CIRCUIT TO PROTECT RF OUTPUT AMPLIFIER AGAINST MISMATCH DAMAGE

• US 3,852,669 A
• Filed: 06/26/1973
• Issued: 12/03/1974
• Est. Priority Date: 06/26/1973
• Status: Expired due to Term

• Current Assignee: Dennis W. Bowman, Robert E. Horn
• Original Assignee: Dennis W. Bowman, Robert E. Horn
• Inventors: Horn, Robert E., Bowman, Dennis W.
• Primary Examiner(s): Safourek, Benedict V.
• Assistant Examiner(s): Ng, Jin F.
• Application Number:
  US05/373,830
• Time in Patent Office: 525 Days
• Field of Search: 325/150,151,159,186,187 317/20,48 324/58B 330/27P
• US Class Current: 455/117
• CPC Class Codes: G01R 27/02 Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant by measuring phase angle only G01R25/00
• CPC Class Codes: H03F 1/52 Circuit arrangements for protecting such amplifiers monitoring arrangements G01R31/28; increasing reliability in communication systems, e.g. using redundancy H04B1/74
• CPC Class Codes: H03F 1/56 Modifications of input or output impedances, not otherwise provided for

Abstract

A mismatch protection circuit to protect power output transistors of a communications transmitter from damaging excessive reflected power, as might occur when the antenna is disconnected or broken, which circuit continuously monitors and compares parameters indicative of forward and reflected power or VSWR and when there is excessive mismatch, instantaneously cuts back the dc power delivered to the output transistors to a safe low level, but a level sufficient for continued cutback operation of the protection circuit, when and for so long as the excessive mismatch continues.

Claims

1. In combination with an output rf power amplifier and a dc power source for the rf power amplifier, a load for the rf power amplifier and rf power delivering means coupling the rf power amplifier to the load, a resistor and a normally off electronic switch connected in series across the dc power source, regulator means including a zener diode connected across said electronic switch and short-circuited by said electronic switch when the latter is turned on, for delivering power from the dc power source to the rf power amplifier at one level when the electronic switch is off and at a lower level when the electronic switch is on, means for sampling selected parameters in the rf power delivering means that are representative of magnitude of forward power and reflected power, respectively, power cutback means including a pair of essentially equal gain emitter followers, one of said emitter followers having a potentiometer output, means for coupling the selected parameter representative of forward power to the input of the emitter follower with the potentiometer output and means for coupling the selected parameter representative of reflected power to the input of the other emitter follower, means coupled to the outputs of both emitter followers to turn on said electronic switch only when and for so long as the selected parameters representative of reflected power and forward power, respectively, exceed a predetermined ratio for instantaneously reducing the dc power to said rf power amplifier to substantially reduce the level of forward rf power while continuing operation of said power cutback means.

2. In combination with an output rf power amplifier and a dc power source for the rf power amplifier, a load for the rf power amplifier, and rf power delivering means coupling the rf power amplifier to the load, a resistor and a normally nonconductive electronic switch connected in series across the dc power source, regulator means connected to said dc power source and said rf power amplifier and including zener diode connected across said electronic switch and short-circuited by said electronic switch when the latter is conductive for delivering power from the dc power source to the rf power amplifier at one level when the electronic switch is OFF and at a lower level when the electronic switch is ON, means for sampling selected parameters in the rf power delivering means that are representative of magnitude of forward power and reflected power, respectively, power cutback means coupled to said sampling means and also coupled to said electronic switch and including a pair of emitter followers to amplify the samplings equally, one of said emitter followers having a potentiometer output, means for coupling the selected parameter representative of forward power to the input of the emitter follower with the potentiometer output and means for coupling the selected parameter representative of reflected power to the input of the other emitter follower, a differential amplifier, a pair of diodes coupling the output of the emitter follower with the potentiometer output to the respective inputs of the differential amplifier, another diOde coupling the output of the other emitter follower to one of inputs of the differential amplifier whereby the two diodes that are coupled to one input function as an OR gate, a transistor whose base electrode is connected to that output of the differential amplifier corresponding to the OR gate input and one of its emitter and collector electrodes is connected to the other output of the differential amplifier and the other of its emitter and collector electrodes is connected to said electronic switch, whereby said power cutback means amplifies the samplings equally and provides a selected fraction of the amplified forward power parameter and turns on said electronic switch only when, and for as long as the amplified reflected power parameter exceeds the selected fraction of the amplified forward power parameter, for instantaneously reducing the dc power to said power amplifier to substantially reduce the level of forward rf power while continuing operation of said power cutback means.

3. The combination defined in claim 2 wherein said electronic switch is a transistor, one of the emitter and collector electrodes of the second-mentioned transistor is connected to said resistor and the other of the emitter and collector electrodes is connected to the power source, and a capacitor connected between the base electrode and the other electrode, and the base electrode is connected to the first mentioned transistor, whereby said electronic switch is close circuited when a positive potential is applied to its base electrode by the first-mentioned transistor.

4. The combination defined in claim 3 wherein said means for sampling selected parameters in the rf power delivering means is a reflectometer.

5. The combination defined in claim 3 wherein said means for sampling selected parameters in the rf power delivering means is a directional coupler having a pair of output ports that provide equal samplings of the forward voltage and the reflected voltage respectively.

6. The combination defined in claim 5 further comprising rf detector means for coupling the forward voltage sampling output of the directional coupler to the input of said emitter follower having the potentiometer output and rf detector means coupling the reflected voltage sampling output of the directional coupler to the input of the other emitter follower.