Amplificadores de Potência

Aula 2

Amplificadores de Potência

�  Requer transistores de elevada potência;

�  Dissipação de calor é um ponto crítico do projeto;

�  São divididos em classes;

�  Sinal de saída pode conter distorções.

Amplificadores de Potência

�  Eficiência

Amplificadores de Potência �  Classe A – Sinal de saída varia por um ciclo

completo de 360o para um ciclo completo do sinal de entrada.

�  Amplificador com alimentação-série

Amplicador Classe A

Amplificador Classe A �  Eficiência máxima

and we denominate their efficiency to the relationship η = PSAL / PENT of where it is also deduced PDIS = PENT - PSAL = PSAL ( η-1 - 1 ) Lineality of the amplification To study the behavior here of the exit transistors with parameters of low sign doesn't make sense. It will be made with those of continuous. It is also important to the transistors to excite them with courrent and not with voltage, since their lineality is solely correct with the first one. To improve all lineality of the amplifications there is that feed-back negatively. The percentage of harmonic distortion D decreases practically in the factor 1+GH. Maximum dissipated power When a TBJ possesses an operation straight line like sample the figure, the power among collector-emitter goes changing measure that the work point moves, and there will be a maximum that we want to find. Their behavior equations are the following ones IC = ( V - VCE ) / R PCE = IC VCE = V VCE / R - VCE2 / R ∂ PCE / ∂ VCE = V / R - 2 VCE / R [ ∂ PCE / ∂ VCE ] PCEmax = 0 ⇒ PCEmax = V2 / 4 R

and we denominate their efficiency to the relationship η = PSAL / PENT of where it is also deduced PDIS = PENT - PSAL = PSAL ( η-1 - 1 ) Lineality of the amplification To study the behavior here of the exit transistors with parameters of low sign doesn't make sense. It will be made with those of continuous. It is also important to the transistors to excite them with courrent and not with voltage, since their lineality is solely correct with the first one. To improve all lineality of the amplifications there is that feed-back negatively. The percentage of harmonic distortion D decreases practically in the factor 1+GH. Maximum dissipated power When a TBJ possesses an operation straight line like sample the figure, the power among collector-emitter goes changing measure that the work point moves, and there will be a maximum that we want to find. Their behavior equations are the following ones IC = ( V - VCE ) / R PCE = IC VCE = V VCE / R - VCE2 / R ∂ PCE / ∂ VCE = V / R - 2 VCE / R [ ∂ PCE / ∂ VCE ] PCEmax = 0 ⇒ PCEmax = V2 / 4 R

and we denominate their efficiency to the relationship η = PSAL / PENT of where it is also deduced PDIS = PENT - PSAL = PSAL ( η-1 - 1 ) Lineality of the amplification To study the behavior here of the exit transistors with parameters of low sign doesn't make sense. It will be made with those of continuous. It is also important to the transistors to excite them with courrent and not with voltage, since their lineality is solely correct with the first one. To improve all lineality of the amplifications there is that feed-back negatively. The percentage of harmonic distortion D decreases practically in the factor 1+GH. Maximum dissipated power When a TBJ possesses an operation straight line like sample the figure, the power among collector-emitter goes changing measure that the work point moves, and there will be a maximum that we want to find. Their behavior equations are the following ones IC = ( V - VCE ) / R PCE = IC VCE = V VCE / R - VCE2 / R ∂ PCE / ∂ VCE = V / R - 2 VCE / R [ ∂ PCE / ∂ VCE ] PCEmax = 0 ⇒ PCEmax = V2 / 4 R

and we denominate their efficiency to the relationship η = PSAL / PENT of where it is also deduced PDIS = PENT - PSAL = PSAL ( η-1 - 1 ) Lineality of the amplification To study the behavior here of the exit transistors with parameters of low sign doesn't make sense. It will be made with those of continuous. It is also important to the transistors to excite them with courrent and not with voltage, since their lineality is solely correct with the first one. To improve all lineality of the amplifications there is that feed-back negatively. The percentage of harmonic distortion D decreases practically in the factor 1+GH. Maximum dissipated power When a TBJ possesses an operation straight line like sample the figure, the power among collector-emitter goes changing measure that the work point moves, and there will be a maximum that we want to find. Their behavior equations are the following ones IC = ( V - VCE ) / R PCE = IC VCE = V VCE / R - VCE2 / R ∂ PCE / ∂ VCE = V / R - 2 VCE / R [ ∂ PCE / ∂ VCE ] PCEmax = 0 ⇒ PCEmax = V2 / 4 R

Eficiência Máxima 25 %

Amplicador Classe A �  Deduza a eficiência máxima com base no circuito

abaixo:

Máxima VCE (p-p) = VCC

Máxima IC (p-p) = Vcc/Rc

Calcule �  A potência de entrada, a potência de saída e a

eficiência do circuito amplicador da figura, para uma tensão de entrada que resulte em uma corrente de base de 10 mA pico.

Po (ca) = 0,625 W Pi (cc) = 9,6 W η = 6,5 %

Amplificador Classe A com Transformador

Eficiência Máxima 50 %

Classe A com Trafo R = 0 para DC

Classe A com Trafo

VCE pk = 2 *VCC Eficiência Máxima 50 %

Amplificador Classe B

�  O transistor fica polarizado em um valor que o mantém na região de corte, sendo ligado somente quando o sinal é aplicado.

Configuração Push-Pull

Configuração Push-Pull

Configuração Push-Pull

Configuração Push-Pull

Crossover (Distorção)

�  Eficiência

Amplificador Classe B

�  Potência de Entrada – PCC = VCC *ICQ

ICC = 2/π * Ip PiCC = (2/π * Ip)* VCC

�  Potência de Saída – Po = VL(rms)2 /RL

�  Eficiência máxima – VL(pico) = VCC

Eficiência Máxima 78,5 %

Exercício �  Para um amplificador classe B que forneça um

sinal de 20 V de pico para uma carga de 16 ohms (alto falante) e uma fonte de alimentação de Vcc = 30 V, determine a potência de entrada, a potência de saída e a eficiência do circuito.

Resp:

Pi = 23,9 W

Po = 12,5 W n(%) = 52,3 %

Dissipador para Transistores

�  Silício (temp. máx) = 150 – 200 oC

�  Germânio (temp. máx) = 100 – 110 oC

Analogia Térmica

Junção – invólucro Invólucro- dissipador Dissipador -ambiente

Exercício �  Um transistor de potência de silício é operado com

um dissipador (θsa = 1,5 oC/W). O transistor, operando em 150 W (25 oC), tem θjc = 0,5 oC/W e a isolação de montagem tem θcs = 0,6 oC/W. Qual a potência máxima que pode ser dissipada se a temperatura ambiente for de 40 oC e a temperatura máxima do silício de 200 oC?