Revision: Power Amplifiers Basic Principles Operating modes of amplifiers Power dissipation and thermal effects Design/Analysis Class A, B, AB amplifiers.
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Revision: Power Amplifiers Basic Principles Operating modes of amplifiers Power dissipation and thermal effects Design/Analysis Class A, B, AB amplifiers Use of multiple transistor units (Darlington) Associated circuits (current mirror, V BE multiplier)
EEM3A Paper 1, Jan 2000 4.Figure 2 shows a power amplifier with a complementary pair output stage Q 1, Q 2 and diode biasing implemented using diode connected transistors Q 3 and Q 4. Q 1 -Q 4 are matched. 1 = 2 =50. 1 = 2 =50
a.Identify the class of the power amp and explain the main features of this class. 1 = 2 =50 Class AB amplifier Features: ‘Push-pull’ class B operation with large signals Class A operation with small signals Good efficiency, close to class B (up to 78%) Linear operation (cross-over distortion suppressed)
b.How are the power transistors Q 1 and Q 2 biased ? What are the advantages of this bias configuration ? What is the function of Q 5, Q 6 and R ? 1 = 2 =50 Bias voltage is provided by the diodes Q 3 and Q 4. Approx. 0.7V is dropped across each giving a 1.4V potential difference between the bases of Q 1 and Q 2. Advantages: Thermal stability Simpler than a V BE multiplier Q 5, Q 6 and R form a current mirror. Provides I D to forward bias the diodes as well as quiescent base current for Q 1.
c.What are the allowable positive and negative output voltages and currents to the load, R L ? 1 = 2 =50
d.Determine R to allow proper bias current for the maximum output current. 1 = 2 =50 We want bias voltage to remain constant, so I D should not vary too much (proportionately). Choose I D(min) to be 1 mA (or higher)
e.What are the maximum positive and negative input voltages ? 1 = 2 =50 NB. Both answers could be found by just subtracting V BE2 from the answers to (c)
f.Propose a simple way to protect the output stage against a short circuited load. 1 = 2 =50 Simple way: Add series resistors to the emitters of Q 1 and Q 2. These will at least limit the short circuit current. In addition, an extra short circuit protection transistor can be added (see notes for details)
Other things that could have been asked: Efficiency calculations Power dissipation calculations (especially for Q 1 and Q 2 ) Heatsink requirements Design of a V BE multiplier instead of the diodes See other years’ exam questions for examples!