[sdiy] Is a LM301 == LM301a

[Bob Weigel]

And I seem to recall noting that in the last one I worked on.  The only 
time it's going to make a difference of course is if your output gain is 
really high and you will tend to have so much DC offset due to this that 
you get early clipping on one rail....or you have a DC critical 
application. 
     A little op amp primer would be in order probably.  You will 
usually note a few types of basic configurations for op (operation...as 
in mathmatical operations like addition, multiplication, etc.)amps in 
these circuits.

1) The buffer.  Here there is a wire tied between inverting input and 
output.  The signal is fed into the non-inverting input.  Reason?   The 
inputs have very high impedence (micro amps max on bipolar units usually 
and nano amps on JFET intput like TL0xx, LFxxx and so on...and pico amps 
on mosfet input units like CA3140 it seems like, TLCxxx etc.)  Thus they 
draw very little current and so you can interface something like a weak 
current producing pickup with circuitry that demands several 
Milliamps...or by adding output transistors in the right way, even amps. 
      The principle of operation is pretty simple.  The inputs function 
as a differential amplifier with a gain of over 10,000 usually.  So...if 
for instance the + or non-inverting input is a millivolt above the 
inverting or - input, you might see like 10V on the output.  In the 
buffer however...this is not possible because the - in is tied to 
output.  So whatever the voltage is when the thing is turned on, it 
quickly notices "Oh I'm a tenth of a volt above the + input" and the 
voltage of the output responds quickly by diving until it crosses over 
the point where now the - input is = to the +.  and at that point of 
course there is no differential voltage and if the amp is ideal it holds 
at that voltage.  They aren't of course, quite ideal.  Every circuit has 
a tiny amount of inductance and capacitance and there will be some 
ringing always at high frequencies.... and...

2) The inverting amp.  Here we tied a resistance from the signal to the 
- input.  We ground the + input.  And finally we tied a feedback 
resistor from out to - input. Now, let's just say the resistances are = 
in value.  For starters.  And are much much less than the impedence of 
the inputs.  (10K or 100K for instance.  )   Now, the + isn't moving 
from ground. Sooo..if when we turn on power there was some condition 
where say the output was -.1V but the input signal was 0V.  Immediately 
a current begins to flow through the resistors from signal in to 
output.  Since the resistors are equal and the flow into the input 
negligable....the voltage at the input is -.05V obviously.  ie there is 
an equal voltage drop across the two resistors of equal value since an 
equal current MUST be flowing through both in order for charge to be 
conserved..as we say.
       Ok so that voltage will send the output quickly soaring towards 
the positive rail since it wants to be 10,000 X .05V (the voltage that 
the + is above the - input...ie. 0 - -.05)  =  500V!!  The typical rail 
on an op amp is 18V maximum in most circuits we'll see.  So that's why I 
say it goes soaring towards positive rail in this case....rather than 
500V :-)
        Well...long before it gets there the condition is no long like 
that.  In fact, we see that when the output reaches ground instead of 
-.1V, there is no longer any current flow between the two points and 
thus the voltage at the - in is also zero and there is no longer any 
reason to amplify.  However, in these circuits, as we add parts we see 
more and more overshoot and it is often necessary to put a bypass 
capacitor in the feedback loop as well.
        Consider this as you hold that thought.  Say we now introduce a 
signal on the input.  As the signal goes positive, the - in will tend to 
start to go positive which will cause the output to go negative since 
again the + input is held to zero!  So this effect will basically 
regulate the -input to zero...and you can picture is AS THE FULCRUM OF A 
TEETER TOTTER!
        Now...let's say the feedback resistor is variable.  As we turn 
up the amount of that resistance, what happens?  It's like moving that 
fulcrum!  Now, in order for Zero to be maintained at the - input, the 
output must swing further than it did before.  In fact, if we double the 
resistance, it has to swing twice as far since the voltage drop across 
that resistor is precisely double...since again the current through both 
resistors is the same due to the high input impedences! 
        Now recall our bypass capacitor.  What is it doing?  It's 
basically...short circuiting the gain on the very high friequency 
oscillations that would otherwise result in such an amplifier.  But the 
lower frequencies charge that tiny capacitor up and don't even notice it.

3) Non-inverting amps-  Here, in the simplest form, signal is again fed 
straight to the + in.  There is a feedback resistor to the - in but also 
a drop resistor from the - in to ground.  This simply forms a voltage 
divider network.  So here if the two resistances are equal, you will get 
half the voltage for instance at the - in that exists at the output.  
ie. a non-inverting amp with a gain of 2.  As the signal rises on the + 
in so the output rises in response until it has produced an equal 
voltage at the - in.  1 Volt in means the out must rise to 2V before the 
divider network produces 1V on the - in such that the + isn't at a 
higher voltage and thus continuing to cause the output to rise.

These forms are manipulated in many ways, including using diodes instead 
of resistors in places to create amps that respond log or exponentially, 
or capacitors to perform integration or differentiation functions.  All 
kinds of possibilities exist.  The op amp is a wonderful organizational 
tool for circuit design.  Though I believe some of the coolest audio 
circuitry is where people use...not quite so ideal building blocks :-). -Bob

Paul Schreiber wrote:

> A = lower offset voltage.
>
> Paul S.
>
> ----- Original Message ----- From: "Jeff Farr" <moogah at gmail.com>
> To: <synth-diy at dropmix.xs4all.nl>
> Sent: Monday, July 25, 2005 11:41 PM
> Subject: [sdiy] Is a LM301 == LM301a
>
>
>> I ask because one place has 301's for < 1$ while another has 'em for >
>> 6.50$.  The schematic for my odyssey shows "301a".  Perhaps this is
>> another case where many op-amps will do.
>>
>
>