A unidirectional diode gate which transmits only the positive – going input signals as shown in below figure.

The gate signal which determines the gating or transmission period is a rectangular waveform that makes abrupt transitions between the two negative level -v_1 and -v_2 

The gate signal is also called a control pulse, a selector pulse or an enabling pulse.

When the gating signal is at its lower level -V2, the diode is heavily back biased and there will be no output due to the peak amplitude of the input signal is larger than the magnitude of this back-biasing voltage.

When the gate signal is at its upper level   -V2, a time-coincident signal input pulse may be transmitted to the output.
The gating effect of the upper level of the gating signal on the output is illustrated in the figure.


The input is +10 v pulse. 

In figure (a), when the gate pulse has –v2 = -20v and –v1 = -10v, there is no output pulse at all.

In figure (b), when   –v2 = -20v and –v1 = -5v, the output pulse is a +5v pulse.

In figure (c), when   –v2 = -20v and –v1 = -5v, the output is a +10v pulse.

In figure (d), when   –v2 = -20v and –v1 = 0v, the output is a +10v pulse superimposed on a pedestal of +5v.

Actually the waveforms with vertical edges shown in the figure are unrealistic because the R1C1 network constitutes an integrating network for the gate waveform and therefore the gating signal will have exponentially raising and falling edges as shown in figure (e).

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