Audio amplifier: Do I need feedback?

[Joel Kolstad]

I'm about to (attempt to) construct a simple class AB audio amplifier, and I
was wondering whether or not I actually should make the input stage a
differential pair and use feedback? (As opposed to just making it something
like a common emitter amp...) When, e.g., Sedra & Smith talk about
amplifiers, it's in a separate chapter from that on feedback, and while I
think I'm well aware that the use of feedback is generally desirable to
avoid frequency vs. gain variations -- and just output gain variations from
one unit to the next -- I'm just looking to build one unit of perhaps 50
watts mainly for my own edification so I don't really care what the _exact_
overall gain is anyway.

If I just build the thing open-loop, since the power stage (2N3055/2N2955
complementary pair) is going to have a gain very close to just one anyway, I
don't see that there'd be a large variation in gain vs. frequency anyway?
(Just whatever happens in the input stage...)

I had been planning to drive power stage with something like a BD139/BD140
(a lot of my choice of transistors is based on reading about this guy's
amplifier --> http://sound.westhost.com/project3a.htm ), although it seems
like I could -- just barely -- get away with the ubiquitous 2N3904/2N3906
pair. Comments? (In fact, I ended up getting some BD139/BD140's from On
Semiconductor as samples -- I'd still love to find a place that'll sell
small quantities of them... the power stage transistors came from All
Electronics, for instance!)

Thanks,
---Joel Kolstad

 

[Walter Harley]

I'm about to (attempt to) construct a simple class AB audio amplifier, and I
was wondering whether or not I actually should make the input stage a
differential pair and use feedback? (As opposed to just making it something
like a common emitter amp...)

Just to be clear: you're wondering about global feedback, that is, feedback
from the output stage all the way back to the input. Essentially all audio
amp circuits use feedback of some sort, it's just not necessarily global, it
may be within each individual stage.

In addition to the other things you mention (e.g., gain stability), global
feedback also serves to reduce distortion.

Global feedback does have certain perils, such as the increased possibility
of oscillation, which are well-known, well-understood, and fairly easy to
work around.

You might be able to answer your question by observing that the vast
majority of commercial audio amps do indeed use global feedback. My usual
engineering rule of thumb is to copy known successes unless I have a very
good reason to be different.

Personally, I found Douglas Self's "Handbook of Power Amp Design" (I think
I'm remembering the title correctly) to be very informative in explaining
just why it is that things are the way they are.

 

[Nico Coesel]

I'm about to (attempt to) construct a simple class AB audio amplifier, and I
was wondering whether or not I actually should make the input stage a
differential pair and use feedback? (As opposed to just making it something
like a common emitter amp...) When, e.g., Sedra & Smith talk about
amplifiers, it's in a separate chapter from that on feedback, and while I
think I'm well aware that the use of feedback is generally desirable to
avoid frequency vs. gain variations -- and just output gain variations from
one unit to the next -- I'm just looking to build one unit of perhaps 50
watts mainly for my own edification so I don't really care what the _exact_
overall gain is anyway.

If I just build the thing open-loop, since the power stage (2N3055/2N2955
complementary pair) is going to have a gain very close to just one anyway, I
don't see that there'd be a large variation in gain vs. frequency anyway?
(Just whatever happens in the input stage...)

One of the key elements of feedback is that is will increase the
bandwidth of your amplifier. But it is a good idea to start with an
open-loop amplifier and measure what it does.

I had been planning to drive power stage with something like a BD139/BD140
(a lot of my choice of transistors is based on reading about this guy's
amplifier --> http://sound.westhost.com/project3a.htm ), although it seems

This diagram is quite basic. Be aware though that it lacks one
important feature: current limiting! When the output is shorted, you
may damage the driver stage.

 

[Joel Kolstad]

Just to be clear: you're wondering about global feedback, that is,
feedback from the output stage all the way back to the input.

Ah, thanks for the clarification. Yes, the individual stages have, e.g.,
emitter degeneration resistives to provide some small amount of feedback.

Global feedback does have certain perils, such as the increased
possibility of oscillation, which are well-known, well-understood, and
fairly easy to work around.

From what I've seen on the web, using a Miller capacitor/dominant pole
compensation capacitor (I'm not quite sure what it's probably called)
appears to be a popular technique. I've used this before (for class work,
not real projects) in low power amplifiers

You might be able to answer your question by observing that the vast
majority of commercial audio amps do indeed use global feedback. My usual
engineering rule of thumb is to copy known successes unless I have a very
good reason to be different.

Sounds good -- I'll see how it performs open-loop, and then close the loop
and bang on it even more.

I was planning to run through some SPICE simulations on this, although I'm
not sure how accurate the models for, e.g., 2N3055's and 2N3904's are for
parasitics if I really wanted to know exactly what the phase margin was
going to be in a closed-loop design.

---Joel Kolstad

 

[Walter Harley]

From what I've seen on the web, using a Miller capacitor/dominant pole
compensation capacitor (I'm not quite sure what it's probably called)
appears to be a popular technique.

