Hello Mr Al,
So you mean like in the following "SEPIC" schematic, (where the concerned opamp is now U5) ....I could change it to the other schematic "SEPIC with diode"? (shown below)
Hi Fly,
Yes, that is the basic idea. However, as i mentioned in my previous post there are some important issues that have to be dealt with.
First, the output resistor to ground has to be low enough to swamp any effects from the output load because the op amp section no longer has the ability to pull the output down. The equivalent resistance could be as high as 500 megohms depending on the diode used (like 1N4148).
So that means we have to pay strict attention to the detail of what exactly is going to pull the output down now that the op amp output no longer has any pull down control over the output node we are now using as the op amp 'output'.
This could be as simple as calculating the current through a resistor and making sure the pull down resistor can sink that current and still provide 0.005 volts at the output. But with capacitance as part of the load, we'd have to make sure it can discharge the cap in a fast enough time to not bother the dynamic aspects of the design, which could be dramatic depending on the required feedback response.
For your circuit with U5 and R14 and C6, you should calculate the max voltage that can appear across C6 and then calculate the max current through R14, then make sure that R4 can sink that current and still provide say 0.005v output level. SO if the max voltage for C6 was 3.3 volts with the 3.3k resistor that means a max current of 1ma. And 1ma through 51k is 51 volts, so that's not going to cut it
To get 0.005v with 0.001 amps, a 5 ohm resistor would be required. Obviously that's too low for the U5 op amp to handle, so we'd have to impedance scale that part of the circuit. For example, with R14=33k, we'd need a 50 ohm resistor. With R14=100k, we'd need a 150 ohm resistor. So you see how that works. But then the time constant is altered, so we'd have to change C6 to a smaller value. But then the following time constant is altered, so we'd have to increase R15 and recalculate R16 and whatever other parts are associated with that circuit section.
So it gets a little more complicated with this circuit, but you get the idea