KR20020000106A - Power amplifier having improved linearity using feedback capacitive compensation - Google Patents

Abstract

PURPOSE: An amplifier having an improved linearity is provided, which achieves a further cancellation of a feedback signal, and obtains a high frequency response. CONSTITUTION: A splitter provides the first and the second signal component out of phase from an input signal. The first and the second bipolar transistor(27,28) comprises a base connection connected to receive the first and the second signal component and an emitter connection connected to a common d.c connection and connected each other and collectors of the first and the second junction impedance respectively, and are operated in a push pull mode where the first and the second bipolar transistor amplify each of the first and the second signal component alternatively. Transformers(16,43) comprise the first and the second winding(19,20) to combine signals amplified from the first and the second load impedance and to provide a voltage between each collector of the transistors and the common connection. A bias unit provides a bias voltage to the base connection of the first and the second transistor. And the first and the second capacitor are connected between a collector of one transistor and a base of another transistor, and have a nonlinear capacitance vs. voltage response equal to a capacitance vs. voltage response of the collector base junction of the connected transistor.

Description

Amplifier with improved linearity {POWER AMPLIFIER HAVING IMPROVED LINEARITY USING FEEDBACK CAPACITIVE COMPENSATION}

The present invention relates to a portable telephone communication art. In particular, a power amplifier is provided that operates with improved linearity in class B mode.

The wireless cell phone industry relies on a powered battery, a low power transmitter to communicate with a remote base station that must produce high enough quality, and a modulated radio frequency signal to transmit to the remote base station. have. When operating in a CDMA format, a transmitter in a mobile phone may require a linear amplification of a modulated radio frequency signal so as not to distort the modulation components in the transmission signal.

It is quite difficult to amplify a modulated signal with high linearity so that it is not distorted. In addition, amplification must be made with high efficiency to conserve battery power. One known circuit to provide efficient amplification is a class B mode push pull amplifier. Class B amplification stages provide high efficiency but linearity degrades due to the effect of base collector capacitance in the transistor used in the amplifier. In order to reduce the effect of collector base capacitance on linearity, the prior art proposes the use of capacitors to connect the collector of one transistor to the base of another transistor. Because each of the transistors used in the amplifier operates out of phase with each other, the opposite phase signal is combined with the signal passing through the collector base capacitance at polarity which minimizes both signals. However, attempts to neutralize the effects of collector base capacitance have been unsatisfactory in that the collector base capacitance is nonlinear and varies with the function of the collector base voltage. Connecting a capacitor from the collector of one transistor to the base of another transistor requires an inductor or trace that adds paratisitics to the capacitor, which further limits its use at radio frequencies. Thus, the cancellation of the collector base capacitance current is incomplete, lowering the amplification linearity of the transistor used in the amplifier.

An amplifier circuit operating with improved linearity in class B push pull mode is provided by the present invention. The signal to be amplified, which can be modulated with telephone signal traffic, is received at a signal splitter that makes the first and second signal components out of phase. The first and second bipolar transistors are biased to operate in push-pull class B mode when the first and second signal components that are out of phase are received at their base connection. The transistor has an emitter connection and a common d.c connection connected to each other. The collector of the transistor is connected to the first and second matched impedances. Transformers with first and second windings are provided for coupling signals from the first and second matched impedances. The first and second capacitances with a nonlinear capacitance versus voltage function are collectors from one collector of the transistor to the base of the other transistor, which are substantially the same capacitor versus voltage function as the transistor collector base junction. Thus, the compensation current can be generated through a nonlinear capacitor that is 180 degrees out of phase with the feedback signal passing through the collector base junction capacitance, thereby achieving more complete cancellation of the feedback signal. This is accomplished by making a junction fabricated on the same IC, thus eliminating the need for wiring or long traces and achieving a high frequency response.

