NEC2207 Surname Experiment 4 (Push pull amplifier) PDF

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OBJECTIVESAt the end of this experiment, the learner should be able to: 1. Construct a push-pull amplifier driven by a common-emitter voltage amplifier 2. Predict and measure the performance characteristics of a push-pull amplifier circuit.BACKGROUND INFORMATIONA push pull amplifier is an amplifier ...

Description

NEC2207

Experiment No. 4

Elect 2 Lab.

OBJECTIVES At the end of this experiment, the learner should be able to: 1. Construct a push-pull amplifier driven by a common-emitter voltage

amplifier 2. Predict and measure the performance characteristics of a push-pull amplifier circuit.

BACKGROUND INFORMATION A push pull amplifier is an amplifier which has an output stage that can drive a current in either direction through the load. The output stage of a typical push pull amplifier consists of two identical BJTs or MOSFETs one sourcing current through the load while the other one sinking the current from the load. Push pull amplifiers are superior over single ended amplifiers (using a single transistor at the output for driving the load) in terms of distortion and performance. A single ended amplifier, how well it may be designed will surely introduce some distortion due to the non-linearity of its dynamic transfer characteristics. Push pull amplifiers are commonly used in situations where low distortion, high efficiency and high output power are required. The basic operation of a push pull amplifier is as follows: The signal to be amplified is first split into two identical signals 180° out of phase. Generally, this splitting is done using an input coupling transformer. The input coupling transformer is so arranged that one signal in applied to the input of one transistor and the other signal is applied to the input of the other transistor. Advantages of push pull amplifier are low distortion, absence of magnetic saturation in the coupling transformer core, and cancellation of power supply ripples which results in the absence of hum while the disadvantages are the need of two identical transistors and the requirement of bulky and costly coupling transformers.

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NEC2207

Experiment No. 4

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Figure 4.1 The push-pull amplifier

The circuit illustrated in Figure 4.1(a) has a problem. The base-emitter diode for each transistor requires approximately 0.7 V before it will conduct. The output signal follows the input except for the 0.7 V diode drop on both the positive and negative excursion. This causes distortion on the output called crossover distortion. Crossover distortion can be eliminated by using diodes to bias the transistors into slight conduction as illustrated in Figure 4.1(b). This type of bias is called diode mirror bias because if the diode is matched to the transistor’s base emitter diode, the current into the collector circuit is equal to the current in the diode. The current mirror offers another advantage. If the temperature increases, the output current will tend to increase. If the diodes are identical to the base-emitter junction, any thermal change will tend to be compensated by the diodes, thus maintaining stable bias.

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Experiment No. 4

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MATERIALS 1 – 330  2 – 10 k 1 – 1 µF 1 – 2N3904 1 – 5 k potentiometer

1 – 2.7 k 1 – 68 k 1 – 2N3906 2 – 1N4148

PROCEDURE 1. Measure and record the resistance of the resistors listed in Table 4.1 Table 4.1

Resistor

Listed Value

R1 R2 R3 R4 RL

10 k 10 k 68 k 2.7 k 330 

Measured Value 10 kΩ 10 kΩ 68 kΩ 2.7 kΩ 330 kΩ

2. Construct the circuit shown in Figure 4.2. The amplifier uses the input signal from the generator to bias the transistor ON. Set the signal generator for a 10 Vpp sine wave @ 1 kHz. Be sure there is NO dc offset from the generator. The split power supply offer the advantage of not requiring large coupling.

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Figure 4.2

3. Sketch the input-output waveform using Plot 4.1. Show the amplitude difference between the peak input waveform and output waveform and note the crossover distortion on your plot.

Plot 4.1

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4. Turn off the power and add the diode mirror bias shown in Figure 4.3. Compute the DC parameters listed in Table 4.2 for the circuit. The DC emitter voltage will be 0 V as shown, if each half of the circuit is identical. Assume that the 0.7 V base-emitter drop is the same as each diode. The current in R1 can be found by applying Ohm’s Law. This current is identical to ICQ because of current mirror action if the diodes match the base-emitter characteristics of the transistors.

Figure 4.3

5. Compute the DC parameters listed in Table 4.3. Assume VS is set to maximum undistorted output voltage. Compute the output maximum voltage and current. Unlike the single power supply case, the output can swing nearly to positive and negative VCC. Then compute the peak output current based on the load resistance. The AC power is found by Pout = 0.5 Ip(out) Vp(out). By substituting for Ip, the AC power out can also be expressed as

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𝑃𝑜𝑢𝑡 =

2 𝑉𝑝(𝑜𝑢𝑡)

Table 4.2

DC Parameters VE VB1 IB2 I1 = ICQ

Elect 2 Lab.

