Abstract:
The present invention breaks up the frequency bands which can be filtered by a simple low-loss band-pass or low pass filter. The second harmonic frequency is reduced by use of a non-linear clipper element which controls the driving waveform symmetry and can reduce the harmonics by as much as 5-15 db which makes the filter much simpler and allows the amplifier to remain wide-band. The output waveform from the amplifier is symmetrical or nearly symmetrical.

Description:
The application claims benefit of provisional application Ser. No. 61/824,047, filed May 16, 2013, for “WIDEBAND AMPLIFIER USING CLIPPER CIRCUITS FOR REDUCED HARMONICS,” the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Wide-band amplifiers can reduce system costs and provide improved performance in many applications. Envelope tracking techniques can provide linear performance of wide-band amplifiers by running them deep into saturation. The improvement in efficiency is substantial with, for example, a 10%-20% improvement with LTE signals. 
     The problem for wide-band, envelope tracking amplifiers is that driving transistors deep into saturation produces substantial harmonics that can interfere with many receiving systems. While filter circuits can be used to reduce such harmonics, this results in reduced bandwidth and requires many discrete surface mount devices (SMDS). The additional filter circuits add power loss to the system and reduce the power amplifier efficiencies. A low pass, or band pass, filter can be used at the output of the amplifier to roll off the harmonics to acceptable levels. The hardest harmonic to filter is the second harmonic since it is the closest in frequency to the fundamental frequency. Typical power amplifier specifications require the second harmonic content to be 30-40 dbc. 
     SUMMARY 
     The present invention breaks up the frequency bands that can be filtered by a simple low-loss band pass or low pass filter. The second harmonic frequency is reduced by use of a non-linear clipper element that controls the driving waveform and can reduce the harmonics by as much as 5-15 db which makes the filter much simpler and allows the amplifier to remain wide-band. The output waveform from the amplifier is symmetrical or nearly symmetrical. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and advantages of the present invention will be apparent to those of ordinary skill in the art in view of the following detailed description in which: 
         FIG. 1  is a schematic diagram of a basic common source amplifier; 
         FIGS. 2-4  depict certain simulated waveforms for the circuit of  FIG. 1 ; 
         FIG. 5  depicts time domain and frequency spectra of square and rectangular waveforms; 
         FIG. 6  is a plot of the second harmonic versus pulse width; 
         FIG. 7  is a schematic diagram of a first illustrative embodiment of the invention; 
         FIGS. 8-12  depict certain simulated waveforms for the circuit of  FIG. 7 ; 
         FIG. 13  is a schematic diagram of a second illustrative embodiment of the invention; and 
         FIG. 14  is a schematic diagram of an element of the circuit of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION 
     Conventional silicon bipolar, HBT, JFET, MESFET and PHEMT devices suffer from the inherent problem that they have a diode element in their input controlling element such as the base or gate terminal. An illustrative prior art PHEMT circuit  100  is shown in  FIG. 1 . Circuit  100  depicts a basic common source amplifier comprising a PHEMT transistor  110  having first, second, and third terminals  112 ,  114 ,  116 , a first DC blocking capacitor  120  at the input of the circuit, a second DC blocking capacitor  130  at the output of the circuit, and a load  140 . The PHEMT transistor has source and drain regions coupled to the first and second terminals  112  and  114  and a gate coupled to the third terminal  116   
     When the RF input voltage swing becomes large enough to forward bias the gate diode, the input voltage is clamped to about 0.7V and the excess voltage is stored across the first DC blocking capacitor  120 . This forces the gate voltage to swing very far negative, such that the “ON” to “OFF duty cycle is not 50%. 
       FIG. 2  is a simulation depicting the fundamental power and second harmonic power where the output power ranges from about 24 dBm to about 30 dBm. Deep into compression the second harmonic power is as much as 17 dbm.  FIG. 3  depicts the gate terminal voltage swinging from about −3.1V to +0.7V.  FIG. 4  depicts the output waveform swinging from about +0.3V to +17V. The output waveform depicted in  FIG. 4  has a narrow bottom and wider top resulting in a second harmonic product which is difficult to filter. 
     Waveforms that are symmetrical have no even order distortion as shown with the square wave in  FIG. 5 . 
       FIG. 6  shows the second harmonic level of the rectangular wave Vs duty cycle (d). As can be seen, if d can be kept to about 0.425 to 0.45, the second harmonic rejection can be ˜10 db. 
