Abstract: A protective circuit effective for use with an output capacitor less type transistor amplifier includes a load impedance detecting circuit and a detecting circuit for detecting an undesirable DC voltage applied to a load. The load impedance detecting circuit comprises first and second DC bridge circuits for detecting a load impedance, and the DC voltage detecting circuit comprises third and fourth DC bridge circuits. Both the load and amplifier are protected by outputs from the first through fourth bridge circuits so as to be free from a load-shorted state and an undesirable DC voltage.

Abstract: A transistor amplifier comprising a bipolar transistor having an input electrode and a pair of output electrodes, one of the output electrodes being coupled to the gate electrode of a field effect transistor, the field effect transistor having triode-type dynamic characteristics. Respective sources of operating potential are coupled to the bipolar and field effect transistors. To stabilize the DC bias current flowing through the field effect transistor in the event of fluctuations of the operating potential supplied thereto, a portion of such fluctuations is injected into the other output electrode of the bipolar transistor. The field effect transistor is adapted to drive a load impedance.

Abstract: A transistor amplifier comprised of a field effect transistor having triode-type dynamic characteristics in combination with a bipolar transistor. The current flowing through the field effect transistor is a function of an input signal. The bipolar transistor is substantially nonconductive until the field effect transistor current reaches a threshold level and then current flows through the bipolar transistor as a function of the input signal. The currents produced by the field effect and bipolar transistors are supplied to a load.

Abstract: A transistor amplifier comprised of a transistor supplied with a first operating voltage and a field effect transistor having triode-type dynamic characteristics and supplied with a second operating voltage. The gate electrode of the field effect transistor is coupled to the transistor collector. In order to stabilize the field effect transistor bias current in the event of fluctuations in the second operating voltage, a voltage control circuit is responsive to such fluctuations to alter the first operating voltage so as to vary the gate voltage applied to the field effect transistor in a direction to stabilize the bias current.