WIDE-BAND AMPLIFIERS USING CLIPPER CIRCUITS FOR REDUCED HARMONICS
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.
- 1. (canceled)
- 2. A power amplifier circuit comprising:
a transistor configured as an amplifier stage of a power amplifier; and a voltage variable clipping element which shapes a peak-to-peak amplitude of a signal applied to the transistor and reduces a second harmonic of the signal, the output of the voltage variable clipping element controlled by a control voltage supplied to the voltage variable clipping element.
- View Dependent Claims (3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17)
- 18. An envelope tracking system comprising:
a power amplifier including a transistor; and
- View Dependent Claims (19, 20, 21)
This application is a continuation application of Non-Provisional application Ser. No. 15/928,639, filed on Mar. 22, 2018, which is a continuation application of Non-Provisional application Ser. No. 15/413,146, filed Jan. 23, 2017, which is a continuation application of Non-Provisional application Ser. No. 14/276,400, filed May 13, 2014, which claims benefit of provisional application Ser. No. 61/824,047, filed May 16, 2013, the entire contents of which are incorporated herein by reference.
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.
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.
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:
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
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%.
Waveforms that are symmetrical have no even order distortion as shown with the square wave in
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
Further results of computer simulation are shown in
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.
As will be apparent to those skilled in the art, numerous variations may be practiced within the spirit and scope of the present invention.