# Bootstrapping low voltage opamp

I was analyzing the below circuit. In this circuit an opamp supply voltage (V+ − V) = 16V is powered by using a 45V supply and a -5V supply.

I understand the circuit to some extent.

Here U1 is supply voltage is varied such that V+ - V- is always kept as 16V.

I need a few clarifications

1. What is the role of the capacitor C3?
2. Why is the U1 output connected to the junction of R2 and R3?
3. How do I do design calculations for circuits like this?
4. How can I perform a stability analysis?

• Unless I am missing some subtle point, this seems extremely complex. Why not use a high voltage OpAmp. such as the OPA454, directly off the supplies? – henros Apr 16 at 11:45
• Because the impedance of the source is 200Tera ohm.So we cant measure it direclty.LMC622's I/P impedance is greater than1Tera Ohm. – HARI T O Apr 16 at 11:53
• What source? What are you working on? Why do you only have 45V and -5V available? – JRE Apr 16 at 12:21
• to measure the o/p of a charge pump present inside the ASIC – HARI T O Apr 16 at 12:37
• Source impedance is clearly important here, so it would have been helpful to have stated it in the question. I also think the source and OpAmp impedances are switched, otherwise the OpAmp loads the source. – henros Apr 16 at 16:44