Digital circuits deal, in principle, with only two values of voltage, whereas analog circuits
process signals with continuous variation of voltage. In fact, of course, no macroscopic signal is truly quantized, so even a digital circuit designer needs some familiarity with analog electronics. These notes will provide a very basic introduction to the capabilities of analog circuitry. You may also need to review the elementary analysis of passive DC and AC circuits as found in a freshman physics text.
process signals with continuous variation of voltage. In fact, of course, no macroscopic signal is truly quantized, so even a digital circuit designer needs some familiarity with analog electronics. These notes will provide a very basic introduction to the capabilities of analog circuitry. You may also need to review the elementary analysis of passive DC and AC circuits as found in a freshman physics text.
A. Transistor action :
Fig. 1 Junction diode structure, circuit symbol and I-V characteristic.
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Transistors of various types are essential to most modern electronics. They, in turn,depend upon the ability to fabricate semiconducting materials to very precise specifications. We begin, therefore, by discussing very crudely how bipolar transistors function. Descriptions of other types of transistors and more quantitative treatments can be found in the suggested readings.
Semiconductors, as the name implies, are not insulators but neither do they conduct as well as metals. (Commercially important semiconductors include the elements silicon and germanium, and compounds gallium arsenide and gallium phosphide.) The reason for this is quite simple: there are relatively few mobile charge carriers in a pure semiconductor at room temperature, and none at absolute zero. Semiconductors are technically useful because the density of charge carriers, and hence the conductivity, is exquisitely sensitive to part-per-million levels of impurities, referred to as "dopants". Furthermore, by appropriate choice of the dopant, either positive or negative charge carriers can be introduced. A specimen with predominantly positive charge carriers is referred to as "p-type", while a specimen with negative carriers is "ntype". (The negative carriers are electrons, as in metals. The positive carriers are "holes", empty states in an otherwise filled sea of electrons. The existence of holes with an effective positive charge is a consequence of the Pauli exclusion principle acting in a crystalline lattice. A complete explanation can be found in any solid state physics text.)
A piece of p-type material in intimate electrical contact with a piece of n-type material forms a "pn junction" in the region of contact. If wires are attached to the sandwich, as indicated in Fig.1, it is found that the current-voltage characteristic is very asymmetric. The resulting device is called a "diode". For circuit use, two features of the diode I-V characteristic are important. First, there is a threshold (about 0.6V in silicon) before significant current flows in the Fig. 1 Junction diode structure, circuit symbol and I-V characteristic.
forward direction. This threshold is the "junction voltage drop" or just "junction drop". For a "forward biased" diode above threshold the current increases very rapidly with little increase in applied voltage. Second, there is a small "leakage current" when the diode is "reverse biased" (n side positive with respect to p side). This phenomenon is usually only a minor nuisance.
A more interesting device can be made by joining three layers as shown in Fig. 2 to create an NPN bipolar transistor. (The term "bipolar" refers to the use of both n and p-type material in the structure. The following description also holds for the analogous PNP transistor if all polarities are reversed. Other designs are possible, but will not be discussed here.) Evidently the base-emitter and base-collector circuits will behave like diodes. The unexpected fact is that if the collector is made positive with respect to the emitter, then a current in the base-emitter circuit can control the current flow across the (reverse biased) collector-base junction. For small base currents the relationship is linear
where the current gain, hFE or , is typically 10-100. This current gain actually represents a power gain, in the sense that a low-power input signal applied to the base can cause a higher power signal to appear in the collector circuit.
Fig. 2 NPN junction transistor structure and circuit symbol
B. Gate circuits :
As a first example of transistor circuitry, consider Fig. 3, which shows the commonemitter configuration of an NPN transistor. The supply voltage VCC is taken to be positive. If Vin is zero or negative there will be no base current, hence no collector current, and the transistor is said to be cut off. When Vin is positive and greater than the junction drop (0.6 V for silicon), base current will start to flow, the collector current will increase according to Eq. 1 and VCE will decrease. For sufficiently large IB, VCE reaches a minimum (0.1-0.2 V for silicon) and IC is limited chiefly by the load resistor R2. The transistor is now fully on, and is said to be saturated. The ability to switch between two well defined states is ideal for implementing digital logic.
As a first example of transistor circuitry, consider Fig. 3, which shows the commonemitter configuration of an NPN transistor. The supply voltage VCC is taken to be positive. If Vin is zero or negative there will be no base current, hence no collector current, and the transistor is said to be cut off. When Vin is positive and greater than the junction drop (0.6 V for silicon), base current will start to flow, the collector current will increase according to Eq. 1 and VCE will decrease. For sufficiently large IB, VCE reaches a minimum (0.1-0.2 V for silicon) and IC is limited chiefly by the load resistor R2. The transistor is now fully on, and is said to be saturated. The ability to switch between two well defined states is ideal for implementing digital logic.
The common-emitter configuration is itself a possible design for a NOT circuit. Suppose we define logic 0 as a zero-volt signal and logic 1 as VCC, a positive voltage. Referring again to Fig. 3, when Vin = 0 the transistor is cut off and Vout = VCC. Conversely, if we choose R1correctly the transistor can be driven to saturation when Vin = VCC. Vout is then nearly zero(actually VCE at saturation). More simply, Vout is the logical converse of Vin, as claimed.
A NOR circuit can be constructed using two transistors in common emitter configurations, as shown in Fig. 4. A logic high applied to either input (or both) turns on Q1 or Q2, pulling the output low, which is the required function. An OR circuit could be made bydriving the NOT circuit just described with the output of the NOR.
The TTL circuits used for the digital exercises also employ transistors as switches, although the circuit configuration is somewhat different to facilitate manufacture. The design rules governing fan-out and logic levels are all determined by the circuit arrangements chosen to implement the various logic functions.
A NOR circuit can be constructed using two transistors in common emitter configurations, as shown in Fig. 4. A logic high applied to either input (or both) turns on Q1 or Q2, pulling the output low, which is the required function. An OR circuit could be made bydriving the NOT circuit just described with the output of the NOR.
The TTL circuits used for the digital exercises also employ transistors as switches, although the circuit configuration is somewhat different to facilitate manufacture. The design rules governing fan-out and logic levels are all determined by the circuit arrangements chosen to implement the various logic functions.
Fig. 3 NPN transistor in common-emitter configuration
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