Power MOSFET amplifier with high voltage of 120VPP

Energy of circuit

To make the amplified waveform similar to the input waveform, the amplifier circuit must use a symmetrical circuit, and the characteristics of the active components must be consistent. This circuit is an amplifier circuit with small waveform distortion. All levels are push-pull symmetrical circuits, which can obtain a pulse response with basically equal time constants on the rising and falling edges and a very high conversion speed.

Power MOSFETs are used as the power components of the output stage to improve high-frequency characteristics and prevent damage to components due to abnormal loads.

How the Circuit Works

This circuit consists of four parts. The input stage is a dual differential circuit of an NPN transistor differential amplifier and a PNP transistor differential amplifier. The two amplifiers have balanced outputs and drive the voltage amplifier stage.

The voltage amplifier stage is an important component that determines the output amplitude and conversion speed, and should be operated at a large bias current as much as possible. Since no phase compensation capacitor is required between the collector and base of TT5 and TT6 , the decrease in conversion speed can be reduced.

TT7 is a bias circuit used to eliminate the base voltage dead zone of the output stage. VR2 can be used to adjust the no-signal current of the output stage.

TT8 and TT9 are circuits used to limit the maximum drain current of the output stage, and play a role in load short-circuit protection. R10 and R21 divide the voltage across R17. If the voltage exceeds 0.5V, the transistor will be turned on, and TT8 will absorb the base current of TT10, thereby limiting the current within a certain range.

The power FET used in the output stage has a high switching speed of 10~25NS, but when used in the amplifier circuit, the input capacitance is large and must be driven with high power. Therefore, TT10 and TT11 are used to form an emitter output circuit for strong driving. R25 and R15 of the output stage gate are used to prevent high-frequency oscillation. Within the range allowed by stability, the resistance values ​​of R25 and R26 should be as small as possible.

Application Notes

Although this circuit is designed to drive a 50 ohm load, it can also drive a load with a lower resistance. The focus is on the allowable power loss of the output stage. When VCO=140V, it cannot work under large drain current conditions, so the power supply voltage is reduced when designing the circuit.

The resistors R7 and R8 used to determine the input stage working current , and the output stage resistors R27 and R28 have a resistance value of 1/10 to 1/20 of their load resistance respectively. Use VR2 to adjust the amplifier stage bias current to about 0.1A.