CLASS-A POWER AMPLIFIERS for ECE ECT.pptx

class a power amplifier

1. CLASS-A POWER AMPLIFIERS Series-fed class-A amplifiers Transformer-coupled class-A amplifiers

2. Class-A Power Amplifiers Class-A power amplifiers are linear amplifiers operating in the class-A region of its characteristic curve, where the output transistors conduct current over the entire cycle of the input signal. This means that the output transistors are always on, even when there is no input signal, and consume significant power.

3. Series-Fed Class-A Amplifiers The class A amplifier operates in the linear region of its characteristic curve, ensuring that the output signal is a faithful reproduction of the input signal. In a series-fed class A amplifier, each transistor operates in the class A region, ensuring that the output signal faithfully reproduces the input signal. Series-fed class A amplifiers are commonly used in audio applications where high fidelity is required, such as in high-end stereo systems, studio equipment, and musical instrument amplifiers. They are also used in radio frequency (RF) applications where low distortion and high linearity are required.

4. The differences in large signal amplifiers. the signals handles are in the range of volts The transistor used is a power transistor ( capable of operating in the range of a few to tens of watts). This circuit is not the best to use as a large-signal amplifier because of its poor power efficiency. The beta of a power transistor is generally less than 100, and can handle large power or current due to power transistor while not providing much voltage gain.

5. If the DC bias collector current is set at one-half the possible signal swing (between 0 and Vcc / Rc ), the largest collector current swing will be possible. Additionally, if the quiescent collector-emitter voltage is set at one-half the supply voltage, the largest voltage swing will be possible.

6. SERIES-FED CLASS A AMPLIFIER – AC OPERATION

7. Power considerations Power considerations in a series-fed class-A amplifier relate to the power consumption and dissipation of the amplifier circuit, as well as its overall efficiency and thermal management. The power into an amplifier is provided by the supply voltage. With no input signal, the dc current drawn is the collector bias current I CQ. The power drawn by the supply is Even with an ac signal applied, the average current drawn from the supply remains equal to the quiescent current I CQ.

8. OUTPUT POWER: The ac signal Vi causes the base current to vary around the dc bias current and the collector current around its quiescent level. The larger the input signal, the larger the output swing, up to the max set by the circuit. The ac power delivered to the load ( Rc ) can be expressed in several ways. Using RMS signals: The ac power delivered to the load ( Rc ) may be expressed using

9. Efficiency Represents the amount of ac power delivered/transferred from the dc source. The efficiency of the amplifier is calculated using Maximum Efficiency It can be determined using the maximum voltage and current swing. For voltage swing it is: maximum Vce (p-p) = Vcc For current swing it is: maximum Ic (p-p) = Vcc / Rc Using the maximum volatage swing

10. The maximum power input can be calculated using dc bias current set to one-half the maximum value: To calculate the maximum efficiency; The maximum efficiency of a class A series-fed amplifier is 25% in ideal case and much less than 25% in practical scenarios.

11. Calculate the input power, output power, and efficiency of the amplifier for an input voltage that results in a base current of 10mA peak.

12. TRANSFORMER-COUPLED CLASS A POWER AMPLIFIERS Transformer coupled class A amplifier uses a transformer to couple the output signal load. It has a maximum efficiency of 50% A transformer can step up or down a voltage or current applied to one side. The voltage transformation is given by. The current transformation is given by

13. Since the voltage and current can be changed by a transformer, an impedance “seen” from either side (primary or secondary) can also changed The impedance R L is connected across the transformer secondary. This impedance is changed by the transformer when viewed at the primary side If we define α = N 1 /N 2 where α is the turns ratio of the transformer, above equation becomes

14. We can express the load resistance to the primary side as

15. Calculate the effective resistance seen looking into the primary transformer connected to an 8-ohm load. What transformer turns ratio is required to match a 16-ohm speaker load so that the effective load resistance seen at the primary is 10K-ohm

16. OPERATION OF AMPLIFIER STAGE Dc Load Line: The transformer (Dc) winding resistance determines the dc load line for the circuit. Typically, this dc resistance is small (ideally 0-ohm) and there is no dc voltage drop across the 0-ohm dc load resistance The load line is drawn straight from the voltage point, Vceq = Vcc

17. Quiescent operating point: The dc bias point sets the conditions for the maximum undistorted signal swing for both collector current and collector-emitter voltage. If the input signal produces a voltage swing less than the maximum possible, the efficiency of the circuit at that time will be less than the maximum of 50%.

18. AC Load Line: To carry out ac analysis, it is necessary to calculate the ac load resistance "seen" by looking into the primary side of the transformer, then draw the ac load line on the collector characteristic. The reflected load resistance (R’L) is calculated using R’L = R L The graphical analysis technique then proceeds as follows. Draw the ac load line so that it passes through the operating point and has a slope equal to -1/R’L (the reflected load resistance. Notice that the ac load line shows that the output signal swing can exceed the value of Vcc In fact, the voltage developed across the transformer primary can be quite large. It is therefore necessary to check that the possible voltage swing does not exceed transistor maximum ratings.  

19. Signal swing and output AC power shows the voltage current signal swings from the circuit. Vce (p-p) = Vce (max) – Vce (min) Ic (p-p) = Ic (max) – Ic (min) The ac power developed across the transformer primary can then be calculated using

20. For an ideal transformer, the power delivered by the secondary to the load is approximately calculated using The output ac power can also be determined using the voltage delivered to the load.

21. For the ideal transformer, the voltage delivered to the load can be calculated using The power across the load can then be expressed as The load current With the output ac power then calculated using

22. Calculate the ac power delivered to the 8-ohm speaker for the circuit. The circuit component values result in a dc base current of 6 mA, and the input signal (Vi) results in a peak base current swing of 4 mA

23. The input (dc) power obtained from the supply is calculated from the supply dc voltage and the average power drawn from the supply: For the transformer-coupled amplifier, the power dissipated by the transformer is small (due to the small de resistance of a coil) and will be ignored in the present calculations. Thus the only power loss considered here is that dissipated by the power transistor and calculated using W here P Q is the power dissipated as heat.

24. When the input signal is very small, with very little ac power delivered to the load, the maximum power dissipated by the transistor. When the input signal is larger and power delivered to the load is larger, less power is dissipated by the transistor. Efficiency Maximum Theoretical Efficiency For a class A transformer-coupled amplifier, the maximum theoretical efficiency goes up to 50%. Based on the signals obtained using the amplifier, the efficiency can be expressed as The larger the value of Vce (max) and the smaller the value of Vce (min), the closer the efficiency approaches the theoretical limit of 50%.

25. Calculate the efficiency of a transformer-coupled class A amplifier for a supply of 12 V and outputs of: a. V(p) = 12 V. b. V(p) = 6 V. c. V(p) = 2 V.