# Patent

## Title
Method and apparatus for detecting an output power of a radio frequency transmitter using a multiplier circuit

## Abstract

A power detection circuit configured to detect an output power of a radio frequency transmitter. The power detection circuit includes a multiplier circuit configured to multiply a first differential input signal and a second differential input signal. The first differential input signal corresponds to a radio frequency signal to be amplified by the radio frequency transmitter. The second differential signal corresponds to an output signal as amplified by an amplifier of the radio frequency transmitter. A bias circuit is configured to generate a bias signal. A differential amplifier is configured to generate, based on the bias signal and the first differential signal and the second differential signal as multiplied by the multiplier circuit, an indication of the output power of the amplifier of the radio frequency transmitter.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present disclosure is a continuation of U.S. patent application Ser. No. 13/713,328 (now U.S. Pat. No. 8,994,362), filed on Dec. 13, 2012, which claims the benefit of U.S. Provisional Application No. 61/576,306, filed on Dec. 15, 2011 and U.S. Provisional Application No. 61/720,844, filed on Oct. 31, 2012. The entire disclosures of the applications referenced above are incorporated herein by reference.


FIELD
The present disclosure relates to radio frequency (RF) transmitters, and more particularly to RF power detection circuit for RF transmitters.
BACKGROUND
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Some radio frequency (RF) transmitters require accurate control of transmitted output power. For example, many RF transmitters need to comply with FCC regulations and wireless standards. Control of output power can be accomplished using an open loop or closed loop control system. In open loop control systems, the RF transmitter relies on accurate gain steps within the transmitter. In closed loop control systems, output power is measured and gain is adjusted accordingly.
An RF power detection circuit is an integral part of any RF transmitter closed-loop power-control system. The RF power detection circuit measures absolute transmitted power. This measurement is preferably independent of variation in temperature, device characteristics due to process spread, and load/antenna impedance.
Some RF power detection circuits assume a resistance value of an output load such as an antenna, measure output voltage and calculate output power based the output voltage squared divided by the resistance value. However, the resistance value of the load such as the antenna may vary during operation. For example, the resistance value of the antenna may be affected when the antenna is near or comes in contact with other objects. As can be appreciated, the RF power calculation will be adversely affected due to the difference between the actual resistance value of the antenna and the assumed resistance value.
SUMMARY
A circuit includes a multiplier circuit including a mixer configured to multiply a first differential input signal and a second differential input signal. The mixer includes a plurality of transistors including control terminals. The control terminals of the plurality of transistors receive a bias signal and the first differential input signal. A bias circuit is configured to generate the bias signal. The bias signal generated by the bias circuit is based on a voltage threshold of one of the plurality of transistors and a product of constant reference current and a bias resistance.
In other features, the mixer includes a Gilbert cell mixer. The bias circuit is configured to generate the bias signal such that a conversion gain of the mixer is substantially constant regardless of variations in process and temperature. The bias circuit includes a current source configured to generate the constant reference current, a bias resistance having the bias resistance and including one end in communication with the first current source, and a first transistor including a first terminal and a control terminal in communication with one end of the bias resistance. The bias signal is generated at a node between the bias resistance and the current source.
A method of operating a circuit includes, using a mixer, multiplying a first differential input signal and a second differential input signal, wherein the mixer comprises a plurality of transistors including control terminals. The control terminals of the plurality of transistors receive a bias signal and the first differential input signal. The method further includes generating the bias signal based on a voltage threshold of one of the plurality of transistors and a product of constant reference current and a bias resistance.
In other features, the mixer includes a Gilbert cell mixer. Generating the bias signal includes generating the bias signal such that a conversion gain of the mixer is substantially constant regardless of variations in process and temperature. The bias signal is generated at a node between a bias resistance and a current source.
Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a functional block diagram and electrical schematic of an example of an RF power detection circuit according to the prior art;
FIG. 2 is a functional block diagram and electrical schematic of an example of a multiplier circuit;
FIG. 3 is a functional block diagram and electrical schematic of an example of a bias circuit according to the present disclosure;
FIG. 4 is a functional block diagram and electrical schematic of an example of a multiplier circuit including a bias circuit according to the present disclosure;
FIG. 5 is a functional block diagram and electrical schematic of another example of a multiplier circuit including a bias circuit according to the present disclosure; and
FIG. 6 is a functional block diagram and electrical schematic of another example of a multiplier circuit including a bias circuit according to the present disclosure.



