Single supply op-amp query

[Don Tillman]

   From: johns at oei.com
   Date: Mon, 13 May 96 15:48:45 

   I have a theremin that gives a weak 2.5V max CV output that I need to 
   boost to around 10V max.  I would like to add a voltage amplifier to 
   scale the voltage to a usable level.  Unfortunately the theremin only 
   uses a single voltage power supply.  So in order to scale the CV 
   higher, I will need to use a some single supply op-amp.  

   I browsed over several National Semi data and application books and 
   could only find that they suggested using a LM3900 for single supply 
   operation.  

Note that the LM3900 is not an opamp.  It's inputs are currents, not
voltages. You probably don't want to use this.

	       The problem with the LM3900 when used as a positive 
   voltage (0V to <+V) amplifier with a single supply is that you can't 
   let the input go below .5V because you need to keep the input diodes 
   forward biased.  So you can see that a hefty portion of the CV range 
   will be wasted if my input range is only 0V-2.5V.  Basicly, the LM3900 
   in only linear on input voltages from .5V to <+V

Actually the input voltage can't go very far above .5 volt either.

   Date: Mon, 13 May 1996 17:04:04 -0700 (PDT)
   From: MACHINE MEDIA <cdmaster at netcom.com>

   An LM 358 will run off a single-supply from 3 to 30 V and can run its 
   inputs an outputs all the to the negative supply (in your case, ground).
   It is an internally compensated dual unit.  

Excellent choice.

   Btw, why is it that some op amps are deemed suitable to be run off a 
   single supply and others aren't??  How does it know if Vee is below 
   ground or not??  Wouldn't it only see the potential between Vcc and Vee??

A typical (non single supply) opamp has a differential pair of NPN
transistors or N-channel FETS at the input.  If one or both of these
inputs goes down to within a couple volts of the Vee level, anomalous
behavior starts happening and the output might likely slam high when
its not supposed to.  

Single supply opamps have PNP input transistors and the rest of the
circuit is designed to allow the unit to keep operating fine as the
input voltages go down to Vee.

The ground isn't really an issue; the opamp generally doesn't know
from ground.

  -- Don