Transmitting/receiving unit having automatic gain control system with temperature compensation

A transmitting/receiving unit comprising:

• a signal transmitting portion;
• and
• a signal receiving portion for generating an output signal, a level of the output signal being determined by a level of a received signal;
• wherein said signal transmitting portion includes first variable-gain amplifying means having an amplified gain which is varied in response to a first automatic gain control voltage;
• wherein said signal receiving portion includes:
• second variable-gain amplifying means having an amplified gain which is varied in response to a second automatic gain control voltage,
• wave-detection means for generating a DC voltage which is proportional to a level of an output signal generated by said second variable-gain amplifying means, and
• temperature-dependent-type automatic gain control voltage generating means for converting said DC voltage into an automatic gain control voltage which is determined by a change in environmental temperature;
• and
• wherein said automatic gain control voltage supplied from said temperature-dependent-type automatic gain control voltage generating means is used as said first and second automatic gain control voltages to compensate a temperature change of said gain so that said level of said output signal corresponds to said level of said received signal regardless of the change in the environmental temperature;
• wherein said temperature-dependent-type automatic gain control voltage generating means comprises:
• a first transistor difference amplifying stage having a common emitter, a first base to which said DC voltage is supplied, and a second base to which first reference voltage is supplied,
• a temperature compensating voltage generating circuit including a thermosensitive device for generating a voltage which depends upon the change in the environmental temperature, and
• a voltage adding stage for summing output voltages from said first transistor difference amplifying stage and said temperature compensating voltage generating circuit to generate the automatic gain control voltage which depends upon the change in the environmental temperature.