Considering class AB and class B bias configuration in power amplifier, for class AB bias design, it provides a better power gain, so it require less driving power, but the drawback is the power transistor pass a quit heavy idle current, for this reason, the overall power efficiency is low. When the output power required is approaching the thermal limit of transistor, the low efficiency cause a problem , pushing the transistor got overheat and burn up before it can generate a good output power.
In this attempt of 40MHz power amplifier, the objective is to squeeze as much power as possible from a BLT50 transistor, the class B bias configuration is selected because of the better efficiency.
Using power supply of 10V, 647mA of current, an output power of 1.6W is achieved.
Two stages is required because the second stage class B amplifier required a pretty large drive power ~20dBm (100mW). L1 is 0.8mm wire with 7mm diameter, 7T. L2 is 0.8mm wire with 7mm diameter, 3T.
Spectrum analyzer shows 26dBm,incorporate with two -3db attenuator, the output power is 32dBm(1.6W). the second harmonic having a level of ~15 db below the fundamental.
The two-stage power amplifier consumes 0.647A of current, at 10V.
Input drive power required 5dBm.
Having adequate heat sink is important because the heavy power dissipation can burn the transistor up.
The waveform for power stage collector. Actual voltage is 4 times larger than the probed voltage, due to the resistor voltage divider.
The waveform for output matching circuit. The circuit consist of L2 and C1 L-section. Actual voltage is 4 times larger than the probed voltage, due to the resistor voltage divider.
The output voltage is measured 28V p-p which is larger than twice of the DC supply voltage 10V, because of the output impedance matching circuit.
