Abstract:
A highly efficient radio frequency (RF) transmitter provides both wide bandwidth and an extended power control range. The RF transmitter includes stage switching, bias adjustment, and drain supply modulation. These components are used to provide fine and coarse power control and EER envelope fluctuations. The RF transmitter is useful in wireless communications to increase both handset talk time and battery life.

Description:
TECHNICAL FIELD 
     The present invention relates generally to radio frequency (RF) transmitters and, more particularly, to RF transmitters with multiple power level capability. 
     BACKGROUND 
     Solid-state amplifiers, such as RF power amplifiers, are widely used for wireless communications. The goal of these power amplifiers is to achieve high operating efficiencies. 
     Various techniques have been employed to provide multiple power level capability for wireless communications. One technique is to place an attenuator before power amplifier to reduce the output signal level from the amplifier. Because, however, the power amplifier is configured to operate efficiently only at a specific instantaneous power level, the amplifier&#39;s operating efficiency is limited. 
     Envelope elimination and restoration (EER) power amplifiers have been known for many years. Generally, the efficiency of the EER amplifier is controlled by using circuit elements to modulate a supply voltage. A common EER amplifier topology uses a switching mode power supply to modulate the drain voltage of a single stage RF power amplifier. A pulse width modulator (PWM) feeds the power amplifier. The output of the amplifier is then passed to a low-pass filter, and the filtered output is used to modulate the drain of a MOSFET configured as a single-stage class D RF amplifier. The PWM applies a simple feedback mechanism to shape its output. A conventional EER amplifier only operates at very low bandwidth signals, such as 3-30 kHz, and does not operate efficiently over a wide range of average power output levels. This can contribute to low operating efficiencies. For low bandwidth signals, a conventional EER amplifier also fails to reduce the amount of in-band quantization (i.e., switching) noise in the output signal. The result is that the output signal must be sampled at very high rates, which causes significant losses in the output signal. 
     SUMMARY 
     In general, the present invention is directed to a highly efficient RF transmitter including a power amplifier circuit with an extended efficient power control range. The preferred RF transmitter may be configured to operate over a wide variety of communication standards. An RF transmitter implementing the invention is highly efficient for nonconstant-envelope modulation formats and can operate over extended control ranges for high frequency/wide bandwidth operations, such as wireless communications. 
     In one aspect, the invention is directed to an RF transmitter that includes a signal processing block. A supply modulator is responsive to an envelope output of the signal processing block, and a FM modulator is responsive to a phase output of the signal processing block. A power amplifier circuit including a plurality of cascaded amplifiers is coupled to the supply modulator, and an attenuator is coupled between the FM modulator and the power amplifier circuit. 
     Implementations of the invention include one or more of the following. The RF transmitter may include switching circuitry configured to bypass selected ones of the amplifiers, when the supply modulator operates in a low power mode. The amplifiers may be used to impart a gain to an output signal of the attenuator. The RF transmitter also includes a modulator connected between the envelope output and the supply modulator, where an output of the modulator further includes a high speed binary pulse stream. 
     In another aspect, the present invention is directed to an RF transmitter that includes a signal processing block configured to convert an input signal into an envelope signal and a phase signal. A supply modulator is responsive to the envelope signal, and a FM modulator receives the phase signal. An attenuator is configured to attenuate the output of the FM modulator, and a power amplifier circuit is coupled between the attenuator and the supply modulator. 
     The preferred transmitter has many benefits. The preferred transmitter may be implemented in a wireless handset. This wireless handset may have longer talk times and smaller and lighter batteries. Further, the preferred transmitter reduces the size and cost of the handset and makes the handset more attractive to the wireless customer. The extended operating range of the transmitter using power control also lengthens the handset&#39;s battery life. 
    
    
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates one embodiment of a RF transmitter with extended efficient power control range. 
     FIG. 2 illustrates a supply modulator for the RF transmitter of FIG.  1 . 
     FIG. 3 illustrates a graph of the oversampling ratio versus signal-to-noise (SNR) ratio for a one bit delta-sigma modulator. 
    
    
     DETAILED DESCRIPTION 
     In general, the present invention is directed to a highly efficient RF transmitter including a power amplifier (PA) circuit with an extended efficient power control range. The RF transmitter may be configured to operate using communication standards, such as the cellular telephone standard IS-95, that operate over a wide bandwidth (e.g., greater than 1 MHz) and an extended power control range (e.g. about 70 dB or more). 
