Protective circuit for a power amplifier

A protective circuit for a push-pull power amplifier including a load current-detecting resistor connected between two transistors jointly constituting said power amplifier for detecting the load current of said power amplifier;

• a load connected to said power amplifier through said load current-detecting resistor;
• a switch for shutting off said load current;
• a load resistance detecting circuit for detecting the resistance of said load;
• and a driving circuit for causing said switch to be opened by an output of said load resistance detecting circuit when the resistance of said load falls to a lower level than prescribed, characterized in that said load resistance-detecting circuit comprises circuit means for drawing out a first signal from one end of said load current-detecting resistor;
• circuit means for drawing out a second signal of lower potential than said first signal from the other end of said load current-detecting resistor;
• and control circuit means, which, when said load has a higher resistance than prescribed, holds the potential difference between said first and second signals to be lower than a predetermined level by causing the potential of said first singal to drop as a result of control on said first signal by said second signal of lower potential than said first signal, and, when said load has a lower resistance than prescribed, increases the potential difference between said first and second signals over said predetermined level;
• and said driving circuit is supplied with said first and second signals, and includes circuit means for opening said switch, when the potential difference between said first and second signals has a larger value than said predetermined value.