Understanding Reverse HFE in Transistors for Audio Circuit Protection

[bitrex]

What is "reverse Hfe"?

Hey everyone, I'm looking over a synthesizer schematic that has a 2SC2878 transistor in shunt with the main output - I think it's connected up in some kind of power on muting circuit for protecting speakers against turn-on thumps. Interestingly, this transistor has something called "Reverse Hfe" - I assume this means for an NPN transistor it will turn on if the base-emitter junction is negatively biased as well as positively? I've looked on the web and can't seem to find a good explanation of how such a transistor would be manufactured.

The datasheet for the device is here: http://www.datasheetarchive.com/pdf-datasheets/Datasheets-38/DSA-757294.pdf

The circuit in question is: http://img9.imageshack.us/img9/9071/alphajuno.th.jpg

Any insight into how this circuit works would be appreciated - I guess it somehow keeps the 2SC2878 turned on until the positive and negative rails stabilize?

 

[cabraham]

It's referring to the hfe value when emitter & collector are swapped. This is referred to as inverse or reverse mode of operation.

The reverse hfe value is typically less than 0.1, more like 0.02 or so. In this mode, the bjt makes a lousy amplifier, but an outstanding switch. In the reverse mode, when driven into saturation, the collector-emitter voltage drop, Vcesat, can be tens of millivolts. That is hard to achieve in the normal, or forward mode.

Claude

 

[jsgruszynski]

Hey everyone, I'm looking over a synthesizer schematic that has a 2SC2878 transistor in shunt with the main output - I think it's connected up in some kind of power on muting circuit for protecting speakers against turn-on thumps. Interestingly, this transistor has something called "Reverse Hfe" - I assume this means for an NPN transistor it will turn on if the base-emitter junction is negatively biased as well as positively? I've looked on the web and can't seem to find a good explanation of how such a transistor would be manufactured.

All transistors have a reverse Hfe as you can always reverse the position of the collector and emitter. It's equivalent (but not strictly identical - Hfe is large signal, BF is small signal hybrid-pi) to the SPICE parameter BR just as BF is equivalent Hfe.

If you operating the amplifier as an amplifier, the lower the reverse gain the better because it represents a "parasitic" reverse BJT operating in the opposite direction. Amplifiers are closer to ideal when they are unilateral in gain. As @cabraham says, having forward and reverse transistor characteristics bilateral (equal) makes for a bad amplifier but a good (bilateral) switch. An asymmetric device can be a good switch if the switched current will be flowing in only one direction (collector to emitter) like an amplifier.

 

[vk6kro]

Because this transistor effectively has AC on its collector, they have chosen a transistor that works well with either polarity on it.

It has AC on it because it is being used to shunt capacitively coupled signals to ground.