4.2 DIODES AS SWITCHES
A physical switch either makes or breaks a contact between two nodes, meaning the resistance between the nodes is either 0 or ∞. When electronic devices are used as switches, no physical contact is either made or broken; the resistance between the nodes is made either too small (ideally 0) or too large (ideally ∞). Diodes can be used as switches. We shall discuss the application of semiconductor and Zener diodes as switches in this section.
A semiconductor diode, used as a switch, is ON when forward-biased and OFF when reverse-biased. As such, this device may be used as a static switch. The voltage between the anode (A) and the cathode (K) is ideally zero; and hence, the resistance is zero when the switch is closed. In practice, however, there is a finite forward resistance (RF), typically of a few ohms, as shown in Fig. 4.1(a). When the switch is open, ideally the current should be zero and the resistance infinity. However, in actuality, a diode will have a finite reverse resistance, Rr which is typically few mega ohms, as shown in Fig. 4.1(b).
Figure 4.2 shows the typical V–I characteristic of a semiconductor diode. Let the reverse saturation current be typically 50 μA. When compared to this, the forward current is as large as 100 mA. Hence, the reverse current is negligible when compared to the forward current. This diode can, thus, be used as a one-way device, i.e., as a switch. Let the diode be an ideal diode. The V–I characteristic of an ideal diode is represented in Fig. 4.3.
FIGURE 4.1 (a) Diode as a closed switch (b) Diode as an open switch
FIGURE 4.2 The V–I characteristic of a practical diode
FIGURE 4.3 The V–I characteristic of an ideal diode
FIGURE 4.4(a) The idealized V–I characteristic of a silicon diode
FIGURE 4.4(b) The idealized V–I characteristic of a germanium diode
FIGURE 4.5(a) A simple diode circuit
FIGURE 4.5(b) Suddenly changing input
FIGURE 4.5(c) Diode currents
During the period 0 to T1, the input forward-biases the diode. If the forward resistance of D is very small when compared to RL, then IF = VF/RL.
At T1, the polarity of the input reverses and the reverse voltage is VR. Thus, IR (= −VR/RL) remains large for a time duration called the storage time ts, though the diode is expected to go into the OFF state at T1. This is because of the presence of a large number of stored charges on either side of the junction in the forward-biased diode. After this time interval, charges get cleaned up, gradually reducing the current to a value Is after a time interval tt called the transition time. At this instant, the diode is said to be switched from the ON state into the OFF state. It can be seen from Fig. 4.5(c) that a finite time elapses before the current is the reverse saturation current. This indicates that a diode is switched from the ON state into OFF state not exactly at T1, when the input reverse-biases the diode but only after a time interval when the reverse current reaches Is. The time interval (ts + tt) is called the reverse recovery time of the diode, trr.
This is the time interval for which the switch is still ON. The reverse recovery time, trr, may thus be defined as the time taken for the diode reverse current to fall to 10 per cent of its forward-current value when the diode is suddenly switched from the ON to the OFF state. If the forward current IF, when the diode is ON, is 10 mA, then the time taken for this current to fall to 1 mA is the reverse recovery time. This can also be termed as the “turn-off” time of the diode. Similarly, when the device switches from the OFF state to the ON state, there is a small turn-on time, which is small when compared with the turn-off time. These time intervals tell us how fast we can switch the diode from one state to the other. Further, a reverse-biased diode has a transition capacitance CT between the anode and the cathode. At low frequencies, this capacitance has no appreciable influence. However, at high frequencies, this offers low reactance, which will have to be taken into consideration when we consider diode series and shunt clippers.
4.2.2 The Zener Diode as a Switch
FIGURE 4.6(a) The V–I characteristic of a Zener diode
FIGURE 4.6(b) Idealized V-I characteristic of a Zener diode
An ideal Zener diode is ON for VF = 0, whereas a practical Si-Zener conducts when VF = 0.7 V. It is also switched ON when reverse-biased and when VR is VZ. Normally, a Zener diode is always operated in the reverse-biased condition. It can be considered as a switch that is open for VR < VZ and closed for VR = VZ. These switches can be used to alter the shape of an input signal.