GB2381681A

GB2381681A - Automatic transmitter bias adjustment between TDMA slots

Info

Publication number: GB2381681A
Application number: GB0126088A
Authority: GB
Inventors: Haim Friedlander; Gadi Shirazi; Perry Bassan
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2001-10-31
Filing date: 2001-10-31
Publication date: 2003-05-07
Also published as: GB0126088D0

Abstract

An amplifier circuit for amplifying signals in a radio transmitter comprises an active amplification device and biasing means for applying to the active device a bias voltage or current to be set at a bias point to control operating properties of the active device; and is characterised by a control loop for controllably adjusting the bias point of the bias voltage or current applied by the biasing means, the adjustment loop comprising a detector for measuring a quiescent current in the active device when the bias voltage or current is applied thereto, a comparator for comparing the measured quiescent current value with a desired value and for producing an error control signal based upon the difference and a connection from the comparator to the biasing means for applying an error control signal thereto to adjust the bias point. The amplifier may be useful as a linear RF amplifier for use in a mobile communication unit. The controller 19 may be a DSP or a microcontroller, and the control loop may be activated between TDMA slots. The amplifier may be class B.

Description

AMPLIFIER CIRCUITS AND THEIR USE IN RADIO TRANSMITTERS Field of the Invention This invention relates to amplifier circuits and their use in radio transmitters. In particular, it relates to linear amplifiers for use in mobile communications units.

Background of the Invention An amplifier circuit is an electronic circuit which accepts an electrical input signal and provides an electrical output signal such that there is a prescribed relationship between the input and output signals. The amplifier circuit requires an active or amplifying device to perform a signal amplification operation. For the majority of active devices commonly in use, such as bipolar junction transistors, field-effect transistors (FETs) etc. , the active devices require a specific bias voltage or current, together referred to herein as'bias point', to be applied to the terminals of the device to ensure that the device operates correctly and provides the desired output signal.

For example, the final amplification stage of a solid-state class-B linear radio-frequency (RF) power amplifier (RFPA) requires accurate setting of the bias point (bias voltage in this case) applied to the active device, e. g. transistor, employed in the stage. This is necessary in order to achieve the desired linearity and efficiency. Setting the bias point at too low a level will impair the linearity of the stage. On the other hand, if the bias point is set at too high a level, the quiescent current through the active device (e. g. as seen at the drain or collector) will be excessively

high, causing efficiency degradation and excessive power consumption.

It is very difficult to set the bias point accurately using a fixed bias configuration. This is due to the given tolerance of the DC parameters of the active device. Also, the electrical properties of the active device will vary with temperature and this will affect setting of the required bias point, causing a drift in the optimum bias point. Furthermore, the electrical properties of the active device will vary with ageing of the device this will also affect setting of the required bias point, causing a further drift in the optimum bias point.

The bias point can be adjusted by post-production trimming (by laser, for example) of one of the biasdetermining components. However, this method has several disadvantages, namely: (a) the time to carry out the production process is lengthened ; (b) production costs are raised; (c) variations of product properties with temperature still occur; (d) variations of product properties due to ageing still occur.

(e) if the active device needs to be replaced when in use, re-trimming can be difficult or impossible.

Another possibility is post-production electronic tuning. This can be accomplished by means of a microcontroller-programmed DAC, whose required value is stored after tuning in a non-volatile memory incorporated in the transceiver. This approach suffers from the following problems:

(a) the time for carrying out the production process is again increased; (b) production costs are again raised; (c) the method relies on all active devices, e. g. transistors, behaving identically over changes in temperature; in general they do not; (d) variations due to ageing still occur; (e) if the active device needs to be replaced in use, re-tuning can be difficult or impossible.

For example, a prior art process of bias point setting which may be used in a production factory or as an occasional procedure in use ('in the field') is that described in GB 2345211A by the Applicant's affiliate Motorola GmbH. It is based on using external test equipment to measure the quiescent current of the final amplification stage of a transmitter, while adjusting and storing the bias point setting within the transceiver as a fixed value. This technique suffers from the disadvantages described earlier with reference to post-production electronic tuning.

Summary of the Invention According to the present invention in a first aspect there is provided an amplifier circuit for amplifying signals in a radio transmitter, the amplifier circuit comprising an active amplification device and biasing means for applying to the active device a bias voltage or current to be set at a bias point to control operating properties of the active device; and characterised by a control loop for controllably

adjusting in use the bias point of the voltage or current applied by the biasing means, the adjustment loop comprising a detector for measuring a quiescent current in the active device when the bias voltage or current is applied thereto, a comparator for comparing the measured quiescent current value with a desired value and for producing an error control signal based upon the difference and a connection from the comparator to the biasing means for applying an error control signal thereto to adjust the bias point.

