Electronics Made Easy - a Complete Introduction to Electronics - Martin Denny [5]
One by-product of the P-N junction is that as the depletion layer increases in width so does the capacitance, therefore the P-N junction becomes a voltage dependant capacitor.
DISCRETE COMPONENTS
The Diode
The characteristic of the P-N junction allows current to pass in one direction (forward bias) while it becomes a high impedance in the other direction (reverse bias). The forward bias necessary to overcome the barrier potential is approximately 0.2V for germanium and 0.6V for silicon. Most modern semiconductor devices use silicon.
The Zener Diode
Diodes which are designed with adequate power dissipation to operate in the breakdown region, are known as avalanche breakdown or zener diodes. The diode is capable of regulating the output voltage from a relatively low current (Iz) to the point at which Iz = Pt/Vb.
The Transistor
The transistor can be considered as two p-n junctions arranged p-n-p or n-p-n. A p-n junction with a forward bias has a low resistance and conversely a high resistance with reverse bias. If a small control current is introduced at the mid point of the two junctions and sufficient current fed in to forward bias one junction and reverse bias the other, then a current should pass through the device. The collector current (Ic) is much greater than the control current (Ib) this factor is known as Gain (hfe),
Before a transistor can act as an amplifier the DC bias conditions and gain must be set so that the resulting waveform is of the correct amplitude. Another consideration is the input and output impedance of the stage. The previous stage must be able to drive the input at the transistor base via a decoupling capacitor, and the transistor drive its output load.
The same calculations are true for NPN, PNP, and even Darlington configured transistors. The Darlington configuration is used when high gain is combined with high power. Power transistors generally have a low gain. If a medium power transistor is combined in the same package high gain and high power output can be achieved. The only significant difference to be taken into account is that the Vbe voltage of the first stage must be added to that of the second.
Fig 5a shows the simplest biasing arrangement; the voltage at the collector is controlled by the value of R2 and the current passing through it (Ic). For maximum output without saturation Vc = 0.5Vs, the maximum value of Ic should not be exceeded when the transistor is fully switched on (ie Vc equal to approximately 0V). The supply voltage must also be taken into account so the maximum Vce voltage is not exceeded.
Calculating the DC Bias of the amplifier. (R1 and R2):
R2 = (Vs - Vc )/ Ic Where: Vs is the supply voltage.
Vc is the collector voltage, (0.5 * Vs).
Current Gain hfe = Ic/Ib Ic is the desired collector current.
Ib is the base current.
Ib = Ic/hfe (see transistor data sheet)
R1 = (Vs - Vbe)/Ib
Note, this configuration limits Vin to less than 1.2V pkpk
The AC Circuit (Voltage Gain):
The ac equivalent circuit of the amplifier can be described as the dc circuit where the 0v and +Vs rails are considered to be at the same potential, and the transistor can be considered as a device with input impedance hie and gain hfeIb, (note, hfe*Ib = Ic). Figure 6 shows a simplified circuit. In this case the values of Re and Cz are zero.
Selection of coupling capacitor and bandwidth:
An approximation of the input impedance can be made by assuming that the effect of hie (data not generally available) is small and the major effect will be that of the collector voltage and current and any load resistors present. For the purposes of selecting a coupling capacitor:
Rin º Vc/Ib*hfe º Vc/Ic
C1 must be chosen to have a value of xc considerably less than the input impedance of the amplifier over the desired frequency range of the amplifier. In this case the lower frequency must be considered.