If a target is hit. .

a weak reflected signal is received
the signa	1 is amplified
the received signal is compared with a reference signal from the transmitter

Unit 9

Find out from your partner how to:

measure the current in this circuit, check a fuse.

R1

R2

This information should help you to advise on your partner's problems.

Task 6

Frequency band

Some uses

Very low (VLF) 3kHz-30kHz Low (LF)

long-wave radio and communication over large distances

Medium (MF) 300kHz-3MHz ? (HF) 3MHz-30MHz

Very high (VHF) 30MHz-300MHz

? (UHF) 300MHz-3GHz

Super high (SHF) (microwaves) ?

short-wave radio and communication, ?

FM radio, police, meteorology devices

TV (bands 4 and 5) and

aircraft-landing

systems

radar, communication satellites, ?

Unit 13

Task 6 Find out from your partner the missing values in these amplifier specifications.

Ask him or her to explain items 4-7.

Provide the information your partner requires. Explain with the help of the passage below what items 1-3 mean. Your partner also has Figs. 1 and 2.

1. voltage gain 40dB

2. frequency response 20Hz to 20kHz at-3dB

3. distortion less than 0.1% THD

4. S/N ratio ?

5. input impedance ?

6. output impedance ?

7. supply voltage ?

Amplifiers

Amplifiers are used in almost all electronic circuits. In audio systems, the very small signal voltages produced by microphones, tape recording heads, magnetic pickup heads, etc. are amplified by a pre-amp. A power amp is then used to enable the signals to drive a 5 loudspeaker.

The gain of an amplifier is measured by comparing its output with its input. If a logarithmic scale is used, the gain is expressed in decibels (dB). In a pre-amp we are interested in the voltage gain, but in a power amp the power gain is more significant.

io The gain of an amplifier is almost constant over a range of input signal frequencies. However, because of capacitance effects, the gain falls by 3dB at the upper and lower cut-off frequencies, as shown in Fig. 1.

Ay max 0.707 A_v max

voltage да

in A_v|

Fig. 1

The useful frequency response of an amplifier is the range of frequencies between these two -3dB cut-off points. The size of this is range is known as the band width.

Ideally, the output signal will be an enlarged copy of the input signal. However, when the input signal level becomes too high, an amplifier no longer behaves in a linear fashion and distortion of the output signal occurs (see Fig. 2). This normally occurs when the output 20 voltage is greater than half the supply voltage.