11.1 Introduction – Pulse and Digital Circuits


A sampling gate is a transmission circuit that faithfully transmits an input signal to the output for a finite time duration which is decided by an external signal, called a gating signal (normally rectangular in shape), as shown in Fig. 11.1.

The input appears without a distortion at the output, but is available for a time duration T and afterwards the signal is zero. They can transmit more number of signals. The main applications of the sampling gates are: (i) multiplexers; (ii) choppers; (iii) D/A converter; (iv) sample and hold circuits, etc. Sampling gates can be of two types:

  1. Unidirectional gates: These gates transmit the signals of only one polarity.
  2. Bidirectional gates: These gates transmit bidirectional signals (i.e., positive and negative signals).

Earlier, we had seen logic gates in which the output, depending on the input conditions, is either a 1 level or a 0 level. That is, the inputs and outputs are discrete in nature. In a sampling gate, however, the output is a faithful replica of the input. Hence, sampling gates are also called linear gates, transmission gates or time selection circuits. Linear gates can use either a series switch, as shown in Fig. 11.2(a) or a shunt switch, as shown in Fig. 11.2(b). In Fig. 11.2(a), only when the switch closes, the input signal is transmitted to the output. In Fig. 11.2(b), only when the switch is open the input is transmitted to the output.

FIGURE 11.1 A sampling gate

FIGURE 11.2 A linear gate (a) using a series switch; and (b) using a shunt switch