100W power amplifier with current limiting protection circuit

Function of the circuit

Power amplifiers with an effective output of 100W are often used in audio or ultrasonic equipment. Since this circuit is composed entirely of discrete components, there is a large degree of freedom in design. Because the emitter follower will cause a large short-circuit current due to a load short circuit, current limiting protection measures must be adopted. In addition, high power output can also be obtained by replacing the output transistor or load resistor .

How the Circuit Works

The heat dissipation of the output transistor is determined by the allowable power loss and the allowable ambient temperature TOP. If TOP = 80 degrees, a heat sink with a thermal resistance of 1.8 degrees/W is required.

Since complementary emitter followers are added in the two-stage differential circuit, large current can be supplied to a small load.

The second-stage differential amplifier circuit TT6T needs to have an output voltage amplitude of 100W, so a current Miller circuit is added to effectively utilize the power supply voltage and convert the differential output into a single-ended output.

TT10 is a bias circuit. In order to eliminate the dead zone of the voltage VBE between the base and emitter of the output circuit, this circuit generates a voltage of 4VBE, and VR1 can be used to adjust the collector current (about 50MA) when there is no signal.

TT11.12 is a current limiting circuit, which divides the voltage generated on the emitter resistor of the output crystal, inputs it into the base, and absorbs the base current of TT12.14 to limit the output current.

The voltage amplification factor AV of this circuit is determined by the formula AV=1+(R11/R10), which is about 40 times. If the resistance value is changed, the amplification factor can also be reduced, but in this case, the capacity of the phase compensation capacitor C4 must be slightly increased.

The CR series circuit connected in parallel at the output end plays a compensation role to prevent the load from affecting the stability of the circuit.