The amplifier circuit may be operable to amplify RF signals. It may provide a power amplifier. The amplifier may comprise a linear amplifier and may be suitable for use as a final amplification stage of a class B amplifier in a RF transmitter chain. The active amplification device may comprise a solid state device such as a transistor, e. g. a bipolar transistor, a MOSFET (metal-oxide-semiconductor field effect transistor or a LDMOS (lateral diffused metal oxide semiconductor device).

The detector for detecting the quiescent current in the active device may comprise a current sampling resistor and means for measuring a voltage drop across the resistor. The detector may however be another form of measuring device known for measuring the current flowing in a given device. For example, it could comprise a current measuring loop placed around wire which feeds DC current to the device.

The output signal from the detector may conveniently be converted for further processing into a digital form by an analogue to digital converter.

The said comparator may comprise a signal processor, e. g. a digital signal processor. The said desired value against which the measured quiescent current value is compared by the comparator may be stored in a memory, e. g. forming part of or operating in conjunction with the signal processor. The desired value may for example have been determined by previous calibration experiments and pre-programmed in the memory. In practice, the measured quiescent current value may comprise a digital number based upon a measured voltage across a current sampling resistor. The number will thus represent the measured current. The stored value will correspond to the optimum value of the number.

Where the comparator comprises a digital signal processor, the error control signal may comprise a digital number representing the positive or negative difference between the measured and stored values representing measured and optimum current values.

Where the error control signal is a digital signal, it may be converted by a digital-to-analog converter into an analogue signal for adjustment of the bias point provided by the biasing means.

According to the present invention in a second aspect there is provided a radio transmitter including an amplifier circuit according to the first aspect. The radio transmitter may comprise a transmitter for use in a mobile communications unit.

The radio transmitter according to the second aspect may include a RF signal producer operably coupled to the amplifier circuit to amplify RF signals produced thereby. The RF signal producer may be operable so that

no RF signal is applied thereby to the active device during a period whilst the control loop for adjusting the bias voltage is operational. The transmitter may otherwise be energised during this period so that the 'on'state characteristics of the transmitter are taken into account when the quiescent current of the active device is measured. Preferably, the quiescent current is measured frequently during periods when no active signal is being sent. For example, the transmitter may comprise a transmitter for use in TDMA (time division multiple access) communications. In such a case, the bias voltage setting procedure may be carried out in a suitable quiescent time occurring periodically between transmission slots (traffic and signalling slots).

The transmitter according to the second aspect of the invention may include a signal controller for controlling application of a signal produced by the signal producer to the active device whereby an RF signal is applied to the active device only during times when the control loop is not operational and no RF signal is applied when the control loop is operational.

The signal controller may be a signal processor and may be combined in a single functional unit with the signal processor referred to earlier.

The invention beneficially allows bias point adjustment of the active device in an amplifier to be achieved by means of in-use training. A fixed quiescent current is required in the active device but the bias point required to achieve this varies with age of the device, temperature etc. The invention allows the bias point to be adjusted in use to achieve the desired quiescent current level. The invention beneficially

eliminates the need for bias trimming or tuning of the active device using an external tuning device, thus saving production time and cost. Moreover, the bias point can always be set optimally, independent of parameter tolerance, temperature, or ageing of the active device. Additionally, the active device can be replaced in use without requiring any re-tuning. Where the amplifier incorporating the invention is used in a RF transmitter, the continual use of optimal bias point setting by the invention unexpectedly and beneficially improves and stabilises transmitter linearity and efficiency performance whilst also simplifying production.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawing in which: Brief Description of the Drawing Figure 1 is a schematic block circuit diagram of a linear transmitter incorporating an amplifier and embodying the present invention.

Description of a specific embodiment of the invention In a transmitter 100 shown in Figure 1, a power transistor 1 is connected to an RF signal producer 3 and provides a stage of linear power amplification to RF signals from the signal producer 3. The transistor 1 is also connected in turn to an RF output circuitry 5 which provides further processing of RF signals amplified by the transistor 1 and an

antenna 7 which sends processsed signals to a remote receiver (not shown) in a known manner.

A bias voltage is applied to the transistor 1 by bias circuitry 9. The transistor 1 is connected to a fixed voltage source 11 via a current sampling resistor 13. The voltage drop across the resistor 13 is measured by a voltage meter 15 providing an output 17 in digital form by an analog-to-digital to converter within the voltage meter 15. The output 17 represents the value of the measured voltage across the resistor 13 and thereby the current through the resistor 13 and transistor 1 in series with it. The output 17 is applied to a signal processor (or microcontroller) 19. The digital number constituting the output 17 is compared by the signal processor 19 with a stored number representing an optimal value of the measured voltage and the quiescent current it represents. The stored number may have been preprogrammed in a memory within the signal processor 19. The signal processor 19 produces from the comparison of the measured and stored values a digital number corresponding to a positive or negative error control signal which is the difference between the two compared values. This digital error control number is provided as an output 21 from the signal processor 19. The output 21 is applied as an input signal to a digital-to-analogue converter 23 which in turns produces a corresponding analog output signal which is applied to the bias circuitry 9 to adjust the bias voltage (bias point) applied thereby to the transistor 1.