1 is a schematic diagram of a first embodiment of the present invention;

Explanation of symbols for the main parts of the drawings

13,15,44,47: choke inductor 44, 47: bypass capacitor

19,20: winding 16: first transformer

43: second transformer 27, 28: amplifying transistor

29, 30: transistor 32, 33: DC isolation capacitor

24, 25: inductor

1 is a schematic diagram of a class B mode amplifier according to one embodiment used to amplify a modulated radio frequency signal. The circuit of FIG. 1 can be used in cellular phone applications to provide high efficiency and low distortion amplification stages. Both amplifying transistors 27 and 28 operate in a push-pull, class B operating mode. Each provides their collector with an amplified signal that is out of phase with each other. Class B configuration is provided to remove even order harmonics from the signal generated during amplification.

Each transistor 27, 28 is also provided with an out of input signal out of phase. The input signal at terminal 12 is coupled to one side of winding 19 of transformer 16 past capacitor 14. Transformer 16 produces two signal components of equal magnitude but different phase in windings 19 and 20.

The out-of-phase signal component is provided to the base connection of the transistor past the first and second matching networks including inductor 24, capacitor 22, inductor 25 and capacitor 23. The out-of-phase signals provided at the bases of the transistors 27 and 28 likewise create an amplified signal level at the collectors of the transistors 27 and 28, which are out of phase with each other. The second matching network is arranged from the transistors 27 and 28 to the second transformer. (43). The out-of-phase signals produced by the transistors 27 and 28 are recombined in the transformer 43 so that a whole amplitude signal is produced at the output 50.

The structure of the class B amplifier described above produces a feedback current through the collector base junction of each of the transistors 27 and 28. The capacitance exhibited by each of these junctions is nonlinear with respect to the voltage. Thus, as the collector voltage decreases in each of transistors 27 and 28, the amount of capacitance seen across collector emitter junctions 27 and 28 increases.

Coupling the radio frequency signal from the collector of each of the transistors 27 and 28 to the base of each of the transistors reduces the signal amplification linearity provided by the transistors 27 and 28. The loss of linearity adversely affects the modulation included in the signal to be amplified and creates distortion components in the output radio frequency signal.

The current capacitively coupled past the collector base junction of each of the transistors 27, 28 is offset or neutralized by providing a current of opposite phase to the base of each of the transistors 27, 28 through similar capacitance. Can be. This neutralization is achieved in the embodiment shown in FIG. 1 by connecting the collector of one of the transistors 27, 28 to the base of the other transistor via capacitance. According to this embodiment, transistors 29 and 30 are connected so that the collector base junction of each transistor provides a neutralizing current to the base of the amplifying transistors 27 and 28, the neutralizing current being the collector of the transistors 27 and 28. The current and phase are capacitively coupled from each other to the base of transistors 27 and 28.

Transistors 29 and 30 provide neutralizing current through the collector base junction, each having a phase opposite to that of the feedback current. A pair of DC isolation capacitors 32 and 33 connect the collectors of the transistors 29 and 30 to the collectors of the transistors 27 and 28, respectively. The collector base junction of transistors 29 and 30 has a capacitance versus voltage function, which is substantially the same as the collector base capacitive voltage function of transistors 27 and 28. Transistor devices 29 and 30 are selected to have substantially the same area, thus becoming similar to the capacitance versus voltage characteristic curves of transistors 27 and 28. Since transistors 27 to 30 are fabricated on the same IC using the same process, long trace wire connections are not needed, and thus are provided by collector base junctions of transistors 29 and 30. The nonlinearity of the capacitance to be matched improves the neutralization of the collector base capacitance of the transistors 27 and 28 by matching the collector base capacitance of the transistor. Thus, the cancellation of the effect of the feedback current from the selector base junctions of the transistors 27 and 28 is substantially more than that which is eliminated when a fixed capacitor is used.

The operating voltage for the circuit of FIG. 1 is provided through a pair of choke inductors 45, 46 with bypass capacitors 44, 47. Choke inductors 45 and 46 and bypass capacitors 44 and 47 help to isolate the supply of the supply voltage V _cc from the output radio frequency signal. The emitters of transistors 27 and 28 are both coupled to a common terminal of the voltage supply. The bias voltage is provided to each of the amplifying transistors 27 and 28 from the source of the bias voltage V _dd . A bias network comprising choke inductors 13, 15 and capacitors 17, 18 supplies a bias voltage to the bases of transistors 27, 28 through transformer windings 19, 20 and matching inductors 24, 25. do. Bias network inductance and capacitance isolate the input radio frequency signal from the bias voltage supply (V _dd ).

