Electronics Made Easy - a Complete Introduction to Electronics - Martin Denny [7]
Let R2 = 30KΩ then i = 86.7µA
Current through R1 = 86.7µA + 13.3µA = 100µA.
Voltage across R1 = (13.6 - 2.6)V
Then R1 = 11/0.1 KΩ = 110KΩ.
Vc = 7.6v to estimate Rin, Rin º Vc/Ic = 7.6/2 KΩ = 3K8Ω
Selecting C1 and C3:
Try 1µF, then xc = 1/2πfc = 1/2π10*1 MΩ
xc = 15K9, If C = 10mF then xc = 1K59Ω.
Select C1 and C3 10mF, as R4 = 1KΩ Let C2 = 22µF (xc = 723Ω)
Current Gain 150, With output loading Rin (stage 2) º 3K8Ω
For voltage gain G1 (no load):
Vout1 = hfeIb * R3 (see fig 6).
When loaded:
Vout2 = hfeIb * R3Rin/(R3+Rin).
With no load G1 ≡ hfe therefore G1 = hfe Vout2/Vout1
G1 = hfeR3Rin/R3(Rin+R3) = hfeRin/(R3 + Rin)
G1 = 150 * 3k8/6K8 = 84 approx
If the same design is used for stage 2: G2 = 150 unloaded
Total Gain = G1*G2 = 150*84 = 12600 and Overall Gain = G1*G2/β
β =R5/(R10 + R5) to (Vr1 + R5)/(R10 + R5 + Vr1)
For gain 250, β = 250/12600 = 1/50.4, For gain 500, β = 1/25.2
Let Vr1 = 200Ω then R5 = 200Ω and gain 10KΩ nominal
Correcting G2: G2 = hfe(R10+R5)/(R8 + R10 + R5) =150*10K2/13K2Ω
G2 = 116 approx
G1*G2 = 9736, β = 1/39 and 1/19.5
β = 200/(R10 + 200)
R10 + 200 = 200*39 therefore R10 = 7K6Ω
R10 = 7K5Ω preferred value.
Component List
As a practical amplifier this design has many drawbacks:
1) Without testing each transistor before use an accurate estimate of gain and input impedance is impossible.
2) The value of Vr1 and R5 must be kept low in comparison R8 and R9.
3) As the transistor has a relatively low input impedance multi-stage design becomes difficult as illustrated in the previous example.
Transistors have a relatively high frequency response and are frequently used in oscillator and in high frequency amplifier design. Other uses include the output stage in power amplifiers where the collector drives the load (loudspeaker) directly or via a capacitor.
The Transistor as a Switch.
For a transistor to operate as a switch the collector voltage should swing between 0 and the supply voltage (Vs), see figure 8.
When Vc is 0v, Tr1 is switched on, Ic = Vs/R2
Ib ³ Ic/hfe min and R1 = (Vin-Vbe)/Ib
In the switch mode the transistor is ideal, the base resistor can be selected to saturate the transistor at the minimum value of hfe and presents a high input impedance.
The transistor in switch mode is used extensively for power regulation (voltage and current), switching relays, and sometimes as a buffer between dissimilar logic circuits.
Power Dissipation.
In the previous examples power dissipation has been low in comparison with the component ratings. When a transistor is operated near its maximum power rating it may become necessary to provide a heat sink. Heat sinks are designed to present the maximum surface area to the surrounding air and are constructed using a good heat conductor (usually aluminium which is light and easy to mould into intricate shapes). If the transistor is housed in a metal box it is sometimes convenient to secure the transistor directly to the case, if this is done care must be taken to isolate the transistor electrically as the transistor body is generally connected to the collector.
The Field Effect Transistor.
The FET is a semiconductor where the control of current is by an electric field. There are two types of field effect transistors, the junction field effect transistor (JFET or FET) and he insulated gate field effect transistor (MOSFET).
The FET has many advantages over conventional transistors:
1) The FET is a unipolar device; it depends on the flow of majority carriers only.
2) It is relatively immune to radiation.
3) It has a high input impedance, typically 1MΩ.
4) It is less noisy than a transistor.
5) It exhibits no offset voltage at zero drain current.
6) It has good thermal stability.
The main disadvantage is its small gain bandwidth in comparison with
conventional transistors in high frequency applications.
The following notation is standard:
Source: The source S is the terminal through which the majority carriers enter the bar. Conventional current entering the bar is designated by Is.
Drain: The drain D is