OpAmp Buffer configuration with max allowed output

Question

I'm designing a circuit, and a section of it is as follows:

^{simulate this circuit – Schematic created using CircuitLab}

What circuit can i use to allow OA1 to follow the Non-inverting input from 0 to 5V exactly, and anything above 5V in the input remains 5V on the output?

(i thought of using 2 resistors or a trimmer to reduce voltage on the input, but i have no use of the 5-10V range, and i want to use the full 0-5V range on the output)

(the 0-10V input is not available for redesign, the OA2 doesn't exists, but i placed it there to signify a high impedance circuit after the output.)

Note: I'm trying to be accurate, i could use a resistor+zener diode on the Output to limit to the closer commercial value for zeners eg. 5.6V, but there must be a better and more precise way to do this..

Answer 1

Here's my solution:

^{simulate this circuit – Schematic created using CircuitLab}

How it works is that the 3rd opamp will compare Vlim against a 5 V reference voltage. As soon as the voltage rises above the 5 V reference, the output of OA3 will go low this basically works as an ideal 5 V zener diode. As soon as Vlim goes below 5 V, OA3's output will become high and the diode will be in reverse mode.

So the diode is to prevent OA3 interfering when Vlim < 5 V

The accuracy of the 5 V clamping level is mainly determined by that level itself and the DC offsets of the OA3. The diode is in OA3's feedback loop so the voltage drop across it does not matter.

Answer 2

A web search for opamp min-max circuits some time ago led me to a fascinating AD8036 Voltage Feedback Clamp Amp from Analog Devices. This device, when used in non-inverting mode can clamp the input voltage between an upper and lower limit.

Figure 2. Clamp-amp block diagram. (See page 16 of the datasheet.) \$ +V_{IN} \$ is applied to A1's non-inverting input until \$ +V_{IN} > V_H \$ at which point S1 applies \$ V_H \$ to A1. Similarly when \$ +V_{IN} < V_L \$ S1 applies \$ V_L \$ to A1.

There are a few limitations to the device - particularly the supply voltages (+/- 3 to +/- 5 V) and 6.3 V max \$ \Delta \$V between \$V_H\$ and \$V_L\$ - but you can work around those by, for example, working in the 0 to 1 V range on your inputs and putting a x 5 amplifier afterwards.

Answer 3

^{simulate this circuit – Schematic created using CircuitLab} Might have to change the resistor values, but the ratio is there and you get the idea.

Answer 4

Just make the positive voltage rail +5V rather than 24V and use a rail to rail op amp.

You could get 5V from 24 with a linear regulator or build your own with a voltage divider buffered by another op amp and a pass transistor.

The circuit may not have great ac characteristics if you're going to be slamming the op amp into the 5V rail.