On Sun, 15 Oct 2017 09:48:52 +0100, John Williamson
wrote:
On 15/10/2017 08:40, Don Pearce wrote:
On 14 Oct 2017 23:47:58 -0400, (Scott Dorsey) wrote:
The minimal op-amp is at least four transistors, maybe five if you want
decent output current drive. Two for the differential input, one for the
intermediate gain stage, then one (or two if you want to make it push-pull)
for the output stage. Add a couple constant current sources, a compensated
reference on the output pair, and the number of components climbs pretty fast.
--scott
Unless you use an integrated op amp. 5532 is probably my favourite
do-everything op amp. The count becomes one op amp and two resistors.
I think that Scott is making the point that inside the black box of the
op amp IC, there are many, many active components, all of which have a
potential effect on noise and linearity.
I know that - but in terms of effort and cost I need to expend to use
it, the op amp wins. And it is the complexity of the op amp that makes
it work so much better than discrete transistors. Almost everywhere
the normal circuit puts a resistor, the op amp has a current source.
That and a hundred other refinements that only come from an unlimited
component count make the op amp work better than the discrete circuit.
It is only really in ultimately low noise circuits like a mic preamp
that I will add several discrete transistors at the input. They will
be reasonably high power transistors (for low internal resistances),
with several placed in parallel until the combined voltage and current
noise hits a minimum at the source impedance of the microphone.
d
---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus