common base mixer

common base mixer Mixing of audio signals is normally effected by means of a so-called virtual-earth mixer, in which the various input signals are applied to the virtual earth, that is, the inverting input of an op-amp, via a series of resistors. The mixer described here R8 N T7 L 7-84 uses a different approach. The circuit is designed on the com- mon base principle in which the input voltages are transformed into alternating currents which, when added together, constitute the collec- tor a.c. component. The emitter of a common base configuration is low impedance and acts as virtual earth, so that crosstalk between the various input signals is virtually impossible. The output signal is taken from the collector of Ti. The amplification of the circuit is equal to R6 = Ri where Ri is the input resistance (= one of the resistors R1 ... R5). A current source, consisting of T2 and T3, has been provided in the emitter circuit of Ti. This current source is high impedance for alternating v...