How to add short circuit protection to a class A amplifier

[victordion]

I was constantly asked of this question how to add short circuit protection to the output stage of a class A amplifier.

Is the output coupling capacitor OK?

 

[WimRFP]

No, as the output capacitor only blocks DC, and not the audio signal. So if you short-circuit the output and have an audio output, large current will flow and this will destroy an unprotected amplifier.

Short circuit protection is mostly implemented by realtime measuring the current in the output (final) transistors. When too high, the drive is reduced to limit the current. To have fast response of the short circuit protection, the protection mostly acts directly at the driver stage. When the protection acts, you will have large distortion, but your amplifier will be fine.

If you have an actual circuit, maybe you can post it.

 

[victordion]

The question does not specify a certain circuit, but just says class A amplifier.

Maybe the following could be used.

Then how to apply short circuit protection to this circuit?

 

[WimRFP]

I was supposing that you had a class A power amplifier in mind.

For this small-signal amplfiier a decoupling capacitor between collector and load is required to separate the DC collector bias voltage from the load. If you use a electrolytic (polarized) capacitor, make sure the the plus lead is at the collector.

OT: your circuit will be unreliable as there is large spread between devices in Ic versus Vbe curve (also temperature dependent). Normally an emitter resistor is added to make the collector current less dependent on transistor properties. To avoid that the emitter resistor reduces the gain, a rather large capacitor is placed across the emitter resistor.

I forgot to mention: using a short circuit (0 Ohms) as load will not harm the amplifier when the capacitor is present, unless the transistor is already operating at maximum junction temperature. When you short circuit the load and there is no capacitor present, the collector resistor has to dissipate more (as more current will flow through it).

 

[victordion]

Thanks.

Do you mean that if the circuit is configured as you said, like above, no short circuit protection is needed?

Then can you illustrate a scenario that a short circuit protection makes sense?

 

[Audioguru]

Your latest class-A amplifier circuit in post #5 has a fairly high output impedance. A speaker will short its output.

ALL audio power amplifiers have negative feedback but yours has none. The negative feedback "tells" the amplifier:
1) Reduce the distortion.
2) Flatten the frequency response
3) Holy Smokes! Something (a shorted output) is reducing the output level. BOOST the output more and more. It will fight your short circuit protection circuit.

 

[WimRFP]

Short circuit protection is required when something goes wrong when a short circuit condition occurs (think of fire, fried components, melted insulation, etc). You may calculate for your own circuit what happens when you short circuit the output. For your circuit very likely nothing breaks down. When you remove the short circuit the circuit behaves as intended.

Protection is mostly required for circuit that can deliver a certain amount of power. Especially circuits with low output impedance may deliver too much current in case of a short circuit. The protection should react before things become too hot. A fuse is mostly to slow to protect semiconductors.