Output Transistors. Current gain / input impedance is a vital parameter of a power amplifier. In the class A analysis, the load impedance is scaled by a factor of b . High power transistors often have a value of b of the order of tens rather than hundreds. Multiple Transistor Units.
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To realise a high current gain with low b transistors, use two cascaded emitter follower stages.
Darlington Pair configuration is available in a single integrated package.
VBE is double that of a normal transistor.
b is typically between 1000 & 10000.
To calculate efficiency, must calculate load power, PL, and the supplied power, PS.
Maximum output current swing is ±IE.
Amplifier efficiency :
So, peak efficiency is when A is at its maximum.
Worst efficiency is when A = 0.
Ambience / Air, TA
Heat only travels from a hot to a cold body Þ TJ > TC > TA
Rate of transfer is proportional to temperature difference (and vice versa)
Temperature difference between the junction and ambience depends on the power dissipation and the thermal resistance between them.
qJA is the thermal resistance between the junction and ambience measured in °C per Watt.
Junction temperature must not exceed Tjmax. Also, power dissipation must not exceed PD0. Combining these limitations gives the power derating curve.
Example : TIP 120
qJA can be broken down into a pair of series resistances.
qJC = thermal resistance between junction and case (fixed)
qCA = thermal resistance between case and ambience.
qCA can be lowered by increasing the surface area of the transistor case, i.e. by adding a heat-sink.
No heat-sink, qJA = 62.5 °C/W
With heat-sink, qJA = 25 °C/W