[hozo]

Sigh.. no one learns analog circuits these days; it's all digital. Talked to several EE friends w/o help. Please excuse my basic electronics knowledge.

I’m trying to understand the following circuit.

from http://headwize.com/projects/showfil...=cmoy3_prj.htm

Can anyone please explain how the differential input stage is getting a differential signal from one input (most examples online show 2 inputs)? Suggestions for available BJT substitutes?

Here’s my understanding
-The diff amplifies the full signal w/ bias.
-The push pull separates it into pos and neg waveform (but there's no cap between the stages?).

Is it possible to simplify it to a two-stage push pull or even one stage?

Not too concerned about sound quality right now...just trying to get a very simple working amp.

 

[tyre]

Dees this layout look okay for the schematic posted above by hozo?

 

[trbl]

Quote:

Originally Posted by hozo Can anyone please explain how the differential input stage is getting a differential signal from one input (most examples online show 2 inputs)?

Both inputs are often used, either because the input signal is balanced, or because the output signal, usually in attenuated form, is being applied to the inverting input to reduce gain and linearize the circuit by applying overall feedback.

In the simple circuit you show, there's no negative feedback applied. Instead the other input just gets a DC voltage that adjustable via P1 to adjust the DC output level of the amp.

This a pretty bare bones circuit. Listing its flaws probably isn't useful, especially as you say you just want to get something that puts out a signal.

In differential circuits like this, the difference action comes from having a (preferabbly) large impedance where P3 is in the circuit you posted. In the olden days this was called a "long tailed pair", the "long tail" being the high value resistance or constant current source from the junction of the two emitters of the input transistor (originally the cathodes of two triodes) to the negative supply rail. Let's assume this really does act as a constant current source, so that the total current through the two input resistors is fixed at some value. Then when one transistor draws more current, the other gets less. With equal load resistances from the collectors to the positive rail, this gives the differential action. One side pulls more current, changing the drop across its load resistor, the other side gets less current by the same amount, changing the drop across its load resistor, so the changes (the output signal) are equal and opposite.

That's the theory of such a differential stage in its simplest form.

Quote:

Originally Posted by hozo -The diff amplifies the full signal w/ bias. -The push pull separates it into pos and neg waveform (but there's no cap between the stages?).

The input stage amplifies the signal and generates a push-pull signal from the single-ended input. The second stage is two single-ended amps, each amplifying one of the outputs of the first stage, and each is biased by the first stage (hence no coupling caps).