Telecommunication Electornics -Question and Answers - L4 - Prof. Del Corso, Past Exams for Telecommunication electronics

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Question and Answers on Analog and communications Electronics - Lesson 4 - Amplifiers and distortions
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Preview3 pages / 3      Telecommunication Electronics – Academic Year 2007-2008 – Author: Salvatore Campione

Question 1) Which are the techniques usable to reduce harmonic content and distortion in amplifiers? (See slide A4 – from page 4 to 7)

A technique usable to limit the effects of non-linearity consists in using a negative feedback; for example, in a mono-transistor amplifier, introducing an emitter resistance leads to a reduction of the actual signal amplitude on the p-n junction. Therefore, it makes the gain stable; it has the same effect at all frequencies so it is suitable for wide-band amplifiers.

Another technique consists in using tuned circuits at the output so that ZC depends on frequency and, consequently, also the gain, by means of the following formula:

Av=Vo Vi = −Zc f  Ze f 

As you can see, if the gain is lower in frequencies we are not interested in, the total effect is an attenuation of the harmonics and other unwanted signals.

Question 2) Is there any difference between the spectral content of collector current and of collector voltage in a tuned amplifier? (See slide A4 – from page 8 to 13)

A tuned amplifier presents a load depending from frequency obtained using LRC tuned circuit, as you can see in the following picture:

Since the ZC depends on frequency, the spectral content of the collector voltage is different from that of the collector current, as shown in this figure:

This change is due to the different behavior of the tuned circuit at different frequencies; if we put the work frequency of this circuit on the fundamental, all the other harmonics will be attenuated of a factor that depends on the distance from the fundamental. In particular, each line of the spectrum is attenuated of a value determined from the following formula:

X=Q∣k−1k ∣ k= ffi In this way, the total attenuation is the sum of this value computed from the tuned circuit and

the value of the spectrum lines of the collector current. The final result is that the output signal is low distorted.

Question 3) Where can be useful a compressing amplifier? (See slide A4 – from page 20 to 21)

04/10/07 Politecnico di Torino Lesson A4 - 1

Telecommunication Electronics – Academic Year 2007-2008 – Author: Salvatore Campione

The compressing amplifiers are used in FM IF amplifiers to remove AM, that is noise in FM receivers.

Question 4) Which parameters describe completely a sine signal? (See slide A4 – from page 29 to 30)

A sine signal has the following form: v t =Vsen  t

where: ● V is the amplitude ● ω is the angular frequency: ω=2πf ● θ is the phase

Moreover, a sine signal is described from other two parameters: spectral purity (components at other frequencies, like harmonics and spurs) and the phase noise.

Presence of other components – Presence of noise

Question 5) Draw the block diagram of a single-transistor sine generator. (See slide A4 – page 31)

The important thing of an oscillator is that it uses positive feedback; therefore, it has the following block diagram:

U=AIU UI = A 1−A

The Barkhausen criterion, only for specified V and ω, permits the wave to be generated; it says that a signal travelling in the loop keeps constant amplitude and phase: that corresponds to constant amplitude oscillation.

∣ A∣=1 phase A=0

04/10/07 Politecnico di Torino Lesson A4 - 2

Telecommunication Electronics – Academic Year 2007-2008 – Author: Salvatore Campione

Question 6) How does a NIC work? (See slide A4 – from page 43 to 48)

A NIC, or Negative Impedance Converter, permits to obtain an impedance that is negative by using a positive feedback. The circuit of a NIC is the following:

iz=vi−k1vi Z

= −k vi

Z =ii

zi= vi ii = −vi

 k vi Z

 =−Z

k

As you can see, this type of circuit allows you to get negative input impedance; the value of zi depends from actual gain k and non-linearity and saturation make ∣ zi∣ decrease as the signal level increases.

Question 7) Is it possible to build a fixed-amplitude sine generator with fully linear devices?

No, because it's impossible to obtain fixed gain with linear devices that respects: ∣ A∣=1 phase A=0

Question 8) Which are the benefits of quartz oscillators? (See slide A4 – from page 53 to 54)

The quartz crystal is a resonator with very high Q. It can be used to build precise and stable oscillators.

04/10/07 Politecnico di Torino Lesson A4 - 3