of a clamping circuit comprising a diode 31 having its anode con nected to the anode of tube 26 (i.e., to point A) and having its cathode connected to a point B that is at substantially the same potential as the normal potential of point A. By normal potential of point A is meant the potential when there is no input signal and no blanking pulse.

The point B is a point on a

potentiometer comprising resistors

32 and 33 connected between the plus B terminal and ground. A capacitor 34 may be connected across resistor 33 to provide lower impedance in the clamping circuit if desired.

The operation of the blanking circuit will now be apparent. Since the signal input to the control grid of tube 26 is always positive, it always lowers the potential at point A; it never raises it above the normal potential as previously defined. Thus the diode 31 is never conducting between blanking pulses and the clamping circuit is inactive. Since there is proper bias on the tube 26 it provides good amplification, there is a good signal-tonoise ratio, and there is no clipping or distortion of the signal at low signal levels.

When a blanking pulse 22 occurs, the tube 26 is driven to cut-off for the duration of the pulse and the point A tries to go to the plus B potential of 300 volts. But this cannot happen because as soon as point A begins to go more positive, the diode 31 conducts and ties the point A to the fixed potential point B. Therefore, since the diode has low impedance, the point A is held to substantially the same potential as the point B during the period the tube 26 is cut off.

It should be understood that the invention is of general application and is not limited to use in a radar system. Also it will be apparent that the invention is not limited to the specific circuit shown in FIG. 2. For instance the tube 26 need not be a pentode. The blanking pulses obviously may be applied to an electrode other than a suppressor grid for driving the amplifier tube to cut-off. The diode 31 may be replaced by any suitable rectifier.

FIG. 3 shows the grid voltage-anode current charac teristic of the tube 26 and illustrates the relation of con trol grid bias and input signal. In the present example the input signal consists of short pulses rather widely spaced in time so that the alternating current axis of the signal is close to the most negative value of the signal. This is apparent in FIG. 3 where the a-c axis of the input signal coincides with the line 36 representing the value of the bias on the control grid of tube 26. The a-c axis and the bias value in this case coincide since the input signal is applied through the capacitor 30, and, therefore, is an a-c signal on the control grid.

As will be evident from FIG. 3, the only substantial effect of the input signal on the control grid is to drive it more positive than the bias value. The amount the control grid is driven more negative between pulses is negligible. It may be noted that in the case where input signal is applied through a coupling capacitor, such as the capacitor 30, the input signal should not be allowed to drive the grid positive with respect to the cathode enough to charge the coupling capacitor to a voltage that is too great. Otherwise, an additional undesired bias will be applied to the grid of the tube as is well known in the art. A small amount of bias due to the grid being driven to grid current conduction for relatively short periods is not necessarily objectionable. It will be apparent that, if, instead of capacitor coupling, direct current or transformer coupling is used, there will be no bias difficulty due to grid current flow. I

In some applications of the invention the input signal to the tube 26 may be of such a nature that it is necessary or desirable to provide the control grid circuit with a clamping circuit to prevent the input signal from driving the control grid in the negative direction. This may be done as shown in FIG. 4 by connecting a diode 37 across the resistor with the cathode of the diode connected to the grid end of the resistor. If at any time the input signal tries to drive the control grid more negative than its bias potential, the diode 37 will conduct so as to hold the control grid at the bias potential.

What 1 claim is:

1. In combination, an amplifier for amplifying applied signals comprising an amplifier tube having an anode, a cathode, and at least one control grid, an anode resistor through which an operating voltage is applied to said anode, means for biasing said tube so that it has good transconductance whereby there is substantial anode current flow in the absence of applied signals, said tube having an input circuit through which signal may be applied to said tube with its polarity in the direction to increase the flow of anode current in said tube, said tube having a blanking circuit through which a blanking pulse may be applied to drive said tube to anode current cut'off, means for applying said blanking pulse to said blanking circuit, said anode being at a certain potential in the absence of applied input signal and applied blanking pulses, and an un-bypassed unidirectional conducting device that is direct-current connected between said anode and a point of substantially the same potential as said certain potential, said device having an anode that is connected to the anode of said amplifier tube to permit current flow from said anode to said point.

2. In combination, an amplifier for amplifying applied signals comprising an amplifier tube having an anode, a cathode, a first grid and a second grid, an anode resistor through which an operating voltage is applied to said anode, means for biasing said tube so that it has good transconductance whereby there is substantial anode current flow in the absence of applied signals, said tube having an input circuit through which positive polarity signal may be applied to said first grid, said tube having a blanking circuit through which a blanking pulse may be applied to said second grid to drive said tube to anode current cut-off, means for applying said blanking pulse to said blanking circuit, said anode being at a certain potential in the absence of applied input signal and applied blanking pulses, and an unbypassed unidirectional conducting device that is direct-current connected between said anode and a point of substantially the same potential as said certain potential, said device having an anode that is connected to the anode of said amplifier tube to permit current flow from said anode to said point.

