The Transistor as a Mixer

This work contains analyses of both point-contact and junction transistors as mixers. Experimental results for signals up to 400 megacycles are presented. The conversion ability of transistor mixers is dependent on the IF and signal frequencies. The IF frequency is limited to the region of amplification of the transistor. The signal frequency range is ultimately limited by the capacitance which shunts the emitter when it is biased in the reverse direction. The analysis is simplified by dividing the signal frequency spectrum into three regions as follows: in the low-frequency range conversion is strongly dependent on alpha; in the medium-frequency range conversion depends primarily on base resistance; in the highfrequency range conversion is mainly dependent on emitter reverse shunting capacitance. The conversion gain influences the noise factor. At low frequencies, a junction transistor mixer equals the performance of a crystal diode mixer followed by a junction transistor amplifier. At vhf frequencies conversion gain may still be obtained if point-contact transistors are used; however, at present, their noise factor is higher than that which can be obtained by the combination of the diode and the transistor amplifier.