Why don't chip amps use emitter resistors?

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The reason I didn't post in "chip amps" is because I'm wondering how to build a descrete circuit to work just like a chip amp, for simplicity and also to learn something.

To keep it simple, I'm comparing a descrete single-pair output stage with a chip amp and am curious if a descrete circuit can be designed and work just like a chip amp.

Chip amps maintain bias current pretty well without emitter resistors, are fairly stable, and can run a wide range of voltages. Although high powered chips seem to run hotter than equivalent descrete designs of the same wattage.

Descrete amps rely on the voltage drop of the emitter resistors for bias stability, and most descretes seem more picky about supply voltage and bias fluctuations. However, descretes are way more rugged and can handle higher powers.

About 2 yrs ago, I tried to make a large descrete with no emitter resistors, it played fine in Class B but as soon as bias was added it went into severe oscillations, and HIGH current draw, until emitter resistors were added, then everything worked fine.

What's your input on this? Is it possible to have a reliable descrete output with no emitter resistors?
speculation - since the output transistors are on a common substrate with the bias control and therefore thermally coupled much more closely than possible with discrete packages on a common heatsink?

Are the bias circuits more complex, taking advantage of the complexity possible with integration?

I'm interested to hear from someone who knows.
The one and only
Joined 2001
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leadbelly said:
1) There are discrete designs with no emitter resistors, look at the JLH amp for example.

This is true. You will notice that later versions did use emitter/
source resistors.

What is interesting is that the topology's driver/splitter transistor
offers a compensating element. More current through it means
less drive for the positive output, and the feedback loop adjusts
the offset, also adjusting the bias.

Having said that, note that the Dartzeel amplifier also has no
emitter resistors.
It's easy to build an amp that doesn't use emitter resistors...all you have to do is match the everlivin' daylights out of the output devices. That way you don't have current hogging and all those other unpleasant things. Assuming that you've got good control of your manufacturing process, you can make chip transistors that match fairly well. Hand matching to equivalent levels can be done--it's just time consuming.
In the real world, you can match the devices to a somewhat lesser degree and use smaller value emitter resistors.



Joined 2005
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mightydub said:

And they weren't very thermally stable. A while back I repaired an NAD receiver with essentially the same amplifier stage that had blown its outputs and drivers. I had to add emitter resistors to get it stable.

I'm sure you replaced with "new" parts that may have slight different characteristics compared to the original ones, the design sure is sensitive.
Tube_Dude said:

I agree with you! I have worked in many NAD 3020 and they are all thermally stable.

we've wandered off topic a bit, but the explanation provided (in another thread from a long time ago) to me was that the original 2955/3055 transistors had essentially a "built in" emitter resistor due to the manufacturing process. Newer parts with a different process apparently don't have that, thus the problem.

Thinking about the original topic a bit more, I suspect that one reason for chip amps not having emitter resistors is power dissipation.
Conrad Hoffman said:
You don't seriously think you know what's really inside those chip amps, or any other IC, do you? Sure, sometimes they give you an equivalent schematic, but not all the tricks are shown.

You do make a good point.

I do notice that when the schematics are shown, they usually don't list ALL the component values. Stuff like zeners or other component of current sources inside the IC.

Also, has anyone here tried to make an MOS output or CFP amp that can clip near the rails like a 12V car radio IC can?
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