Yes, that's what's used in the real world. Your classes were right

I was planning to run through some SPICE simulations on this, although I'm
not sure how accurate the models for, e.g., 2N3055's and 2N3904's are for
parasitics if I really wanted to know exactly what the phase margin was
going to be in a closed-loop design.

Sounds like you already know what you're doing; but anyway, things like
physical layout matter. The SPICE models are probably good enough to catch
the "formal" instabilities, but they won't cover inductances in the supply
lines, parasitic capacitances between adjacent traces, blah blah. First get
it to work in SPICE; then get it to work in the real world. (Douglas Self
makes the interesting point, certainly borne out by my experience, that the
destructive oscillations encountered in practice are often much higher in
frequency than could be accounted for by treating the amp as a phase-shift
oscillator.) Oh, and use a Zobel network on the output if there's any
possibility of a reactive load.

 

[Kevin McMurtrie]

I'm about to (attempt to) construct a simple class AB audio amplifier, and I
was wondering whether or not I actually should make the input stage a
differential pair and use feedback? (As opposed to just making it something
like a common emitter amp...) When, e.g., Sedra & Smith talk about
amplifiers, it's in a separate chapter from that on feedback, and while I
think I'm well aware that the use of feedback is generally desirable to
avoid frequency vs. gain variations -- and just output gain variations from
one unit to the next -- I'm just looking to build one unit of perhaps 50
watts mainly for my own edification so I don't really care what the _exact_
overall gain is anyway.

If I just build the thing open-loop, since the power stage (2N3055/2N2955
complementary pair) is going to have a gain very close to just one anyway, I
don't see that there'd be a large variation in gain vs. frequency anyway?
(Just whatever happens in the input stage...)

I had been planning to drive power stage with something like a BD139/BD140
(a lot of my choice of transistors is based on reading about this guy's
amplifier --> http://sound.westhost.com/project3a.htm ), although it seems
like I could -- just barely -- get away with the ubiquitous 2N3904/2N3906
pair. Comments? (In fact, I ended up getting some BD139/BD140's from On
Semiconductor as samples -- I'd still love to find a place that'll sell
small quantities of them... the power stage transistors came from All
Electronics, for instance!)

Thanks,
---Joel Kolstad

Feedback is for more than frequency stability. It reduces the output
impedance, reduces crossover distortion, and keeps the gain stable as
semiconductors age and change temperature.

The differential pair is a pretty easy setup. For an inverting amp, you
can replace the positive input transistor with a diode and replace the
constant current source with a resistor.

|
|
|
\|
|--- (-) in
<|
GND--->|--+--/
|
|
|
R
|
+----C--- GND
|
R
|
- rail



Fancy amps have the full differential pair. One is used to pull the
output voltage up and the other uses a current mirror to pull it down.
It makes the slew rate more symetrical than the usual design with a
constant current source pulling in one direction.

 

[R.Legg]

one unit to the next -- I'm just looking to build one unit of perhaps 50

watts mainly for my own edification so I don't really care what the _exact_
overall gain is anyway.

If I just build the thing open-loop, since the power stage (2N3055/2N2955
complementary pair) is going to have a gain very close to just one anyway, I
don't see that there'd be a large variation in gain vs. frequency anyway?
(Just whatever happens in the input stage...)

For a 50W output you would need a signal swing of +/- 28V @ 8ohms, or
+/- 20V at 4 ohms.

Because most signal sources don't provide this amplitude, you'd have
to have gain built in somewhere that exceeds unity. Probably a voltage
gain of 5 or 10 would be required.

When this gain is achieved, I expect you will be able to identify
components that determine this gain level, and recognize that they
form a local feedback network.

Adding current buffers to develop output power, without accounting for
their non-linearities, doesn't make a lot of sense.

A tube circuit gets impedance matching through an output transformer,
which can compensate for all sorts of tom-foolery in the driver. For
some reason, it's not fashionable to do the same thing with
transistors.

RL

 

[Leon Heller]

[deleted]

(In fact, I ended up getting some BD139/BD140's from On

Semiconductor as samples -- I'd still love to find a place that'll sell
small quantities of them... the power stage transistors came from All
Electronics, for instance!)

Farnell sells BD139/140s. They have a presence in the US.

Leon

 

[R.Legg]

Some audio power amp designs used transformers when transistors were new and
they were still basing the circuits on existing tube circuits, but you don't
see them any more. Presumably this is because with transistors it's simply
not necessary. It is debatable whether you can successfully engineer around
the various electrical problems of power transformers, but there is no
debate about them being bulky, heavy, and expensive. A high-fidelity output
transformer capable of even 50W is a beast.

The whole idea behind the 'quasi-complementary' output stage (ca
1962)was to remove the need for a high power PNP output device, or
matched devices of opposing polarity. There was good reason for this
at the time. There still is if you're counting your pennies.

Using the quasi configuration for both devices, to swap positions of
the opposing polarity power devices can only be seen as an attempt to
remove the vbe of the hottest parts out of the biasing equation.

Comments about fashion and transformers was an attempt at humour. It
was prompted by the main subject - the fetish of global feedback
avoidance. I believe this is a hang-over from the early days of IC use
in audio (ca 1970).

RL