       FIG. 7  is a schematic diagram depicting a first illustrative embodiment of an amplifier circuit  700  of the invention. Circuit  700  comprises a power amplifier  710 , a negative clipping element  720  at the gate or base terminal of power amplifier  710  to control the duty cycle and even order harmonics at the power amplifier output, and a wide-band band pass filter (BPF) or a low pass filter (LPF)  740  to filter out the remaining harmonics. Circuit  700  also comprises a first DC blocking capacitor  750  at its input, a second DC blocking capacitor  760  at its output, and a load  770 . Amplifier  710  can be either a bipolar transistor or a field effect transistor, as shown, each having first, second and third terminals  712 ,  714 ,  716 . In the case of a bipolar transistor, the first and second terminals are coupled to an emitter and a collector of the transistor; and the third terminal is coupled to a base of the transistor. In the case of a field effect transistor, the first and second terminals are coupled to source and drain regions of the transistor and the third terminal to a gate. In either case, a signal applied to the third terminal controls current flow between the first and second terminals. 
     To make the system wide band, filter  740  is designed to have roughly the same impedance as the impedance of load  770  (within a ˜10 db return loss). 
     The negative clipping element  720  together with the output filter  740  provides a very wide-band amplifier of high output power with good harmonic rejection and low-loss. It is envisioned for instance, an amplifier using this invention could achieve adequate performance to cover EUTRAN bands (5, 6, 8, 12, 13, 14, 17, 18, 19, 20, 26) or EUTRAN bands (1, 2, 3, 9, 10, 25, 33, 35, 36, 37, 39) assuming each band is routed to an appropriate system filter to meet FCC (or similar) specifications. 
     Other applications of the wide-band amplifier of the present invention include its use in envelope tracking systems, in envelope elimination and restoration systems, and in polar modulation systems. 
     As shown in the plots of  FIG. 8 , computer simulations of the circuit of  FIG. 7  using two series PHEMT diodes (˜1.4V) show a second harmonic in deep saturation (&gt;30 dbm) that reduces the signal power from ˜13 dbm to 17 dbm to −5 dbm to −4 dbm.  FIG. 9  depicts the gate terminal voltage swinging from about −2.2 V to 1 V.  FIG. 10  depicts the output waveform swinging from about 0 V to 20 V. As can be seen in  FIG. 10 , the output waveform is symmetrical or nearly symmetrical 
     Further results of computer simulation are shown in  FIGS. 11 and 12 . S Parameters are of a full amplifier with 37 db gain that covers a bandwidth of 1710 MHz-1980 MHz as depicted in  FIG. 11 . The second harmonic distortion gives a rejection of &gt;35 dbc. 
       FIG. 13  depicts a second illustrative embodiment of a circuit  1300  of the invention that uses a voltage variable clipper element. Circuit  1300  comprises a power amplifier  1310 , a voltage variable clipping element  1320  at the gate or base terminal of power amplifier  1310  to control the duty cycle and even order harmonics at the power amplifier output, a voltage generator  1330 , and a wide-band band pass filter (BPF) or a low pass filter (LPF)  1340  to filter out the remaining harmonics. Circuit  1300  also comprises a first DC blocking capacitor  1350  at its input, a second DC blocking capacitor  1360  at its output, and a load  1370 . Again, amplifier  1310  can be either a bipolar transistor or a field effect transistor, as shown, each having first, second and third terminals  1312 ,  1314 ,  1316  coupled to an emitter, collector and base of the bipolar transistor or to the source, drain and gate of the field effect transistor. In either case, a signal applied to the third terminal controls current flow between the first and second terminals. 
     The operation of circuit  1300  is similar to that of circuit  700  but the output of clipping element  1320  and therefore the second harmonic level is responsive to a control voltage supplied by voltage generator  1330 . The output of the voltage generator can be controlled so that it is responsive to a number of factors such as temperature, output power, and/or VSWR under mismatch. Thus, the second order harmonic level can made to depend on such factors as temperature, output power, and VSWR under mismatch. The clipping voltage can be programmed; or the clipping voltage can be supplied by an adaptive feedback loop 
     The voltage variable clipper circuit can be made from any number of diodes, but in this case a GaAs Schottky diode is preferable.  FIG. 14  depicts a diode  1410  in series with a variable voltage source  1420  such as an OP AMP and a capacitor  1430  to GND to make a low impedance voltage source to the negative clipping diode. Diode  1410  can be implemented in a variety of different forms such as a base-emitter, base collector, silicon PN junctions, germanium PN junctions, or JFET diode The resulting clipping voltage will be the built-in voltage plus/minus the voltage source voltage. 
     As will be apparent to those skilled in the art, numerous variations may be practiced within the spirit and scope of the present invention.