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— | university:courses:electronics:electronics-lab-13a [21 Apr 2013 02:16] – [Questions:] Doug Mercer | ||
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+ | ====== Activity 13A. Amplifier Output Stages ====== | ||
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+ | ===== Objective: ===== | ||
+ | |||
+ | To investigate a simple push-pull amplifier output stages (class B and AB). | ||
+ | |||
+ | ===== Background: ===== | ||
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+ | The role of an output stage is to provide power gain. It should have high input impedance and low output impedance. An obvious choice for this stage is the emitter follower. However, in order to provide both current sourcing and sinking capabilities, | ||
+ | |||
+ | ===== Materials: ===== | ||
+ | Analog discovery Lab hardware\\ | ||
+ | Solder-less breadboard\\ | ||
+ | Jumper wires\\ | ||
+ | 2 - 100Ω resistors\\ | ||
+ | 1 - 2.2KΩ resistor\\ | ||
+ | 2 - 10KΩ resistors\\ | ||
+ | 2 - small signal NPN transistors (SSM2212 with matching V< | ||
+ | 2 - small signal PNP transistors (SSM2220 with matching V< | ||
+ | |||
+ | ===== Directions: ===== | ||
+ | |||
+ | Before starting make sure the power supplies on the Analog Discovery hardware are turned off. The circuit and the connections to the Lab hardware are as indicated in figure 1. Scope input 1+ should to be connected to the junction of Q< | ||
+ | |||
+ | {{ : | ||
+ | |||
+ | <WRAP centeralign> | ||
+ | |||
+ | ===== Hardware Setup: ===== | ||
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+ | The waveform generator, W1, should be configured for a 1 KHz sine wave with 3.0 volt amplitude peak amplitude and 0 offset. Channel one of the scope should be connected to display the output of the first generator and both scope channels 1 and 2 should be set to display 1V per division. | ||
+ | |||
+ | ===== Procedure: ===== | ||
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+ | Next, apply power and adjust the waveform generator so that W1 is a 100 Hz triangle wave with 0V offset and 3.0 V amplitude values. Use the oscilloscope in the x-y mode to observe the voltage-transfer curve of the circuit. Record the curve on paper, label all breakpoints, | ||
+ | |||
+ | Switch the scope to just the time display mode, and adjust the waveform generator so that W1 is a 1 kHz sine wave with the amplitude = 0 V. | ||
+ | |||
+ | • Starting with the amplitude = 0 V, gradually increase it until you just begin to see an signal on scope channel 2 appear at the output. For what range of amplitude values of W1 can we say that both BJT's are essentially off? Confirm this by observing the voltages of the current-sensing resistances R< | ||
+ | |||
+ | • Raise W1 to 3.0V amplitude value, and record the amplitude of the output waveform as well as the collector currents of the BJTs, which can be found via Ohm's law from the voltages across R< | ||
+ | |||
+ | • Repeat, but with W1 raised a 5.0V amplitude value; and comment. | ||
+ | |||
+ | Simulate the circuit of figure 1 using QUCS, compare with your lab findings, and justify any differences. | ||
+ | |||
+ | ===== Questions: ===== | ||
+ | |||
+ | Add questions here: | ||
+ | |||
+ | ====== Reducing Output Distortion: ====== | ||
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+ | The large amount of distortion at the zero-crossings in the basic push-pull stage of figure 1 is a result of a dead zone when both the NPN and PNP emitter followers are off. The waveform' | ||
+ | |||
+ | ===== Directions: ===== | ||
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+ | With the power turned off, assemble the circuit of figure 2, keeping leads as short and neat as possible. NPN transistors Q< | ||
+ | |||
+ | {{ : | ||
+ | |||
+ | <WRAP centeralign> | ||
+ | |||
+ | If we examine, in figure 2, the loop formed by the base emitter voltages of Q< | ||
+ | |||
+ | ===== Questions: ===== | ||
+ | |||
+ | • Display the input / output transfer curve of the circuit of figure 2, record it on paper, label all breakpoints, | ||
+ | |||
+ | • Apply a 1-kHz sine wave of 0 V offset and various amplitudes, and verify that the circuit yields Vout ? Vin all the way down to small amplitudes. What is the upper limit on the amplitude of W1 before the circuit starts to distort? Justify quantitatively in terms of the transfer curve just observed. | ||
+ | |||
+ | Using your scope input, measure Vout as well as the voltage across R< | ||
+ | |||
+ | Measure the input impedance by inserting a 10K? resistor in series with the signal generator (between W1 and the emitters of Q< | ||
+ | |||
+ | Simulate the circuit of figure 2 using QUCS, compare with your lab findings, and justify any differences. | ||
+ | |||
+ | ====== Another Configuration: | ||
+ | |||
+ | Remembering the loop formed by the base emitter voltages of Q< | ||
+ | |||
+ | {{ : | ||
+ | |||
+ | <WRAP centeralign> | ||
+ | |||
+ | ===== Questions: ===== | ||
+ | |||
+ | • Display the input / output transfer curve of the circuit of figure 3, record it on paper, label all breakpoints, | ||
+ | |||
+ | • Apply a 1 kHz sine wave of 0 V offset and various amplitudes, and verify that the circuit yields Vout ? Vin all the way down to small amplitudes. What is the upper limit on the amplitude of W1 before the circuit starts to distort? Justify quantitatively in terms of the transfer curve just observed. | ||
+ | |||
+ | Using your scope input, measure Vout as well as the voltage across R< | ||
+ | |||
+ | Measure the input impedance by inserting a 10K? resistor in series with the signal generator (between W1 and the bases of Q< | ||
+ | |||
+ | Simulate the circuit of figure 3 using QUCS, compare with your lab findings, and justify any differences. | ||
+ | |||
+ | ==== For Further reading: ==== | ||
+ | Output Stages [[http:// | ||
+ | [[http:// | ||
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+ | **Return to Lab Activity [[university: | ||
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