The principle of RF amplifier circuit

The circuit principle of RF power amplifier is that the directional coupler at the output end of the transmitter power amplifier detects the reverse power voltage output. After being processed by the relevant circuits in the power amplifier stabilizing power supply, it is sent to the transmitter control circuit XP1/12A. Then, after passing through the reverse power detection voltage compensation circuit, it is added to the N20B amplifier, which is then added to the in-phase input terminal of N20A. When the voltage at this input terminal reaches about 300mV (when the power amplifier output and antenna impedance are mismatched, the voltage standing wave ratio is greater than 2.5:1), the output voltage of N23A is greater than 6.2V, causing the VD5 voltage regulator and V3 to conduct. V3 conduction causes VD6 conduction, which lowers the level of the in-phase input terminal of N23B and the voltage at the output terminal of N23B. As a result, the DC control voltage applied to the Vx input terminal of the analog multiplier N24 drops to a certain level, ultimately reducing the output power of the RF amplifier to a certain value and protecting the RF amplifier.

When the voltage standing wave ratio is not greater than 2.5:1, the reverse power detection voltage is small, and the voltage output by N23A is not enough to make VD5 conductive. Therefore, the circuit VD6 of the RF power amplifier is also turned off due to reverse bias. Only the normal power DC control voltage set on the front panel is applied to the N23B in-phase input terminal, and the voltage applied to the input terminal of the analog multiplier Vx is also normal. The transmitter RF power amplifier outputs power normally.