Class B BJT Amplifiers
Discrete Semiconductor Devices and Circuits
|Don’t just sit there! Build something!!|
Learning to mathematically analyze circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way.
You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the “answers” instead of a book or another person. For successful circuit-building exercises, follow these steps:
- Carefully measure and record all component values prior to circuit construction, choosing resistor values high enough to make damage to any active components unlikely.
- Draw the schematic diagram for the circuit to be analyzed.
- Carefully build this circuit on a breadboard or other convenient medium.
- Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.
- Mathematically analyze the circuit, solving for all voltage and current values.
- Carefully measure all voltages and currents, to verify the accuracy of your analysis.
- If there are any substantial errors (greater than a few percent), carefully check your circuit’s construction against the diagram, then carefully re-calculate the values and re-measure.
When students are first learning about semiconductor devices, and are most likely to damage them by making improper connections in their circuits, I recommend they experiment with large, high-wattage components (1N4001 rectifying diodes, TO-220 or TO-3 case power transistors, etc.), and using dry-cell battery power sources rather than a benchtop power supply. This decreases the likelihood of component damage.
As usual, avoid very high and very low resistor values, to avoid measurement errors caused by meter “loading” (on the high end) and to avoid transistor burnout (on the low end). I recommend resistors between 1 kΩ and 100 kΩ.
One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once.
The circuit shown here is a standard push-pull amplifier, comprised of a complementary pair of bipolar junction transistors:
Trace current in this circuit during periods of time when the instantaneous voltage of the signal source (Vin) is positive, and for those periods when it is negative. Determine at which times each of the transistors is “on” (conducting current).
A student builds the following push-pull amplifier circuit, and notices that the output waveform is distorted from the original sine-wave shape output by the function generator:
Thinking that perhaps this circuit requires DC biasing, just like Class A amplifier circuits, the student turns on the “DC offset” feature of the function generator and introduces some DC voltage to the input signal. The result is actually worse:
Obviously, the problem will not be fixed by biasing the AC input signal, so what causes this distortion in the output waveform?
A simple yet impractical way to eliminate crossover distortion in a Class B amplifier is to add two small voltage sources to the circuit like this:
Explain why this solution works to eliminate crossover distortion.
Also, explain what practical purpose this push-pull amplifier circuit might serve, since its voltage gain is only 1 (0 dB).
One way to greatly boost the current gain (AI) of a Class B push-pull amplifier is to use Darlington pairs instead of single transistors:
The only problem with the Darlington pair amplifier circuit shown is that the original biasing network will no longer be sufficient. Unless something else is changed in this circuit, the amplifier will exhibit some crossover distortion.
Draw the necessary modifications to the circuit to properly bias the new transistors, and explain why these modifications are necessary.
High-power PNP transistors tend to be scarcer and more expensive than high-power NPN transistors, a fact which complicates the construction of a high-power complementary push-pull amplifier circuits. An ingenious solution to this problem is to modify the basic Darlington push-pull circuit, replacing the final PNP transistor with an NPN transistor, like this:
The cascaded combination of an NPN and PNP transistor is called a Sziklai pair, or complementary Darlington pair. In this case, the small PNP transistor controls the larger NPN power transistor in the Sziklai pair, performing the same basic function as a PNP Darlington pair.
Modify the circuit shown here to use diodes in the biasing network instead of just resistors. The solution is not quite the same for this circuit as it is for a conventional Darlington push-pull circuit!
A popular variation of the Class B amplifier is the Class AB amplifier, designed to eliminate any trace of crossover distortion. What makes the difference between a Class B and a Class AB amplifier? Why do Class AB amplifiers have less crossover distortion than Class B amplifiers? And, is there any disadvantage to changing from Class B to Class AB operation?
An interesting addition to the basic Class B push-pull amplifier circuit is overcurrent protection, in the form of two more transistors and two more resistors added to the circuit:
This form of overcurrent protection is common in voltage-regulated DC power supply circuitry, but it works well in amplifier circuitry, too. Explain how the additional transistors and resistors work together to protect the main power transistors from damage in the event of an overload.
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