Re: can anybody help me to understand how this circuit works
R1, R2, R3 and R4 in voltage divider configuration are used to control base voltages of Q1 and Q2 ..
For example, if the output of the opam is -15 (or close to) this should cause the Ube of Q1 to be ≈0.7V, so it should be fully open Q1, and the output voltage should be close to +125V, and the opposite way around, +15V should close Q1 and open Q2 ..
However, you have 1MΩ R5 (same with R6, but at the other end) which role is, as negative feedback, to lower the output voltage from +/-125 to possibly +/-100V ..
Q3+R11 and Q4+R12 limit the output current to ≈ +/-27mA ..
Other "funny" thing about this circuit is that you need an input voltage below -10V to change the output form -100 to +100, as there is quite strong positive feedback between the output and the opamp (+) input (R13) ..
In other words, for any input voltage above -10V the output will be always -100V (or so)
Regards,
IanP