2𝑅𝐿

Computed Value

Measured Value

0V

0V

0.7 V -0.098 μA 0.83 mA

0.6 V -0.097 μA NA

Table 4.3

AC Parameters Vp(out) Ip(out) P(out)

Computed Value 9V 27.27 mA 122.73 mW

Measured Value 9V NA NA

6. With the signal generator OFF, turn ON the power supply and measure the DC parameters listed in Table 4.2 7. Turn ON the signal generator and ensure that there is no dc offset. While viewing Vout, adjust the generator for the maximum unclipped output. Enter Vp(out) in Table 4.3 8. One common method in applying a signal to a push-pull amplifier is shown in Figure 4.4. The signal is amplified by transistor Q3, a CE amplifier. The quiescent current in the collector circuit is designed to produce the same DC conditions as in the circuit of Figure 4.3. The bias adjust allows the DC output voltage to be set to zero to compensate for tolerance variations in the components. Compute the DC parameters listed in Table 4.4. Assume the bias potentiometer is to 3 k and apply the voltage divider rule to find VB3. Take note that the voltage across the divider string is the difference between +VCC and -VCC.

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Figure 4.4

9. Compute the voltage gain of Q3 by taking into account the load presented to the collector circuit by Q3 by the push-pull amplifier and by dividing by the resistance of the Q3 emitter circuit. The voltage gain of the push-pull amplifier is nearly 1.0, so the total voltage gain of the amplifier is approximately equal to the gain of Q3. That is, 𝐴𝑣(𝑇𝑂𝑇) = 𝐴𝑣(𝑄3) =

𝑅1 // {𝛽𝑄1(𝑅𝐿 + 𝑟𝑒𝑄1 )} (𝑟𝑒(𝑄3) + 𝑅4 )

10. Connect the circuit as shown in Figure 4.4. Measure the DC voltage across the load resistor and vary the bias adjust potentiometer for 0 V. Measure the parameters listed in Table 4.4. Set the signal generator for a voltage that produces the maximum unclipped output voltage across the load resistor. Then measure the total voltage gain of the circuit. Record the computed and measured gain in Table 4.5

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Table 4.4

DC Parameters VB3 IE3 ICQ3

Computed Value

Measured Value

-5.79 V

-5.83 V

2.40 mA 0.122 mA

2.42 mA NA

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Elect 2 Lab.

COMPUTATIONS FOR EXPERIMENTAL DATA VBE = 0.7 V VB = VE + VBE = 0 V + 0.7 V = 0.7 V ICQ = (VCC - 0.7 V) / R1 = (9-0.7) / 10 kΩ = 0.83 mA IB2 = (VCC - VBE)/RB = (-9-0.7)/100000000 = -0.097 μA

VE3 = -6.49 VBE3 = 0.7 V VB3 = VE3 + VBE3 = -6.49 V + 0.7 V = -5.79 V IE3 = (VB3 - 0.7 V) / R4 = (-5.79-0.7) / 2.7 kΩ = 2.404 mA ICQ3 = (VCC - 0.7 V) / R1 = (9-0.7) / 68 kΩ = 0.122 mA

VP(OUT) = VCEQ = VCC = 9 V

IP(OUT) = VCC/RL = 9 V/330 ohms = 27.272 mA P(OUT) = V2P(OUT)/2RL = 92/2(330) = 122.73 mW

PAIBA ITSURA NG INYO PLS 8::::::::::::D----3

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QUESTIONS AND PROBLEMS 1. With no signal applied, what power is provided by the power supplies in Figure 4.3? Note: the current in the diodes is equal to the current in the transistors _________________________________________________________________ ___ ___ _______ ______ P (OUT) = V2__ /2RL________________________________________________ P(OUT)

= 92 /2(330) = 122.727 mW __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________

2. Assume that the circuit in Figure 4.3 has a positive half-wave rectified output. What failure(s) could account for this? __________________________________________________________________

it should be on the diode and voltage source __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________

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3. If one of the diodes in Figure 4.3 shorts, what symptoms will it produce? The terminal voltage goes up and the diode will heat up which then shall affect the surrounding components

4. In procedure number 10, you found that the total voltage gain was fairly low for the circuit of Figure 4.4. What changes to the circuit would you suggest to increase the voltage gain? I would suggest to add more bypass capacitor.

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EXPERIMENTAL DISCUSSION

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CONCLUSION _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________

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REFERENCES (APA Format): Books ______________________________________________________ _______ _______________________________________________________

Instructor’s Initial: _____________ Date Performed: ______________

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REFERENCE: Floyd, Thomas, Electronic Devices 9th Edition Buchla, David, Experiments in Electronics and Electric Circuit Fundamentals 4th Edition

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