Current Mode Switching

AC linked current mode switching. 

Circuit 1:

R1 and R2 control the current flowing through D1, altering its impedance (eg. 26 Ohms at 1 mA). This reduces the impedance the emitter of Q1 sees thereby increasing its gain depending on the current through D1.

Circuit 2:

An AM modulator making use of gain modulation by the current through Q3.

AC linked differential pair.  The value of C1 has to be very high for audio frequencies in order to transmit the low impedance of the emitter of Q2 to Q1 and vice-versa. 

Current mode circuits typically show a Tanh gain response.  This is useful for low distortion oscillator circuits where the reduction in average gain with signal strength. This avoids the extreme distortion of supply rail clipping.

The reduction in gain with signal strength is also useful for controlled positive feedback circuits such a regenerative radio receivers.  Reduction in gain means reduction in regeneration (positive feedback), preventing the circuit from suddenly popping into supply rail limited oscillation and allows smooth control of the regenerative effect.