EXPERIMENT NO.(3) TRANSISTOR POWER AMPLIFIERS

Transcription

1 Electronics II aboratory EXPERIMENT NO.(3) TRANSISTOR POWER AMPIFIERS OBJECT: To study some important aspects of different power amplifiers configurations. EQUIPMENTS AND COMPONENTS: 1. Signal generator. DC Power supply 3. Oscilloscope 4. AVO meter 5. Transistor and components as shown in Fig.() and Fig.(4). THEORY: An amplifier designed to deliver electric power to a desired load is known as a power amplifier. Power amplifiers find applications in transmitter, servomotor amplifiers, industrial control circuits, and audio amplifiers. In general, power amplifiers designed to del1iver the maximum power output at the highest efficiency. Since power amplifiers inherently involve excursions in voltage and current, the transistor may operated in the non-linear regions of the characteristic curve resulting distortion in the output. Furthermore, the transistor subjected to large values of current and voltage, thermal instability may become a problem and thus the power amplifier must biased to guard against thermal runaway. Jassim K. Hmood

2 Electronics II aboratory Depending on their operation, power amplifiers can be grouped into four main classes: - CASS A operation - CASS AB operation - CASS B operation - CASS C operation The output signal as result of a sinusoidal input signal for each of the four classes can show in Fig.(1). In class A operation, the entire input signal is reproduced faithfully at the output resulting minimum distortion. The power delivered by the power supply is constant and not affected by input signal. This means that, power being dissipated by the circuit even through no signal is present. Further more, the maximum lower dissipated in the transistor under up signal condition. Therefore the efficiency in class A operation is poor. Fig.( ) shows the typical circuit used for class A power amplification. In class AB operation output (collector) current flows for more than half of the input signal cycle. Hence, more than half of the signal is amplify and appears at the output. Class A Class AB Class B Class C Fig.(1): Signal waveform for Classes (A, AB, C, D) In class B operation, exactly half of the input signal appears amplified at the output. Transistor is biased such that Ic =0. Therefore the amplifier dissipates power only when it is being used to amplify signal Jassim K. Hmood 3-011

3 Electronics II aboratory (input signal present). Hence, the efficiency is higher but the distortions are considerable. In order to obtain high efficiency and low distortion, circuit shown in Fig.(3) is used. This is known as PUSH PU amplifier. In positive half cycle one transistor provides output current while in negative half cycle other transistor provides output current. By this way output current is continuous. 1 Rs + Vs - C1 RB + VBB R 1k Q1 NPN C Vo + Vs - Trans Q NPN R1 1k Vo a) Direct coupled load b) Transformer coupled Fig.() : shows type of coupling In class C operation, somewhat less than half of the input signal appears amplified at the output. The output signal waveform is high distorted and rich in harmonic. Generally, in class C amplifier, load is a tuned circuit, which selects the fundamental or the desired harmonic rejecting all other frequency components. Efficiency in class C amplifier is the highest. These generally used to amplify radio-frequency (r.f.) single in transmitters. Jassim K. Hmood

4 Electronics II aboratory Class (A) Operation a.c. output power 1 ( Icmax) R R Power supplied = I cq c max i efficiency I Q R I Q Class B operation a.c. output power R Vop.p 8R Power supplied = I cav = / R For ideal class B operation: Power ac Power dc R R R Icav max or ideal efficiency RI Q Class C operation efficiency sin 4 sin cos Where conduction angle Jassim K. Hmood

5 Electronics II aboratory PROCEDURE: 1. Connect the circuit as shown in Fig.(a). Adjust V BB to obtain Ic=4.5mA. Then measure VBE and VCE.. Draw the load line on the given output characteristics. ocate Q point. 3. Apply a sinusoidal input signal of 1kHz and adjust the input voltage to obtain maximum undistorted output. Draw input and output waveform accurately. 4. Calculate efficiency and distortions. 5. Adjust V BB equal to the half value set in step 1. Measure Ic, VBE, and VCE. 6. Apply a sinusoidal input signal of magnitude applied in step 3. Draw input and output waveform. Then measure rms output voltage (hence calculates rms and average current). Repeat step Adjust V BB to obtain Ic = 0 (class B operation). Repeat step Adjust V BB = -0.5 volt. Measure VBE, VCE and Ic. 9. Apply a sinusoidal input signal of magnitude apply in step 3. Draw input and output waveform. Then calculate efficiency. 10. Connect the circuit as shown in Fig.(3). Measure the dc current pass through each transistor. 11. Measure the ac and dc current pass through the load. Then calculate efficiency. Discussion 1. Compare between the theoretical and your practical results.. Derive the laws of efficiency and distortion used in calculations. 3. Discuss the various types of distortion in amplification and how they can be minimized? Jassim K. Hmood

6 Electronics II aboratory 4. Design an audio frequency power amplifier to boost the power by a factor of Select a suitable class of operation and circuit. Give justification. R1 Q1 NPN R 5.6k Vs - 1k R 5.6k R1 100 Q PNP Fig.(3): Push Pull transistor amplifier Jassim K. Hmood