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In my class, we are to design a current amplifier with short circuit protection.

In our design, we utilized 2 emitter follower amplifiers (Q3, and Q1). We then added Q4 and Q7 to lighten the current load on Q6 when the output RL is shorted. This worked fine so that none of the output transistors (Q4,Q7, and Q6) were over the maximum power dissipation.

Here is the problem... When the output is shorted, the collectors of Q3 and Q1 have about 500mA going through them. This is clearly too high. If we add resistors, it adds noise to our output. What can we do to fix our amplifier? Do you know of an easier short circuit protection or how to fix ours?

http://tinypic.com/r/166akxd/5

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I would think about Polyfuse somewhere :-)

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  • \$\begingroup\$ I think I found my mistake. We put 100 ohm resistors on the bases of transistors Q7 and Q4 (and the corresponding ones on the bottom half). This made the current go down to like 80mA. which is SAFE :) \$\endgroup\$ – Unsure Pony Mar 4 '12 at 4:06
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Detect overcurrent by series resistance, either in current design or add a low value (0.1ohm, etc.) resistor, and have that turn on another transistor which steals current from the bases of the offending transistors or/and their drivers. This is called foldback current limiting.

See 6.05 Foldback current limiting in AoE for a simple example.

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