DESCRIPTION
Referring now to FIG. 1, part of an output circuit 10 of a prior art transmitter is shown. The output circuit 10 includes a power amplifier (PA) 20 that receives a radio frequency (RF) signal to be amplified and transmitted. The PA 20 outputs an amplified RF signal to a primary side of a transformer 24. One end of a secondary side of the transformer 24 is connected to an antenna 26, which may be arranged on a printed circuit board (PCB). Another end of the secondary side of the transformer 24 is connected to a reference potential such as ground. In this example, the antenna is the load, which has a load impedance.
The output circuit 10 also includes an RF detection circuit 32 that detects an output power level of the PA 20. The RF detection circuit 32 includes an amplifier 40 that receives and amplifies inputs to the PA 20 and outputs an amplified signal to first inputs of a multiplier circuit 42. A voltage divider 44 is connected to outputs of the PA 20 (or to nodes 45A and 45B on the secondary side of the transformer 24) and outputs signals to second inputs of the multiplier circuit 42. Outputs of the multiplier circuit 42 are connected to inputs of an amplifier 46, which has first and second feedback resistances RFB connected to respective inputs and outputs of the amplifier 46. The amplifier 46 outputs a power detect voltage signal VPD, which is based on detected output power.
The transmitted RF power is measured by multiplying the output voltage and current of the PA 20. The result is independent of load/antenna impedance (R) or voltage standing wave ratio (VSWR). The output voltage of the PA 20 is sensed through the voltage divider 44 (kv*VPA). The output current of the PA 20 is replicated by using a scaled down replica PA (kI*IPA).
In FIG. 2, an example of the prior art multiplier circuit 42 is shown. The multiplier circuit 42 includes a mixer 50, such as a Gilbert cell mixer, including transistors M1, M2, M3, and M4. First terminals of transistors M1 and M2 receive current Ip. First terminals of transistors M3 and M4 receive current In. Control terminals of transistors M2 and M3 receive a first bias signal VB and the sensed output voltage VPA (or VB−1⁄2 VPA). Control terminals of transistors M1 and M4 receive the bias signal VB and the sensed output voltage VPA (or VB+1⁄2 VPA). A second terminal of transistor M3 is connected to a second terminal of transistor M1. A second terminal of transistor M2 is connected to a second terminal of transistor M4.
The multiplier circuit 42 has a conversion gain Gc. The mixer 50 performs V*I multiplication. Transistors M1 thru M4 are biased in the linear region. Current Ip divides into two parts, Ip1 and Ip2. The ratio depends on the admittances of transistors M1 and M2 (gds1 and gds2). Similarly, current In is also divided into two parts, In1 and In2, depending on gds3 and gds4. While a virtual GND termination is assumed for ease of derivation, it is not necessary.



  
    
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From FIG. 1, the output (voltage) of the power detection circuit is equal to:

VPD=kV·kI·Gc·(VPA·IPA)·RFB 

From FIG. 2, the multiplier conversion gain Gc is :





  
    G
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Therefore the output of the power detection circuit is equal to:





  
    V
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The value of the on-chip resistance RFB depends on temperature and process variation (manufacturing). MOS threshold voltage VT also depends on temperature and process variation (manufacturing). kv and kI (PA voltage and current division ratio) can be accomplished using a ratioed Gilbert cell, which is independent of temperature, process and load impedance.
According to the present disclosure, (VB-VT) is set equal to Iref* Rbias. Resistors RFB and Rbias can be implemented as scaled versions of each other, e.g. RFB=A*Rbias. The ratio of resistances A remains constant and independent of process and temperature variation, therefore the output of the power detector is:




  
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The constant reference current Iref does not depend on process or temperature. The constant reference current Iref is usually already available on-chip. The constant reference current Iref can be generated by using a combination of a bandgap voltage and an external high-precision resistance.
Referring now to FIG. 3, a bias circuit 100 for generating a bias voltage VB=VT+Iref*Rbias is shown. The bias circuit 100 includes a current source Iref that is connected to one end of a bias resistance Rbias. Another end of the resistance Rbias is connected to a first terminal and a control terminal of a transistor M5. A second terminal of the transistor M5 is connected to a reference potential such as ground. Assuming:

