Do power amps require transistors with a low beta value?

Question

If so, why? Also why can't you use transistors with a high beta? Will it cause distortion?

Answer 1

"Medium" Beta transistors (50 to 100) will suffice for consumer electronics.

Leakage current and junction temperature are the enemies of Bipolar Junction Transistors and therefore beta is an issue in Transistors because:

1) C-B Leakage current increases with temperature. A high Beta transistor operating at any voltage will drift as it's junction temperature increases because higher leakage TIMES a high Beta will encourage thermal runaway.

2) High voltage transistors (ie: Horizontal deflection amplifiers) have typically an extremely LOW Beta because the leakage current that can result with a high voltage Collector pulse can also upset the Q point and push the transistor into Thermal runaway.

3) Beta changes with collector current. It's not a linear quantity.

Shunting the BE circuit with a low resistance WILL provide a path for CB leakage currents but the low resistance is just the same as a low Beta.

Alternately, use a MOSFET if possible. They are immune to thermal runaway and the only loading you have to worry about it capacitive loading from the G-S leads.

Answer 2

Beta is a function of collector current. At higher collector currents, the beta drops and the circuit approaches saturation.
There are very high gain, Darlington connected, transistors which will handle up to about 100 watts, but you really don't need them. Just keep the device in its linear region and drive it harder when it approache saturation.

Doug