Emitter Coupled Clipper Circuit:

An emitter coupled clipper using two transistors is shown in Fig. 30.71. It is a two level clipper in which the input is applied to the base of one of the transistors and the base of the other transistor is kept at a fixed voltage.

Consider initially that input voltage v_in is negative enough to ensure that transistor Q₁ is in cutoff. In this condition only transistor Q₂ carries current. Let us also consider that V_BB2 has been adjusted so that transistor Q₂ is in its active region and current in the emitter resistance R_E remains constant. This will be valid only if

As the input voltage rises, transistor Q₁ will eventually come out of cutoff, both transistors will be carrying current, but current in transistor Q₂ decreases while current in transistor Q₁ increases, the sum of currents in the two transistors remaining unchanged and equal to emitter resistance current I. The input signal appears at the output, amplified but not inverted.

As input voltage v_in continues its excursion in positive direction, the common emitter follows the base of transistor Q₁. As the base of transistor Q₂ is fixed, a point will be reached when the rising emitter voltage cuts off transistor Q₂. In summary, the input signal is amplified but limited twice, once by the cutoff of transistor Q₁ and once by the onset of cutoff in transistor Q₂.

The total range Δv_out over which the output can follow the input is IR_E and is, therefore, adjustable through the adjustment of current I.

The absolute voltage of the portion of the input waveform selected for transmission may be selected through an adjustment of a biasing voltage on which v_in is superimposed or through an adjustment of V_BB2. The total range of input voltage Δv_in between clipping limits is Δv_out/A, where A is the gain of the amplifier stage. The transfer characteristic of an emitter coupled clipper is given in Fig. 30.72.