Feed forward amplifier

• US 5,455,537 A
• Filed: 08/19/1994
• Issued: 10/03/1995
• Est. Priority Date: 08/19/1994
• Status: Expired due to Fees

• Current Assignee: Radio Frequency Systems Incorporated (Amphenol Corporation)
• Original Assignee: Radio Frequency Systems Incorporated (Amphenol Corporation)
• Inventors: Wills, David L., Larkin, Robert S.
• Primary Examiner(s): Mullins, James B.
• Application Number:
  US08/292,869
• Time in Patent Office: 410 Days
• Field of Search: 330/52, 330/149, 330/151
• US Class Current: 330/52
• CPC Class Codes: H03F 1/3235 using a pilot signal
• CPC Class Codes: H03F 2201/3212 Using a control circuit to adjust amplitude and phase of a signal in a signal path

Abstract

In a feed forward amplifier (100), a main amplifier (125) is coupled between an input (102) at which an input signal is received and an output (173) at which an output signal is provided. The output signal is an amplification of the input signal. A sample of the output signal is compared (144) with a sample of the input signal to produce a difference signal indicative of distortions introduced by the main amplifier (125). An error amplifier (165) provides a distortion signal which is an amplification of the difference signal, and the distortion signal is combined (169) with the output signal to compensate for main amplifier distortions. A random frequency, constant amplitude sine wave pilot signal is combined (117) with the input signal, and a correlation power detector (194,195,202,212,213) is provided to determine the magnitude of the pilot signal in the output signal. The gain and phase of the difference signal is then adjusted (155) to minimize the magnitude of the pilot signal in the output signal. Additionally, a distortion signal power signal, which is indicative of the magnitude of the difference signal, is provided (151,158), and the gain and phase of the input signal is adjusted (112) to minimize the magnitude of the distortion signal power signal.

Claims

1. A feed forward amplifier comprising:

a main amplifier coupled between an input at which an input signal is received and an output at which an output signal is provided, said output signal being an amplification of said input signal, and said main amplifier being responsive to said input signal for providing an amplified input signal;

sampling means responsive to said amplified input signal for providing a sample of said amplified input signal;

comparison means for comparing said sample of said amplified input signal with a sample of said input signal to produce a difference signal indicative of distortions introduced by said main amplifier;

an error amplifier responsive to said difference signal for providing a distortion signal which is an amplification of said difference signal;

combiner means for combining said distortion signal with said amplified input signal to provide said output signal, thereby compensating for said distortions;

means for combining a variable frequency, constant peak amplitude pilot signal in an input signal path between said input and said sampling means;

a correlation power detector having;

(a) coupler means responsive to a sample of said output signal for providing a pair of sample signals having equal amplitude and ninety (90) degrees phase difference;

(b) a pair of mixers, each mixer being responsive to a sample of said pilot signal and to one of said sample signals for providing a multiplication signal, each multiplication signal being the product of said sample of said pilot signal and one of said sample signals;

(c) a pair of averaging circuit means each responsive to one of said multiplication signals for providing an averaged signal, each averaged signal being respectively indicative of the average of a corresponding multiplication signal;

(d) means for providing a pilot signal power signal as the sum of the squares of said averaged signals, said pilot signal power signal being indicative of the magnitude of said pilot signal in said output signal; and

means responsive to said pilot signal power signal for adjusting the gain and phase of said difference signal to minimize the magnitude of said pilot signal power signal.

2. A feed forward amplifier according to claim 1 further comprising:

means responsive to a sample of said difference signal for providing a distortion signal power signal indicative of the magnitude of said difference signal; and

means responsive to said distortion signal power signal for adjusting the gain and phase of said input signal to minimize the magnitude of said distortion signal power signal.

3. A feed forward amplifier according to claim 2 wherein said means responsive to said pilot signal power signal and said means responsive to said distortion signal power signal are signal processing means, said signal processing means providing gain and phase signals to electronically adjustable attenuator and phase shifters for adjusting the gain and phase of said difference signal and said input signal, respectively.

4. A feed forward amplifier according to claim 1 wherein said pilot signal is combined in said input signal path and then removed from said input signal path to provide for the detection and removal of bias voltages in said correlation power detector.

5. A feed forward amplifier according to claim 1 wherein said mixers are multiplication functions.

6. A feed forward amplifier according to claim 1 wherein said averaging circuit means are low pass filters.

7. A feed forward amplifier according to claim 1 wherein said pilot signal has a random frequency.

8. A feed forward amplifier according to claim 1 wherein said pilot signal is a sine wave of random frequency.

9. A feed forward amplifier according to claim 8 wherein the frequency of said sine wave is determined in response to a random noise generator.

10. A method for removing distortions from an output of a feed forward amplifier comprising the steps of:

coupling a main amplifier between an input at which an input signal is received and the output at which an output signal is provided, said output signal being an amplification of said input signal, said main amplifier providing, in response to said input signal, an amplified input signal;

sampling said amplified input signal at a sampling location at an output of said main amplifier, and providing an amplified input signal indicative thereof;

comparing said sample of said amplified input signal with a sample of said input signal to produce a difference signal indicative of distortions introduced by said main amplifier;

amplifying said difference signal and providing a distortion signal indicative thereof;

combining said distortion signal with said amplified input signal to provide said output signal, thereby compensating for said distortions;

prior to said step of sampling said amplified input signal, combining a variable frequency, constant amplitude pilot signal in an input signal path between said input and said sampling location;

providing a correlation power detector responsive to a sample of said output signal and to a sample of said pilot signal for providing a pilot signal power signal, said correlation power detector performing the steps of;

(a) providing, in response to a sample of said output signal, a pair of sample signals having equal amplitude and ninety (90) degrees phase difference;

(b) providing a pair of multiplication signals, each multiplication signal being provided as the product of a sample of said pilot .signal and one of said sample signals, respectively;

(c) providing a pair of averaged signals, each respectively indicative of the average of a corresponding multiplication signal;

(d) providing said pilot signal power signal as the sum of the squares of said averaged signals; and

adjusting the gain and phase of said difference signal to minimize the magnitude of said pilot signal power signal.

11. A method according to claim 10 further comprising the steps of:

providing a distortion signal power signal indicative of the magnitude of said difference signal; and

adjusting the gain and phase of said input signal to minimize the magnitude of said distortion signal power signal.

12. A method according to claim 10 further comprising the steps of:

combining said pilot signal in said input signal path and then removing said pilot signal from said input signal path; and

detecting and removing bias voltages in said correlation power detector.

13. A method according to claim 10 further comprising the step of providing said pilot signal as a random frequency signal.

14. A method according to claim 10 further comprising the step of providing said pilot signal as a sine wave signal having a random frequency.