I am sorry if my original post caused confusion. Yes, I am looking for
an op amp because of the DC requirement. The LM386 I tried to no
avail.

Phrasing it another way, why is there no predefined op amp/Darlington
configuration, or off-the-shelf power op amp, that can do the all same
things as the LM386 does with audio, but down to DC?

Single supply
Wide supplyvoltage range
DC coupling
Easily variable gain
Low power
Etc.

At first, I thought such specs might be mutually exclusive. However,
if there are such design limitations, they seem to be receeding fast.

For example, there is the "new" OPA569, rated at 2A. It meets all my
requirements except it only operates up to 5.5V. I need a range of 4
to 12V.

http://focus.ti.com/docs/prod/folders/print/opa569.html

Can someone explain, why can't something like this be done with a
CA3130 and a couple of transistors.

Trevor Morton





On Fri, 23 Mar 2007 17:23:09 +0000, Eeyore
wrote:



Trevor Morton wrote:

I am still developing my abilities and find myself all too often
struggling with op amp design.

Can anyone here suggest an "all purpose" amplifier built around a
readily available op amp and transistor follower that will operate
within the specifications below? Preferably without changes in
componentry except to upgrade the power transistor depending upon load
requirements.

Is there any particular reson you want to use discretes for the output stage ?
You'll increase complexity lots.


DC coupled with frequency response DC to 20KHz

Do you mean audio ?


Input signal level 1-2V
4 to 12 volts single supply

If it's a single supply you'll have to AC couple the signal ! Unless you want a
bridge tied load of course (more complicated).


Low power DIP op amp, suited to battery operation if required

Well, power dissipation is likely to be predominantly determined by any quiesent
current in the output stage (to avoid cross-over distortion) actually.


Variable gain, unity to 10 via trimpot (dependent on rail V)
Swing as close to rail as practical without using a rail-to-rail amp
Drive resistive load 20 to 200 ohms (dependent on power transistor
used.)

Other specs are non-critical.

Are any of these, such as the wide supply range, inherently
contradictory to a predominantly fixed value circuit?

Any help would be very much appreciated.

The simplest best bet might be to look at some car audio amplifier chips.

Graham