     In a preferred implementation, a highly efficient envelope elimination and restoration (EER) type power amplifier circuit is used. To achieve high efficiency in this type of power amplifier circuit, a modulated signal is polar decomposed into an envelope component and a phase component. The phase component is FM modulated around a desired carrier frequency to produce a constant envelope component. A supply voltage (e.g., the drain voltage for a FET or the collector voltage for a BJT) of the PA circuit is modulated by the envelope signal, and an input of the PA circuit (e.g., the gate for a FET or the base for a BJT) is driven by the constant-envelope signal component. For an EER amplifier, the overall operating efficiency is highly dependent on the efficiency of the circuit elements that modulate the supply voltage. This is because the circuitry modulating the supply voltage must process the energy that the PA circuit converts to RF energy. In particular, the overall PA circuit efficiency is the product of the supply modulator efficiency and the RF PA circuit efficiency. 
     FIG. 1 illustrates a preferred RF transmitter having an in-phase input  3  and a quadrature input  4 . The inputs  3  and  4  may be digital signals arriving from, for example, a baseband IC (not shown). The inputs  3 ,  4  are received by a signal processing block  5 , which converts the in-phase input  3  and the quadrature input  4  into an envelope component signal  7 , and converts the in-phase input  3  and the quadrature input  4  into a phase component signal  8 . Like the inputs  3 ,  4 , the envelope and phase component signals  7 ,  8  are usually digital signals. The signal processing block  5  includes digital delay elements that deliver integer clock-cycle delays to delay the envelope component signal  7  and the phase component signal  8  with respect to each other. This provides coarse compensation for delay differences between the component signals. 
     In FIG. 1, the phase component signal  8  is delivered to an FM modulator  10 . The FM modulator  10  modulates the phase component signal  8  onto any suitable RF carrier frequency. For example, the carrier may be 800 MHz for a cellular band and 1.9 GHz for a PCS band. In general, the FM modulator  10  tunes the RF carrier frequency to a desired operating channel. For example, the operating channel may be about 1.25 MHz apart for the IS-95 standard and 30 KHz apart for the cellular telephone standard IS-136. The output  11  of the FM modulator  10  is a constant-envelope signal having a low power level, typically about 30-40 dB below the peak output power level of the transmitter  1 . A common value for the peak output power level is about 30 dB. 
     To transmit at low average power of about 20-30 dB below the maximum operating power of the transmitter, the transmitter  1  also includes a voltage control attenuator (VCA)  15  that AM modulates the constant-envelope output  11  into an AM-modulated (envelope modulated) signal  17  that feeds a power amplifier circuit  23  described below, when the power amplifier  23  is bypassed for very low average power levels. These power levels may be below 0 dBm. 
     In the illustrative embodiment, the power amplifier circuit  23  is a three-stage power amplifier circuit including first and second driver amplifiers  20 ,  21 , and an additional output stage amplifier  22 . 
     The first-stage driver amplifier  20  includes a drain input terminal  60  and a switch  30 . The second stage driver amplifier  21  also includes a drain input terminal  61  and a switch  31 . The output stage amplifier  22  also includes a drain input terminal  62  and a switch  32 . Each of the amplifiers  20 ,  21 ,  22  is used to impart gain to the output signal  17  from the VCA  15 . 
     The transmitter  1  uses a voltage supply modulator  50  connected to the drain input terminals  60 ,  61 ,  62  and the voltage control input of the VCA  15 . In this configuration, operating at full power, the supply modulator  50  supplies a constant DC voltage to the driver amplifiers  20 ,  21  and a modulated signal to the output stage amplifier  22 . 
     FIG. 2 shows the supply modulator  50  in more detail. The supply modulator  50  includes a delta-sigma (Δ-Σ) modulator  75  having a clock input  79 . The Δ-Σ modulator  75  receives the digital envelope component signal  7  from the signal processing block  5  and up-samples and Δ-Σ modulates this signal. The envelope component signal  7  provided to the Δ-Σ modulator  75  is usually a multi-bit envelope word fed through a delta-sigma D/A converter (not shown). The modulator  75  outputs a high speed binary pulse stream that is delayed for time alignment with the output signal  11  from the FM modulator  10 . The output signal from the Δ-Σ modulator  75  is then delivered to the multiplexer  80  and filtered and presented to the driver amplifiers  20 ,  21  and output stage amplifier  22 , as described above. The multiplexer  80  provides input to delay lines  71 - 74 . Each of the delay lines  71 - 74  connects to a corresponding one of several switching transistors  81 - 84 . The switching transistors  81 - 84  each supply a selected current level to one of several low-pass filters  91 - 94 . This causes the digital signals from the above generated pulse stream to be converted to analog drain voltages to be fed to the drain terminals  60 ,  61 , and  62 . 
     The delay lines  71 - 74  are used to achieve precise time alignment of the envelope component signal  7  and the phase component signal  8  throughout the transmitter  1 . 
