Temperature compensated amplifier

A radio device, a transmitter of which has a temperature compensated amplifier with variable gain to set the transmitting power and to keep the transmitting power independent of the temperature, the amplifier comprising at least one differential pair, total current of which being arranged to be dependent on input signal of the amplifier and one collector current of which being output signal of the amplifier, the amplifier further comprising a control circuit to change a control of transistors of the differential pair

• so that the ratio of said one collector current to the total current, or current gain, changes corresponding to changes in a gain control signal and
• as a function of the temperature so that the effect of the temperature change on the collector currents inside the differential pair is substantially compensated, wherein;
• the control circuit has a differential output, first terminal of which is connected to base of first transistor of the differential pair and second terminal of which is connected to base of second transistor of the differential pair,
• the control circuit comprises a first and a second current source, the current difference of which sources can be varied on both sides of zero by means of said gain control signal,
• output voltage of the control circuit is proportional to said current difference, the output voltage can thus be varied on both sides of zero to set the current gain to be more or less than 0.5,
• the currents of the first and second current source are arranged to be dependent on the temperature, wherein a change in the output voltage of the control circuit, being due to a temperature change, depends on both temperature difference and said current difference and causes changes in the collector currents of the differential pair, which changes have opposite sign compared with the immediate effect of said temperature change on said collector currents inside the differential pair, and
• the control circuit is further arranged such that the above-mentioned changes with opposite signs in each collector current of the differential pair are substantially equal regarding their absolute values.