unsigned op amp output?
Figure 1. LT1078 Absolute Value Circuit (Full-Wave Rectifier).


  For negative inputs, the first section operates as a closed-loop inverter (A=-1) and the second stage is simply a buffer for the positive output. When the input signal is positive, the first opamp output remains saturated near ground and the diode becomes high-impedance, allowing the signal to pass directly to the buffer stage non inverted. The composite effect is a full-wave rectified waveform at the output of the buffer.


It seems as though this will satisfy the basic requirement of your question.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 2. 9 V DC offset removal.

Your original question suggested that there is a 9 V bias on the input signal that needs to be removed. The 'adder' circuit of Figure 2 will accomplish that by adding -9 V to the input signal. This circuit is inverting but that won't matter to a full-wave rectifier that follows.