Vgs5=VT+VI;

If Vdsat5<<VT;

Then Vgs5≈VT

This can be done by biasing the transistor M5 with a very low current density. The transistor M5 is preferably a scaled version of transistors M1-M4 for best matching.
Referring now to FIGS. 4 and 5, an example of the multiplier circuit 200 according to the present disclosure is shown. In FIG. 4, the multiplier circuit 200 includes a mixer 206, such as a Gilbert cell, with transistors M1, M2, M3, and M4. The sampled voltage VPA is connected to first terminals of capacitances C1 and C2. Second terminals of the capacitances C1 and C2 are connected to control terminals of transistors M1, M2, M3, and M4 and to first terminals of resistances R1 and R2. Second terminals of the resistances R1 and R2 provide a bias voltage VB to the bias circuit 100. First terminals of first and second transistors M1 and M2 and third and fourth transistors M3 and M4 are connected to IPA. A second terminal of transistor M3 is connected to a second terminal of transistor M1. A second terminal of transistor M2 is connected to a second terminal of transistor M4.
An amplifier 220 has a non-inverting input connected to the second terminals of the transistors M1 and M3 and to one end of a first feedback resistance RFB. The amplifier 220 has an inverting input connected to the second terminals of the transistors M2 and M4 and to one end of a second feedback resistance RFB. An inverting output of the amplifier 220 is connected to another end of the first feedback resistance RFB and to a first inverting input of an amplifier 230. A non-inverting output of the amplifier 220 is connected to another end of the second feedback resistance RFB and to a second inverting input of the amplifier 230. In FIG. 5, a common mode input of the amplifier 230 is connected to a second terminal of the transistor M5 and one end of a common mode feedback resistance RCMFB.
Transistors M1-M4 are biased with a constant voltage (Vgs−VT). The circuit accommodates a non-zero common-mode input voltage level. Iref*RCMFB sets the common-mode voltage reference. A common-mode feedback amplifier sets V+=V−=VCMREF. Therefore, transistors M1-M4 are still biased with (Vgs−VT)=Iref*Rbias.
While the preceding discussion involved a power detector using a passive mixer, the present disclosure can also use an active mixer as well. The active mixer transistors may be biased with a constant overdrive voltage=Iref*R. As can be appreciated, while the foregoing description relates to RF detection circuits, the multiplier circuit can be used in other systems. Additionally, the input does not have to correspond to voltage and current delivered to a load.
PA load impedance is unknown and can vary with the environment ZL=|Z|·e−jφ. Knowing the value of load impedance is useful because PA output matching can be optimized to allow the PA to operate most efficiently. PA load impedance can be measured if we have the following two measurements:

Po=VPA*IPA 

Vsq=VPA*VPA=VPA*IPA*|Z|*e−jφ

|Z| and φ can be solved using these two measurements. The voltage Vsq can be generated in multiple ways, one of which is shown in FIG. 6.
Referring now to FIG. 6, the voltage VPA is input to a transconductance amplifier 260, which receives VPA. The transconductance amplifier 260 transforms a voltage input to a current output. The transconductance amplifier 260 generates an output current GmVPA, which is input to the first terminals of the transistors M1 and M2 and transistors M3 and M4 instead of IPA as in FIGS. 4 and 5.
The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure.

## Claims

1. A power detection circuit configured to detect an output power of a radio frequency transmitter, the power detection circuit comprising:
a multiplier circuit configured to multiply a first differential input signal and a second differential input signal, wherein (i) the first differential input signal corresponds to a radio frequency signal to be amplified by the radio frequency transmitter, and (ii) the second differential signal corresponds to an output signal as amplified by an amplifier of the radio frequency transmitter;
a bias circuit configured to generate a bias signal; and
a differential amplifier configured to generate, based on (i) the bias signal and (ii) the first differential signal and the second differential signal as multiplied by the multiplier circuit, an indication of the output power of the amplifier of the radio frequency transmitter.