     When one of the driver amplifiers  20 ,  21  or the output stage amplifier  22  is in a non-AM modulated mode, a static voltage is supplied to the input drain terminal of that power amplifier by a device, such as a battery  90 . The corresponding switching transistor  81 - 84  remains closed. 
     In certain embodiments, the Δ-Σ modulator  75  and the FM modulator  10  are delta-sigma Pulse Density Modulator (PDM) converters. These PDM converters can be used to transform the envelope and phase component signals  7 ,  8  into one-bit, high-rate samples. FIG. 3 shows that the oversampling rate depends on the order of the converter and the desired signal-to-quantization noise of the resulting signal. These modulators also push the majority of the quantization noise of the envelope and phase component signals  7 ,  8  outside of the operating bandwidth, where it can be filtered away. 
     The phase component signal  8  provided to the FM modulator  10  is usually a multi-bit phase word that is sigma-delta up-sampled by a converter  98  to a high-speed, one-bit stream. This stream is presented to a control input of a synthesizer  95  having an N, N+1 divider. The synthesizer output causes a voltage controlled oscillator  96  to generate a phase-modulated carrier signal centered at a selected frequency. For example, the carrier frequency could be 800 MHz or 1.9 GHz. An optional loop filter  97  removes out-of-band noise and spurs generated by the converter  98 . A phase detector  100  coupled to a reference frequency source  101  may be used to perform the modulation in the FM modulator  10 . 
     The transmitter  1  may operate at low nominal signal powers, for example, 30 dB below the maximum output power of the output stage  22 . In general, these signals cannot be realized by bypassing the driver amplifiers  20 ,  21  or the output stage amplifier  22 . Thus, in a preferred configuration, the VCA  15  reduces its nominal power output and processes the input signal  11  imparting AM modulation. The input  11  may also be envelope modulated using the signal  16  from a supply modulator  50 . At this stage, the output  17  may include an AM and FM modulated signal that can bypass the driver amplifiers  20 ,  21  and the output stage amplifier  22 , and the resulting signal is then sent to an antenna (not shown) via the output stage amplifier  22 . 
     The cascaded driver amplifiers  20 ,  21  and output stage amplifier  22  allow the transmitter  1  to also operate at maximum average power, which may be about 30 dBm for the IS-95 standard. However, for efficient power control with sufficient linearity over the 30 dB power control range, bypassing of the driver amplifiers  20 ,  21  and the output stage amplifier  22  in stages is necessary. This is because envelope restoration through drain modulation and variable gate biasing of the driver circuits  40 ,  41 ,  42  is linear for a single stage of the power amplifier circuit  23  only for a limited range of gains. This range may be about 6-20 dB. 
     The bypassing of one or more of the driver amplifiers  20 ,  21  or the output stage amplifier  22  is as follows. To bypass the driver amplifiers  20 ,  21  and the output stage amplifier  22 , the switches  30 ,  31 ,  32  are closed to create a single path around the amplifiers  20 ,  21 , and  22 . As a result, the output amplifier stage  22  is shut off for power levels around 10 dB below the maximum output level of the transmitter  1  by bypassing the output amplifier stage  22  through disconnecting the drain terminal  62 . As a result, the driver circuit  41  can be drain modulated through the drain terminal  61  to superimpose the AM modulation to the constant-envelope output signal  18  from the driver circuit  40 . To impart an attenuation of about 20 dB below the maximum output level, the output stage amplifier  22  and the second stage amplifier  21  can be bypassed through disconnecting the drain terminals  62  and  61 , respectively. To impart an attenuation of about 30 dB below the maximum output level, the drain terminals  62 ,  61 , and  60  can be disconnected to bypass the amplifiers  22 ,  21 , and  20 , respectively. Switching in this manner increases the power output level in increments of about 10 dB, which provides for coarse power control. Fine power control (e.g., sub-dB increments) can be achieved by adjusting the DC operating level of the output-stage power amplifier  22 . Fine power control may also be achieved by fine tuning any of the drain bias signals via drain terminals  60 ,  61 , and  62  from the supply modulator  50 . 
     The transmitter described above is efficient for wireless communications. One benefit is that wireless handsets have longer talk times and smaller and lighter batteries. The preferred transmitter reduces the size and cost of the handset and makes the handset more attractive to the wireless customer. The extended operating range of the transmitter using power control also lengthens the handset&#39;s battery life. Moreover, using the drain input terminals and the FM modulator to modulate the driver amplifiers and the output amplifier reduces the die size and external component counts in existing architectures using the IS-95 and IS-136 standards. As a result, the size and cost of wireless transceivers are significantly reduced. The transmitter also generates power output levels for CDMA applications that lead to longer talk times than current GSM handsets. 
     A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment, but only by the scope of the appended claims.