In a frequency-modulated (fm) wave, the audio signal is combined with the rf carrier wave to vary the frequency of the carrier (see Fig. 2).

AF signal

RF carrier

FM carrier

Fig. 2

The block diagram of a radio is shown in Fig. 3 below. The tuner selects the required RF wave from those picked up by the aerial. The selected RF wave is amplified and passed to the detector, which separates the audio modulation from the RF carrier wave. The audio frequency amplifier then amplifies the audio signal to make it strong enough to drive the loudspeaker.

aerial

modulated RF carrier amplified modulated RF carrier

AF

amplified AF

RF tuner

RF amplifier

detector or demodulator

AF amplifier

Af power amplifier

loudspeaker

Fig. 3

Tuner

A typical radio tuner circuit consists of an inductor and capacitor connected in parallel (see Fig. 4). The size of the aerial inductance coil can be kept small by winding it on a ferrite rode core.

aerial

Fig. A

The RF waves fed to the tuner cause the circuit to oscillate. The impedance of the circuit is smallest and the oscillation is greatest at a 20 particular frequency known as the resonant frequency. This frequency is determined by the values ofthe inductance and the capacitance. By using a variable capacitor, the circuit can be tuned to the required radio frequency, and the selected RF wave passed on to the RF amplifier.

Task 10 Identify the waves. Use these terms:

1 FM carrier 2 AM carrier 3 AF signal 4 RF carrier

Task 11

Explain what happens at each stage in this flowchart, which shows how a radio works. The first and last stage are done for you.

Component Function

1 aerial

2 RF tuner

3 RF amplifier

4 detector

5 AF amplifier

6 loudspeaker

Writing Describing a process

When describing a process, it can be useful first to make a flowchart like the one in Task 11, showing the stages in the correct sequence. You can then expand the flowchart to include a brief description of what happens at each stage.

The next step is to turn your flowchart into a written description. You can help your readers by marking the order of the stages with sequence markers. The most common markers are:

Firstly, Next, Following that,

Then, After that, Finally,

We can summarize this advice with a flowchart:

Make a flowchart showing the
main stages.
	r
Add a brief description of each
stage.
	r
Turn the flowchart into a full
description.
	f
Add sequence markers to guide
your readers.

Describe how a radio deals with a radio signal. Base your description on the flowchart in Task 11.

₁₁

_{Transistor characteristics}

Tuning-in

Task 1 Study this table from a suppliers' catalogue of transistor characteristics. What

do these abbreviations mean? Compare your answers with your partner.

IV 2 mA 3 mW 4 MHz 5 AF Table 1

Small-signal low-frequency silicon transistors

Type	Price	Case	Material	V "ceo	^сво		lc(max)	p tot	TyphFE	TypfT	Application
no.	each	style		(max)	(max)	(max)	mA	(max)	н	(MHz)
				V	V	V		mW
BC107B	20p	T018	NPN	45	50	6	100	300	290 a 2mA	300	AF driver (comp to BC177)
BC108C	20p	T018	NPN	20	30	5	100	300	520 @ 2mA	300	General purpose (comp to ВС 178)
BC109C	20p	T018	NPN	20	30	5	100	300	520 @ 2mA	300	Low noise, high gain amp (comp to BC179)
BC168C	14p	T092	NPN	20	30	5	100	300	650(5 2mA	85	General purpose
BC169C	14p	T092	NPN	20	30	5	50	300	6 50 <s2mA	150	High gain. low noise amp
BC183L	Юр	T092	NPN	30	45	5	200	300	>125@2mA	150	General purpose (comp to BC213L)
BC184L	12p	T092	NPN	30	45	5	200	300	>125@2mA	150	Low noise, high gain amp (comp to BC214L)
BC212L	Юр	T092	PNP	-50	-60	-5	200	300	>60 № 2mA	200	AF driver (comp to BC182L)
BC213L	Юр	T092	PNP	-30	-45	-5	200	300	>80e2mA	200	General purpose (comp to BC183L)
BC2I4L	12p	T092	PNP	-30	-45	-5	200	300	> 140 @ 2mA	200	Low noise, high gain amp (comp to BC184L)

Reading Locating and applying information

Task 2

Locating information about components quickly from a table and using that information to solve a practical problem are useful skills for technicians and engineers. In the task which follows, note the time you take to answer the questions. Compare times within your class.

Use Table 1 and these explanations of the column headings to answer the questions which follow.

Heading

Case style V_CE0 (^max)

V_CB0 (^max) V_EB0 (max)

I_c (max) P_T0T (max) Typh_FE

@I_C

Typf_T

Application

Explanation

transistor case and pin connections

maximum value of collector-emitter voltage with base left open-circuit

maximum value of collector-base voltage with emitter left open-circuit

maximum value of (reverse) emitter-base voltage with collector left open-circuit

maximum value of collector current

maximum total power dissipation

typical small-signal current gain (in common-emitter configuration)

value of collector current at which the small-signal current gain is quoted

transition frequency (i.e. the frequency at which the common-emitter current gain falls to 1)

recommended use for the device

1 What's the recommended use for a BC214L?

2 What's the maximum collector current for a BC169C?

3 Which transistor can be used in a complementary configuration with a BC183L?

4 Could you use a BC109C in a circuit where the collector-emitter voltage will be in the range 10-30 volts?

1 with a collector current greater than 100mA and a collector-emitter voltage between -10V and -35V?

2 with a common-emitter current gain greater than 100 and a collector current greater than 100mA?

3 which requires a common-emitter current gain greater than 200 at a frequency of 100MHz?

4 which is a low noise preamp with a collector current greater than 80mA?

5 with a collector current of 70mA and a common-emitter current gain greater ' than 600?

5 What is the common-emitter current gain for a BC168C at a frequency of 85Mhz?

Writing 1 Describing transistor characteristics

Complete this description of a BC108C using the information in Table 1 on page 63 and the explanations in Task 2.

The BC108C is an NPN transistor with a ¹ style case. It can pass a

maximum collector current of² mA.

With the base left open, the collector-emitter junction would break down at

! V. The collector-base breakdown ⁴ , with the emitter

disconnected, is 30V. The L._ junction breakdown voltage is 5V

when the collector is left open. These are the highest usable voltages for this transistor.

The transistor dissipates a ⁶__ power of 300mW. When connected in

common-emitter configuration, the small-signal current gain is 520 measured

at a collector current of mA. Typically, this gain would be

reduced to 1 at a frequency of MHz. The transistor is a

Oy

device which can be used in complementary configuration with

10

a

Now write your own description of a BC214L, based on the text you have just completed in Task 4.

Task 6

Writing 2 Ordering components Study this letter.

Satex S.p.A.