3. In combination, an amplifier for amplifying applied signals comprising a pentode amplifier tube having an anode, a cathode, a control grid, a screen grid, and a suppressor grid, an anode resistor through which an operating voltage is applied to said anode, means for biasing said tube so that it has good transconductance whereby there is substantial anode current flow in the absence of applied signals, said tube having an input circuit through which positive polarity signal may be applied to said control grid, said tube having a blanking circuit through which a blanking pulse may be applied to said suppressor grid to drive said tube to anode current cut-off, means for applying said blanking pulse to said blanking circuit. said anode being at a certain potential in the absence of applied input signal and applied blanking pulses, and an un-bypassed unidirectional conducting device that is direct-current connected between said anode and a point of substantially the same potential as said certain potential, said device having an anode that is connected to the anode of said amplifier tube to permit current flow from said anode to said point.

4. An amplifier for amplifying applied signals comprising in combination, an amplifier tube having a cathode, an anode and at least one control grid, said tube being biased to have good transconductance whereby there is substantial anode current flow in the absence of applied signals, said tube having an input circuit and an output circuit, said output circuit including an anode resistor through which an operating voltage is applied to said anode, said tube to be operated with signal applied to said input circuit with positive polarity whereby said anode does not go substantially more positive as a result of application of input signal, means for applying to an electrode of said tube a blanking pulse of such polarity and amplitude as to drive said tube to anode current cut-off, and un-bypassed unidirectional conducting device having an anode and a cathode which has its anode direct-current connected to the anode of said amplifier tube and which has its cathode direct-current connected to a point having a potential that is substantially the same as the potential that said amplifier tube anode has when neither an input signal nor a blanking pulse is being applied to said amplifier tube.

5. An amplifier for amplifying applied signals comprising in combination, an amplifier tube having a cathode, an anode, a first grid and a second grid, said tube being biased to have good transconductance whereby there is substantial anode current flow in the absence an un-bypassed unidirectional conducting device having an anode and a cathode which has its anode directcurrent connected to the anode of said amplifier tube and which has its cathode direct-current connected to a point having a potential that is substantially the same as the potential that said amplifier tube anode has when neither an input signal nor a blanking pulse is being applied to said amplifier tube.

Claims

1. In combination, an amplifier for amplifying applied signals comprising an amplifier tube having an anode, a cathode, and at least one control grid, an anode resistor through which an operating voltage is applied to said anode, means for biasing said tube so that it has good transconductance whereby there is substantial anode current flow in the absence of applied signals, said tube having an input circuit through which signal may be applied to said tube with its polarity in the direction to increase the flow of anode current in said tube, said tube having a blanking circuit through which a blanking pulse may be applied to drive said tube to anode current cut-off, means for applying said blanking pulse to said blanking circuit, said anode being at a certain potential in the absence of applied inpuT signal and applied blanking pulses, and an un-bypassed unidirectional conducting device that is direct-current connected between said anode and a point of substantially the same potential as said certain potential, said device having an anode that is connected to the anode of said amplifier tube to permit current flow from said anode to said point.

2. In combination, an amplifier for amplifying applied signals comprising an amplifier tube having an anode, a cathode, a first grid and a second grid, an anode resistor through which an operating voltage is applied to said anode, means for biasing said tube so that it has good transconductance whereby there is substantial anode current flow in the absence of applied signals, said tube having an input circuit through which positive polarity signal may be applied to said first grid, said tube having a blanking circuit through which a blanking pulse may be applied to said second grid to drive said tube to anode current cut-off, means for applying said blanking pulse to said blanking circuit, said anode being at a certain potential in the absence of applied input signal and applied blanking pulses, and an un-bypassed unidirectional conducting device that is direct-current connected between said anode and a point of substantially the same potential as said certain potential, said device having an anode that is connected to the anode of said amplifier tube to permit current flow from said anode to said point.

3. In combination, an amplifier for amplifying applied signals comprising a pentode amplifier tube having an anode, a cathode, a control grid, a screen grid, and a suppressor grid, an anode resistor through which an operating voltage is applied to said anode, means for biasing said tube so that it has good transconductance whereby there is substantial anode current flow in the absence of applied signals, said tube having an input circuit through which positive polarity signal may be applied to said control grid, said tube having a blanking circuit through which a blanking pulse may be applied to said suppressor grid to drive said tube to anode current cut-off, means for applying said blanking pulse to said blanking circuit, said anode being at a certain potential in the absence of applied input signal and applied blanking pulses, and an un-bypassed unidirectional conducting device that is direct-current connected between said anode and a point of substantially the same potential as said certain potential, said device having an anode that is connected to the anode of said amplifier tube to permit current flow from said anode to said point.

5. An amplifier for amplifying applied signals comprising in combination, an amplifier tube having a cathode, an anode, a first grid and a second grid, said tube being biased to have good transconductance wherEby there is substantial anode current flow in the absence of applied signals, said tube having an input circuit an an output circuit, said output circuit including an anode resistor through which an operating voltage is applied to said anode, said tube to be operated with signal applied through said input circuit to said first grid with positive polarity whereby said anode does not go substantially more positive as a result of application of input signal, means for applying to said second grid a blanking pulse of negative polarity and of sufficient amplitude to drive said tube to anode current cut-off, and an un-bypassed unidirectional conducting device having an anode and a cathode which has its anode direct-current connected to the anode of said amplifier tube and which has its cathode direct-current connected to a point having a potential that is substantially the same as the potential that said amplifier tube anode has when neither an input signal nor a blanking pulse is being applied to said amplifier tube.