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Calculul simplificat al preamplificatoarelor audio

Jul 12 2008 07:32 AM | donpetru in Audio

În acest articol vom lua spre exemplificare una din cele mai utilizate scheme de preamplificatoare audio şi anume schema cu tranzistor bipolar în conexiunea emitor comun (fig.1).

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Pentru început, considerăm că ne interesează o amplificare de aproximativ 10 cu o impedanţă de intrare de cel puţin 30 kOhmi şi o impedanţa de ieşire de 10 Kohmi.
Este cunoscut faptul că amplificarea este raportul dintre tensiunea de ieşire şi tensiunea de intrare, iar la etajele cu emitorul comun aceasta nu depinde practic de tranzistor ci de produsul S*RS, unde S reprezintă panta tranzistorului iar Rs rezistenţa de sarcină a etajului.
Rezistenţa RE2 nu intervine în calcul, avînd în paralel un condensator de mare capacitate CE, singurul element de circuit care influenţează fiind rezistenţa RE1 care introduce o reacţie negativă.
Deci, se poate aproxima:
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Această relaţie este destul de precisă pentru amplificării mari în tensiune. Ca să se asigure impedanţele de intrare şi ieşire impuse, ca să avem un zgomot de fond mic şi bune performanţe la frecvenţe înalte, alegem un curent de colector IC destul de mic (în general, pentru obţinerea acestor performanţe, se recomandă curentul de colecto...
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Proiectarea amplificatoarelor audio

Mar 14 2009 03:00 PM | donpetru in Audio

Pentru proiectarea amplificatorului audio din figura 1 vom porni de la următoarele ipoteze:

rezistenţa de sarcina R_L=32 Ohmi;
tensiunea de ieşire variabilă în domeniul 0...5V_V-V.
tensiunea de intrare variabila in domeniul 0...350mV.

Amplificatorul are rolul de a asigura o tensiune corepunzătoare pe R_L la curentul de sarcină cerut I_{S_efectiv}.Amplificatorul audio se va utiliza pentru reproducerea domeniului de frecventa cuprins intre f_0min=20Hz şi f_0max=20kHz.

Soluţie:

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unde „0,35" reprezintă coeficientul care face legătura dintre o valoare vârf la vârf şi una efectivă a unei tensiunii electrice sinusoidale.

Deoarece I_{S_efectiv} cerut de sarcina R_S=32Ω este mai mare ca valoarea curentului nominal al unui amplificator operaţional de tipul NE5532 dată de catalog atunci vom realiza un amplificator final ca cel din figura 1.
Imagine postata

Amplificatorul din figura 1 este un amplificator clasa B. La ieşirea amplificatorului final se conectează sarcina de 32Ω.

Tensiunea de alimentare a amplificatorului trebuie să sadisfacă următoarea relaţie pentru a permite variaţia tensiunii în plaja mai sus menţionată:

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unde V_{V...

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