Transfer characteristic of circuit
Hints: 







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. (a) Redrawn circuit. (b) Rearranged but equivalent circuit.


If you redraw the circuit (Figure 1a) it should become clearer what the D1/D2 junction voltage is without the sine.
If you are using real diodes rather than ideal then rearrange as shown in Figure 1b.


Now you should be able to replace the batteries, D1, D2, R1 and R2 with a single voltage source and resistor.