I am designing a low-power opamp (differential amplifier) that will be used in the low-power consumption devices, e.g., power management circuits, portable devices, or energy harvesting circuits. I am new to this area and not any expert, but this idea just came to my mind after taking an Analog Circuits course at my university.
I thought the power consumption would be around
200 uW, and operating voltage would be around
1.3-5 V. Thus the maximum quiescent current would be
200uW/5V = 40 uA. The following is the general idea of the above features rewritten:
- Maximum quiescent current ~
- Operating voltage
- Gain bandwidth product ~
I have searched through many textbooks and have done several SPICE circuit designs, but still couldn't achieve the above features with a good stability characteristic. (For example, the bias and start-up circuit in my design consumes a significant amount of quiescent current even operating at
1.3 V. Or when I tried to improve the stability by adding compensation network, the gain bandwidth dropped below a few of
10 kHz.) I also tried to reverse engineer some chip like TSU111 but the schematic is not revealed (unlike the ubiquitous
741 opamp which is quite well-known ?). It seems to me that it is almost impossible to reverse engineer the commercial chips out there.
- Is this just a secret of the company or is it just an ongoing topic in the research area?
I know this question is too broad. But I would like to ask some of the good practice or the general procedure in "designing/reverse engineering" the low-power opamp like in the commercial chips. The answer could be any that answers the following questions:
- What considerations should I take into account in the low-power design? (Operating in the subthreshold region to get a low quiescent current? Adding more stages to get more gain, but that means more quiescent current and thus power consumption?)
- Where in the circuit should I first start sizing the
W/Lratio of the
MOSFET? (I usually start at the bias circuit and try to size the
W/Lratio to get the current that meets the specifications that I want. Any other considerations?)