0195136047.pdf

380 SEMICONDUCTOR DEVICES

• MOSFET small-signal equivalent circuit (for low frequencies) and its application for simple
  circuit configurations.


• Basic notions of integrated circuits.

7.7 PRACTICAL APPLICATION: A CASE STUDY

Electronic Photo Flash

A simplified schematic diagram of the electric circuit of an electronic photo flash typically used

on cameras is shown in Figure 7.7.1. By passing a high current through the flash tube, a bright

flash of light is to be produced while the camera shutter is open. The power level is quite high,

as much as 1 kW. The total energy delivered, however, is only on the order of 1 j, since the flash

lasts less than 1 ms.

The electronic switch alternates between opening and closing approximately 10,000 times

per second. Energy is delivered by the battery over a period of several seconds and stored in the

capacitor. The stored energy is extracted from the capacitor whenever needed. The battery source

causes the current in the inductor to build up while the electronic switch is closed. Recall that the

current in an inductor cannot change instantaneously. When the switch opens, the inductor forces

current to go through the diode in one direction, charging the capacitor. Thus the diode allows the

capacitor to be charged whenever the electronic switch is open, and prevents the flow in the other

direction when the electronic switch is closed. The voltage on the capacitor eventually reaches

several hundred volts. When the camera shutter is opened, another switch is closed, enabling

the capacitor to discharge through the flash tube. Nowadays several practical electronic circuits

employ diodes, BJTs, FETs, and integrated circuits.

Batterysource Electronicswitch C Flash

Switch that

closes when

camera

shutter

opens

R L Diode

Figure 7.7.1Simplified schematic diagram of the electric circuit of an electronic photoflash.

Problems

7.2.1Explain the action of apn-junction with bias. Con-

sider both the forward bias and the reverse bias,

and use sketches wherever possible.

7.2.2Assuming the diode to obeyI=IS(eV/^0.^026 − 1 ),

calculate the ratioV/Ifor an ideal diode withIS=

10 −^13 A for the applied voltages of− 2 ,− 0. 5 ,

0. 3 , 0. 5 , 0. 7 , 1 .0, and 1.5 V, in order to illustrate

that a diode is definitely not a resistor with a

constant ratio ofV/I.

*7.2.3A semiconductor diode withIS= 10 μA and a

1-kresistor in series is forward-biased with a

voltage source to yield a current of 30 mA. Find the

source voltage if the diodeI–Vequation is given

byI=IS(e^40 V− 1 ). Also find the source voltage