Class AB - Protection A short circuit output causes the current demand to rise beyond the design limit. In practice, it rises just far enough to destroy the output device. Protection is required.

Short Circuit Protection Normally, V RE .05 V (<< 0.5V) so Q 4 stays off and does not interfere. If I C1 rises above the normal maximum, however, V RE rises and Q 4 turns on, diverting I bias and, therefore, switching off Q 1.

Scary Example –  A741C Op-Amp

Class C Operation Time I out One device conducts for a small portion of the a.c. cycle. Conduction angle << 180 .

A Class C Output Stage Class C power amplifier relies on a narrow band tuned load. Transistor only conducts when v in > V BIAS + V BE. For the remainder of the a.c. cycle, current circulates around the tuned load.

Class C Output Waveforms Input Output

Transistor Dissipation Cut-OffActiveSaturated V CE >0 I C = 0 V CE > 0 I C > 0 V CE = 0 I C > 0 P D = 0P D > 0P D = 0 Three operating regions:

Class C Efficiency For most of the a.c. cycle, the transistor is off – i.e. zero power dissipation. When it is on, it is close to saturation – i.e. V CE is low implying low power dissipation. Average power dissipation is much lower than output power so efficiency can be > 99 %.

Class D Operation A Class D amplifier varies its output voltage using pulse width modulation. The mark-space ratio of a high frequency square wave is set to be proportional to the required output. The average output voltage can be recovered by a low pass filter.

A Class D Output Stage L.P.F RLRL P.W.M.

Class D Efficiency Each of the two transistors in a class D output stage is always either fully on (saturated) or fully off. When on, V CE is close to zero so power dissipation is approximately zero. When off, I C is zero so power dissipation is zero again. In theory, efficiency ≈ 100 %.

Class D Inefficiency Most power dissipation will be during the transitions between states. ICIC V CE t t t PDPD

Minimising Class D Inefficiency Fast switching is essential for low power dissipation. Low PWM frequency implies fewer transitions and lower power dissipation… …but PWM frequency shouldn’t be so low that the output is distorted.

Output Stage Selection Class A – low complexity, good linearity but poor efficiency. Usually suitable for low power only. Class B – High efficiency, suitable for switching applications. Class AB – High efficiency and linearity. Good general purpose power amp. Class C – Very efficient for high-Q loads. Class D – Very efficient but complex.

Power Amplifiers – Key Issues Output Stage Selection. Linearity / Distortion Efficiency Power Dissipation. Heatsink Requirements. Thermal resistance Junction temperature Biasing / Thermal Runaway. Class AB design of V bias Protection.