DIODE LOGIC

Diodes

A semiconductor diode is fabricated from two types of semiconductor material, called p-type and n-type that are brought into contact with each other as shown in Fig.1 (a). This is basically the same material that is used in p-channel and n-channel MOS transistors. The point of contact between the p and n materials is called a pn junction. (Actually, a diode is normally fabricated from a single monolithic crystal of semiconductor material in which the two halves are “doped” with different impurities to give them p-type and n-type properties.)

The physical properties of a pn junction are such that positive current can easily flow from the p-type material to the n-type. If the circuit is built as shown in Fig.1 (b), the pn junction acts almost like a short circuit. However, the physical properties also make it very difficult for positive current to flow in the opposite direction, from n to p. Also in the circuit of Fig.1 (c), the pn junction behaves almost like an open circuit. This is called diode action.

Although it’s possible to build vacuum tubes and other devices that exhibit diode action, modern systems use pn junctions—semiconductor diodes—which we’ll henceforth call simply diodes. Fig.2 (a) shows the schematic symbol for a diode. In normal operation significant amounts of current can flow only in the direction indicated by the two arrows, from anode to cathode. In effect, the diode acts like a short circuit as long as the voltage across the anode-to-cathode junction is nonnegative. If the anode-to-cathode voltage is negative, the diode acts like an open circuit and no current flows.

The transfer characteristic of an ideal diode is as shown in Fig.2(b). If the anode-to-cathode voltage, V, is negative, the diode is said to be reverse biased and the current I through the diode is zero. If V is nonnegative, the diode is said to be forward biased and I can be an arbitrarily large positive value. But V can never get larger than zero, because an ideal diode acts like a zero-resistance short circuit when forward biased.

A nonideal, real diode has a resistance that is less than infinity when reverse biased, and greater than zero when forward biased, so the transfer characteristic looks like as shown in Fig.2(c). When forward biased, the diode acts like a small nonlinear resistance; its voltage drop increases as current increases, but not strictly proportional. When the diode is reverse biased, a small amount of negative leakage current flows. If the voltage is made too negative, the diode breaks down, and large amounts of negative current can flow; in most applications, this type of operation is avoided.

 A real diode can be modeled very simply as shown in Fig.3(a) and (b). When the diode is reverse biased, it acts like an open circuit; we ignore leakage current. When the diode is forward biased, it acts like a small resistance, Rf, in series with Vd, a small voltage source. Rf is called the forward resistance of the diode, and Vd is called a diode-drop.

Careful choice of values for Rf and Vd yields a reasonable piecewise-linear approximation to the real diode transfer characteristic, as in Fig.3(c). In a typical small-signal diode such as a 1N914, the forward resistance Rf is about 25 W and the diode-drop Vd is about 0.6 V.

A real diode does not actually contain the 0.6-V source that appears in the model. Due to the nonlinearity of the real diode’s transfer characteristic, significant amounts of current do not begin to flow until the diode’s forward voltage V has reached about 0.6 V.

In typical applications, the 25-W forward resistance of the diode is small compared to other resistances in the circuit, so that very little additional voltage drop occurs across the forward-biased diode once V has reached 0.6 V. Hence practically, a forward-biased diode may be considered to have a fixed drop of 0.6 V.

Diode Logic

Diode action can be exploited to perform logical operations. Consider a logic system with a 5-V power supply and the characteristics as shown in Table-1. Within the 5-volt range, signal voltages are partitioned into two ranges, LOW and HIGH, with a 1-volt noise margin between. A voltage in the LOW range is considered to be logic 0, and a voltage in the HIGH range is logic 1.

With these definitions, a diode AND gate can be constructed as shown in Fig.4(a). Consider both inputs X and Y are connected to HIGH voltage sources, say 4 V, so that VX and VY are both 4 V as in (b). Then both diodes are forward biased, and the output voltage VZ is one diode-drop above 4 V, or about 4.6 V. A small amount of current, determined by the value of R, flows from the 5-V supply through the two diodes and into the 4-V sources.

Suppose that VX drops to 1 V as shown in Fig.4(c). In the diode AND gate, the output voltage equals the lower of the two input voltages plus a diode drop. Thus, VZ drops to 1.6 V, and diode D2 is reverse biased (the anode is at 1.6 V and the cathode is still at 4 V). The single LOW input “pulls down” the output of the diode AND gate to a LOW value. Two LOW inputs create a LOW output as shown in Fig.4(d) and is repeated in terms of binary logic values in Fig.4 (e) for an AND gate.

Fig.5 (a) shows a logic circuit with two AND gates connected together and the equivalent electrical circuit with a particular set of input values is as shown in Fig.5 (b) which explains the necessity of diodes in the AND circuit D3 allows the output Z of the first AND gate to remain HIGH while the output C of the second AND gate is being pulled LOW by input B through D4.

When diode logic gates are cascaded as shown in Fig.5, the voltage levels of the logic signals move away from the power-supply rails and towards the undefined region. Practically, a diode AND gate normally must be followed by a transistor amplifier to restore the logic levels; this is the scheme used in TTL NAND gates.

  

With these definitions, a diode OR gate can be constructed as shown in Fig.6. Consider both inputs A and B are connected to HIGH voltage sources, say 4 V, so that VA and VB are both 4 V. Then both diodes are forward biased, and the output voltage across output is one diode-drop below 4 V, or about 3.4 V. A small amount of current, determined by the value of R, flows to the GND through the two diodes.

Suppose that VA drops to 1 V, in the diode OR gate, the output voltage equals the higher of the two input voltages minus a diode drop. Thus, output drops to 4-0.6V = 3.4V. The single LOW input “pulls up” the output of the diode OR gate to a HIGH value.

Two LOW inputs will produce a LOW output as the output will be at 1V-0.6V = 0.4V (LOW).