Power MOSFET amplifier with improved high frequency characteristics

Function of the circuit

MOSFET power amplifiers for high power have faster switching speeds than single-stage transistors and are suitable for high-frequency operation. This circuit uses an exciter that determines the switching speed, and also uses MOSFETs at the output stage to improve high-frequency characteristics. The output power depends on the power supply voltage and load. This circuit can output 100~150W continuously and can withstand load short circuits.

How the Circuit Works

The basic circuit composition is the same as the 100W power amplifier of the current limiting protection circuit. The second differential stage is the power MOSFET for output, 2SJ77, and the current Miller circuit uses 2SK214. Although the working current is only 6MA, because the power supply voltage is as high as ±50V, the transistor will heat up, so a small heat sink is installed.

The output stage adopts direct drive mode. Since there is no emitter output buffer, the load of the drive circuit is increased. If the conversion speed needs to be increased, the drain bias current can be increased as much as possible within the allowable loss range of TT5-6.

Power MOSFETs often generate high-frequency oscillations, and it is difficult to suppress the oscillations. A simple measure is to connect a resistor RO in series with the gate, but this will sacrifice some high-frequency characteristics. The resistance value of RO varies with the components used, usually between 50 and 500 ohms.

Compared with unipolar transistors, the voltage loss of the output circuit is large, and the power supply voltage should be increased. Since the loss voltage of the output circuit depends on the on-resistance of the power MOSFET, this problem cannot be ignored when working under high current conditions, so a multi-tube parallel connection method is used to solve it.

Component selection

Because the VOB~ID characteristics of FET are less consistent, TT5 and TT6, TT7 and TT8, TT12 and TT13, TT14 and TT15 should try to use products with different transfer characteristics, and the even-numbered high-efficiency harmonics will increase.

The circuit can work even without source resistors R2A~R31. However, if the circuit is used for current detection and uses color-coded wirewound resistors, the inductance component cannot be ignored and it is not suitable to work under high frequency conditions.

Adjustment

The operating current of the first differential can be adjusted by the bias current of the input stage. If R7 is reduced, the current will increase and the conversion speed will be faster. If R1 is equal to R11, the offset voltage will decrease. To completely adjust to zero, a variable resistor must be added to the emitter circuit of TT1.2.

The bias current of the output stage is adjusted by VR1, and the drain current of each output transistor is about 100mA on average.