A method of operation for periodically setting the bias voltage (bias point) of the transistor 1 may be carried out as follows.

(a) The transmitter 100 is turned on but the signal processor 3 disables the RF signal producer 3. Thus no RF signals are delivered from the RF signal producer 3 to the transistor 1. Control signals from the signal processor 19 to the RF signal producer 3 are sent via a connection indicated by a dashed line 25.

(b) The signal processor 19 programs the digitalto-analog converter 23 with an initial (default) value for the output 21.

(c) The signal processor 19 activates the bias circuitry 9 via a connection indicated by a dashed line 27.

(d) A bias voltage is applied to the transistor 1 based on the initial (default) value of the digital output 21 from the signal processor 19 and corresponding analog signal from the digital-to-analog converter 23.

The transistor 1 begins drawing an initial quiescent current.

(e) The quiescent current flows through the current sampling resistor 13, thus producing a voltage drop across the resistor 13 proportional to the quiescent current.

(f) The voltage drop representing the quiescent current is sampled by the voltage meter 15.

(g) A corresponding digital output 17 is provided and the value of the output 17 is read by the signal processor 19.

(h) Any difference between the read digital value and a stored value as determined by the signal processor

19 is provided as a revised digital output or error control signal 21.

(i) The process is repeated until a sufficiently small difference (i. e. a difference less than a predetermined amount) is detected in the signal processor 19 between the measured input value (output 17 from the voltage meter 15) and the corresponding stored optimal value with which it is compared. In this condition an optimal setting of the bias voltage (at the bias point) applied to the transistor 1 has been achieved.

(j) Once the bias voltage applied to the transistor 1 has been so adjusted, the signal processor 19 activates the RF signal producer 3 by another control signal sent via the connection 25 and transmission of RF signals can begin.

Claims

CLAIMS 1. An amplifier circuit for amplifying signals in a radio transmitter, the amplifier circuit comprising an active amplification device and biasing means for applying to the active device a bias voltage or current to be set at a bias point to control operating properties of the active device ; and characterised by a control loop for controllably adjusting in use the bias point of the voltage or current applied by the biasing means, the adjustment loop comprising a detector for measuring a quiescent current in the active device when the bias voltage or current is applied thereto, a comparator for comparing the measured quiescent current value with a desired value and for producing an error control signal based upon the difference and a connection from the comparator to the biasing means for applying an error control signal thereto to adjust the bias point.

2. An amplifier circuit according to claim 1 and wherein the detector for detecting the quiescent current in the active device comprises a current sampling resistor and means for measuring a voltage drop across the resistor.

3. An amplifier circuit according to claim 1 or claim 2 and wherein the detector includes an analogue-to-digital converter for producing an output signal in digital form.

4. An amplifier circuit according to any one of the preceding claims and wherein the said comparator comprise a digital signal processor and the said desired value against which the measured quiescent current value is compared by the comparator is stored in a memory.

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5. An amplifier circuit according to claim 4 and wherein the error control signal produced in operation by the digital signal processor comprises a digital number representing the positive or negative difference between the measured and stored values representing measured and optimum quiescent current values.

6. An amplifier according to claim 5 and including a digital-to-analog converter for converting a digital error control signal produced by the digital signal processor into an analogue signal for application to the biasing means in order to adjust the bias point of the voltage or current provided by the biasing means.

7. An amplifier circuit according to any one of the preceding claims and which comprises a linear RF power amplifier.

9. A radio transmitter including an amplifier circuit according to any one of the preceding claims.

10. A radio transmitter according to claim 9 and which comprises a transmitter for use in a mobile communications unit.

11. A radio transmitter according to claim 9 or claim 10 and which includes a RF signal producer operably coupled to the amplifier circuit to amplify RF signals produced thereby, the RF signal producer being operable so that no RF signal is applied thereby to the active device during a period whilst the control loop for adjusting the bias point is operational.

12. A radio transmitter according to claim 11 and including a controller operable to produce control signals to switch the signal producer between alternate states in which it is connected to and disconnected from the active device.

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13. A radio transmitter according to claim 12 and wherein the signal controller comprises a digital signal processor which is combined in a single functional unit with a signal processor providing the function of the said comparator.

14. A radio transmitter according to any one of claims 9 to 13 and which is operable according to a TDMA (time division multiple access) protocol and whereby the quiescent current is repeatedly measured and the bias point if necessary is adjusted during periods between traffic and signalling transmission slots.

15. An amplifier circuit according to claim 1 and substantially as described herein with reference to the accompanying drawing.

16. A radio transmitter according to any one of claims 9 to 14 and substantially as described herein with reference to the accompanying drawing.