Can someone please have a look at this diagram and advise if it is
workable?
I've not seen a diagram that isn't workable. Not while it is just a diagram.
The object is to output two identical signals, one with a +12V shift
above DC/GND, and the other a -12V shift.
IOW there is a 24V differential between the mid-points of the two
waveforms, with respect to each other, centered on the GND reference
of the input signal.
http://www.4shared.com/photo/9Ixd-Ldp/DC-offset-amps.html
Your diagram is somewhat confusing. The two op-amps are on one IC, U1.
What is the U2 box? and why isn't its mating pair labeled?
Are those separate boxes just the power connections to the op-amp IC?
It seems like the circuit you're trying to design actually has two separate
op-amp IC's, U1 and U2. You're trying to give each of these a single voltage
supply: one from 0 to 12V and the other from -12 to 0.
Firstly, a dual supply based on batteries is risky. The reason is that the
batteries may not discharge evenly over time, leading to unequal voltage rails.
The same comment applies to the +12/-12 midpoint on each side between
BT4 and BT3, and BT2 and BT1.
Secondly, batteries are not ideal voltage sources and have an internal
resistance. As AC signals leak back into the power supply circuit, the signal
voltage will then appear in the power rails, thanks to that resistance. You
need some bypass capacitors to combat this problem.
Third, your + connections to the op-amps are left floating. You cannot do this;
the + connections must be connected to a reference voltage, otherwise the
op-amp circuit is meaningless. If you have a chip with multiple op-amps and
some of them are not used, even the unused op-amps have to be properly
connected, never mind used ones.
You have no feedback to control the gains. Basically your circuit is quite
incomplete. (Please post what you think is a complete circuit.) The reference
voltage is different for the different op-amps because you want a DC offset
between them. Thinks about what you want the output to be when the input is at
0V with respect to ground. How are you going to make the top op-amp put out 12V
(the midpoint between 0 and 12) when the signal at the - input is at 0V?
One obvious way to make the implied circuit topology to do the required job is
to generate a +12V voltage reference at the + input of the top op-amp, and a -12V reference at the + input of the other op amp, with some crappy voltage
dividers (that are well bypassed with capacitors). (You have the battery
dividers for this already, but see the note of caution above: if BT4 discharges
faster or slower than BT3, that reference level will shift!)
Add negative feedback to stabilize the op-amps (are you looking for gain, or
just unity buffering?). Then, use capacitors to couple the input signal to
the - inputs (with separate capacitors, one in front of each op-amp). Your
input will be centered on 0V, and the coupling capacitors will take care of
bridging the AC component of the input signal into the 12V and -12V DC levels
present at the op-amp inputs.
