900MHz, 1W silicon power amplifier-MAX2235

MAX2235 to 900MHz, 1W silicon power amplifier (PA), using its power output automatic slope control structure can effectively reduce the FSK / TDMA system, transient noise and the direct modulation system "VCO pulling" and therefore the MAX2235 suitable for two-way paging, AMPS, GSM and other cellular micro-900MHz ISM band applications.

900MHz, 1W silicon power amplifier-MAX2235

MAX2235's operating frequency range of 800MHz ~ 1000MHz, with 2.7 ~ 5.5V single power supply, directly from the three Nicd or a Li + battery-powered, when the supply voltage of 3.6V, the output power typically 30bBm (1W ); the power supply voltage of 2.7V, output power up to +28 dBm. MAX2235 power gain may be an external regulator, adjustable range is 37dB. Power control pin can control the gain and bias circuitry, and thus obtain the highest efficiency and the effective extension of the system's battery life. 30dBm output power, the efficiency is 47%; use of the shutdown mode the chip can further reduce power consumption. 1. MAX2235 Block Diagram

Figure 1 shows the MAX2235's internal structure diagram, which consists of RF input stage, driver stage, power amplifier stage and bias circuit, power management components, RF input level for the variable gain amplifier (VGA), by changing the gain control pin GC of the voltage regulation amplifier gain, gain control range of 37dB, the relationship between gain and VGC curve shown in Figure 2. Enter the equivalent of a transistor transconductance, can maintain a fixed bias current. Class A input stage for the amplifier in standby mode to ensure the input voltage standing wave ratio (VSWR) of the variation is not greater than 1.5:1, in order to improve input VSWR, should be an external input matching inductor.

900MHz, 1W silicon power amplifier-MAX2235

Driver Circuit for power by the output stage to provide enough signal to make the output stage power transistor working in saturation. Drive circuit in C, in the standby mode will be shut down. Drive level and power output stage of the matching circuit built-in chip, used to improve the load and power transmission characteristics, and can save circuit board size, matching network bandwidth, Yun Xu PA working in a wide frequency range.

Power output stage to provide a 50Ω load 30dBm output power, in order to obtain high power-added efficiency (PAE), the level of work in the E class circuit. In the appropriate external output matching network to optimize output power, therefore, when the Yi Ban at the output using an external one with low equivalent series resistance of the pull-inductance, the minimum inductor current rating of 1.5A.

900MHz, 1W silicon power amplifier-MAX2235GC, RAMP, REF is the bias circuit and power control pin, 900MHz, 1W silicon power amplifier-MAX2235 pin voltage falls below 0.5V,, MAX2235 internal circuit all closed; 900MHz, 1W silicon power Amplifier-MAX2235 pin voltage higher than 2.0V when, MAX2235 open at this time, if the 900MHz, 1W silicon power amplifier-MAX2235 0.6V ~ 2.3V in the range of changes, we can continuously control the input gain and output power level sizes. When the 900MHz, 1W silicon less than 0.4V when the power amplifier-MAX2235, MAX2235 in standby mode, when only the input stage to maintain effective working condition. When VGC floating, 900MHz, 1W silicon power amplifier-MAX2235> 2.0V, the chip maximum output power. MAX2235 work pattern in Table 1.

2. MAX2235 Application Circuit

MAX2235 typical application circuit shown in Figure 3, the RF input and RF output match at 824MHz ~ 849MHz, the figure capacitor C1 ~ C18 are selected ceramic capacitors, in order to reduce radiation and insertion loss, should be as short as possible the pin connection circuit board , in particular, the transistor emitter lead inductance the most serious in its emitter current to generate the feedback voltage will cause the power amplifier gain and output power of a serious decline. Figure 4 for the circuit test results, respectively, under different supply voltage, output power with input power change and power amplifier efficiency and output the corresponding curve.

900MHz, 1W silicon power amplifier-MAX2235

900MHz, 1W silicon power amplifier-MAX2235

Figure 3, the connection pin and VREF pins in the RAMP capacitor C8 between the amplifier is used to set off in the open and the output power during the rise and fall times, the role of C8 in the capacitor, the output will slow to rise or fall speed, which can effectively reduce the output noise and transient scattering spectrum. In addition, direct modulation system, VCO's load impedance amplifier with the power and the rapid changes, which will result in VCO frequency or phase shift, this shift must be phase-locked loop circuit (PLL) correction, correction moment (so-called "VCO pulling") will lead to erroneous data bits, there may even be excessive spurious emissions. VCO pulling caused by another factor is the amplifier's output power to the VCO leakage (through radiation or conduction), the power amplifier during the transient phase shift caused by the same need for PLL correction. MAX2235 input impedance of the device is turned on only minor changes, thus preventing the load change caused by VDO traction. In addition, the automatic slope control structure is given in a period after power-on control of output power, as long as the rise time is designed (by an external capacitor C8) 低于 loop bandwidth, PLL tracks referrals from amplifier output can be any feedback, but transient phase shift does not occur. As can be seen from Figure 5, when in power, the voltage standing wave ratio of the change is very small, and thus can effectively solve the direct modulation system, the VCO pulling problem.

900MHz, 1W silicon power amplifier-MAX2235