2. The power detection circuit of claim 1, wherein:
the multiplier circuit includes a mixer, wherein the mixer includes a plurality of transistors, and wherein each of the plurality of transistors includes a control terminal in communication with (i) the bias signal and (ii) the first differential signal.

3. The power detection circuit of claim 2, wherein:
to generate the bias signal, the bias circuit is configured to generate the bias signal further based on a voltage threshold of one of the plurality of transistors.

4. The power detection circuit of claim 3, wherein:
the bias circuit includes a bias resistance; and
to generate the bias signal, the bias circuit is configured to generate the bias signal further based on a product of (i) a constant reference current and (ii) the bias resistance.

5. The power detection circuit of claim 1, wherein:
the bias circuit comprises
a current source configured to generate a constant reference current,
a bias resistance having a bias resistance and including one end in communication with the current source, and
a first transistor including a first terminal and a control terminal each in communication with one end of the bias resistance; and

to generate the bias signal, the bias circuit is configured to generate the bias signal at a node between the bias resistance and the current source.

6. The power detection circuit of claim 1, wherein:
the bias circuit is connected to the multiplier circuit via a first resistance and a second resistance; and
the first resistance and the second resistance provide a bias voltage to the bias circuit.

7. The power detection circuit of claim 1, further comprising:
a common mode feedback amplifier connected to an output of the differential amplifier, wherein the common mode feedback amplifier includes a common mode input connected to the bias circuit.

8. The power detection circuit of claim 1, wherein, to generate the bias signal, the bias circuit is configured to generate the bias signal such that a conversion gain of the multiplier circuit is substantially constant regardless of variations in process and temperature.

9. The power detection circuit of claim 1, wherein:
the multiplier circuit includes a plurality of matched transistors; and
the bias circuit includes a first transistor that is scaled with respect to the plurality of matched transistors.

10. The power detection circuit of claim 9, wherein:
the plurality of transistors includes a first transistor, a second transistor, a third transistor and a fourth transistor;
the bias signal and a first polarity of the first differential input signal are input to respective control terminals of the first transistor and the fourth transistor; and
the bias signal and a second polarity of the first differential input signal are input to respective control terminals of the second transistor and the third transistor.

11. The power detection circuit of claim 10, wherein:
a first polarity of the second differential input signal is input to first terminals of the first transistor and the second transistor; and
a second polarity of the second differential input signal is input to first terminals of the third transistor and the fourth transistor.

12. The power detection circuit of claim 9, further comprising:
a first resistance; and
a second resistance,
wherein
the plurality of transistors includes a first transistor, a second transistor, a third transistor and a fourth transistor,
respective terminals of the first transistor and the third transistor are in communication with a non-inverting input of the differential amplifier and a first end of the first resistance, and
respective terminals of the second transistor and the fourth transistor are in communication with an inverting input of the differential amplifier and a first end of the second resistance.

13. The power detection circuit of claim 1, further comprising:
a transconductance amplifier configured to (i) receive the first differential input signal and (ii) output the second differential input signal to the multiplier circuit.

14. The power detection circuit of claim 1, wherein the bias circuit is configured to generate the bias signal based on a constant reference current provided to the bias circuit.

15. A radio frequency transmitter comprising the power detection circuit of claim 1.

16. A method for detecting an output power of a radio frequency transmitter, the method comprising:
multiplying a first differential input signal and a second differential input signal, wherein (i) the first differential input signal corresponds to a radio frequency signal to be amplified by an amplifier of the radio frequency transmitter and (ii) the second differential signal corresponds to an output signal as amplified by an amplifier of the radio frequency transmitter;
generating a bias signal; and
generating, based on (i) the bias signal and (ii) the first differential signal and the second differential signal as multiplied, an indication of the output power of the amplifier of the radio frequency transmitter.

17. The method of claim 16, wherein:
multiplying the first differential input signal and the second differential input signal includes using a multiplier circuit comprising a plurality of transistors; and
generating the bias signal includes generating the bias signal further based on a voltage threshold of one of the plurality of transistors.

18. The method of claim 16, wherein generating the bias signal includes generating the bias signal further based on a product of (i) a constant reference current and (ii) a bias resistance.

19. The method of claim 16, wherein generating the bias signal includes generating the bias signal such that a conversion gain associated with a power detection circuit is substantially constant regardless of variations in one or more of process and temperature.