The present invention has been shown and described in the full text of the present invention. In addition, although the disclosure of the present invention has shown and described only preferred embodiments of the present invention, it is to be understood that the present invention can be used in various other combinations, modifications, and environments, as described above, and that it is within the spirit of the invention described herein. It should be understood that variations and modifications that are the same as the teachings and / or techniques or knowledge in the relevant art are possible. The presently described embodiment also describes the best mode disclosed for practicing the present invention, and those skilled in the art can use the present invention in various modifications by special application or use of the present invention in this or other embodiments. It was intended to be. Accordingly, the invention is not limited to only the forms disclosed herein. Also, the appended claims are intended to be construed, including other embodiments.

It is quite difficult to amplify a modulated signal with high linearity so that it is not distorted. The amplification must also be of high efficiency to conserve battery power. The disclosed class B amplifiers provide high efficiency but linearity reduces the effect of base collector capacitance in the transistor used in the amplifier. To solve this problem, the prior art proposes the use of a capacitor to connect a collector of one transistor to another base. However, it was not satisfactory in that the collector base capacitance was nonlinear and varied with the function of the collector base voltage. Also, connecting a capacitor from the collector of one transistor to the base of another transistor requires an inductor or trace that adds paratisitics to the capacitor, further limiting its use at radio frequencies. Therefore, the cancellation of the collector base capacitance current is incomplete, which causes a problem of lowering the amplification linearity of the transistor used in the amplifier.

The present invention provides an amplifier circuit that operates with improved linearity in class B push pull mode. Compensation current is produced through a nonlinear capacitor that is 180 degrees out of phase with the feedback signal across the collector base junction capacitance, thereby achieving more complete cancellation of the feedback signal. This is also achieved by making junctions fabricated on the same IC, thus eliminating the need for wiring or long traces and achieving a high frequency response.

Claims (10)

A splitter for providing first and second signal components out of phase from the input signal; A base connection connected to receive the first and second signal components out of phase, an emitter connection connected to each other and a common dc connection, and a collector of first and second matched impedances, respectively; First and second bipolar transistors operating in push-pull mode in which the first and second bipolar transistors alternately amplify each of the first and second signal components that are out of phase; A transformer having first and second windings for combining amplified signals from the first and second load impedances and for providing a voltage between the collector of each transistor and the common connection; Bias means for providing a bias voltage to the first and second transistor base connections; First and second capacitors, each connected between a collector of one transistor and a base of the other of the transistors, and having a nonlinear capacitance-to-voltage response substantially equal to the capacitance-to-voltage response of the collector base junction of the connected transistor; Containing Amplification circuit with improved linearity. The method of claim 1, The capacitor having a collector base connection connected from the collector of one of the transistors to the base of the other transistor. The method of claim 1, And first and second impedance matching circuits for matching the input impedance of the base of the transistor to the signal splitter. The method of claim 1, The bias means comprises an amplifier comprising first and second radio frequency chokes connecting a bias voltage source to the base connection of the transistor The method of claim 1, The power splitter is a transformer having first and second windings for providing signal components out of phase to a source of radio frequency signal components. The method of claim 2, And the collector of the collector base junction is capacitively coupled to the collector of the transistor. An amplifier that operates with improved linearity in class B mode, A signal splitter for providing first and second signals out of phase from the input signal; First and second bipolar transistors having collector emitter connections connected in series to alternately amplify the first and second signals out of phase; Signal coupling means for coupling a signal made from the first and second transistors, Nonlinear capacitor means connected between a collector of each of said bipolar transistors and a base of another of said transistors, said nonlinear capacitor means providing a capacitance versus voltage function substantially equal to said collector base capacitance of each of said transistors, An amplifier in which the nonlinear effect of the collector base junction is compensated by means of the nonlinear capacitor. The method of claim 7, wherein And said non-linear capacitor means is a transistor having a collector base impedance substantially equal to said collector base impedance of said transistor. The method of claim 8, The capacitor means having the same area as the bipolar transistor. The method of claim 7, wherein And said signal coupling means is a transformer.